TryBuildCalcFrench Drain / Land Drain Materials Calculator (Stone, Pipe, Geotextile, Fittings & Capacity)
Size a French drain from the real trench — including the fall dug into it.
🕒 Last updated: September 24, 2026
Inputs
The Trench
Applies to every dimension on this form — trench lengths in feet, and widths, depths, bedding and overlaps in inches. Switching converts the figures already entered rather than reinterpreting them.
ℹ️The single biggest difference between this calculator and every other one. On flat ground the fall has to be dug into the trench, so the outlet end finishes deeper than the inlet end and the dig is a wedge, not a box.
The ground surface is level (or falls less than the drain needs), so the trench floor drops away from it. The outlet end finishes deeper than the inlet end by the full length times the slope. This is the normal case for a wet lawn or a low spot.
Drain Runs
Add a row per straight run. A main with laterals coming into it is best entered as the main plus one row per lateral — each has its own length, depth and fall, and the fabric and pipe are bought per run.
Run 1
💡Measured at the inlet end — the outlet end is worked out from the fall.
💡1% is the standard minimum — 1/8 in of drop per foot of run.
Pipe
ℹ️Changes the capacity by more than a factor of two at the same fall. The ribbed bore of cheap single-wall coil is the single most common reason a correctly-dug drain still backs up.
The cheap black flexible coil sold for yard drainage. Easy to lay around curves, and hydraulically the worst of the three — its ribbed bore carries barely 40% of what rigid pipe carries at the same fall.
ℹ️4 in / 100 mm covers almost all residential work. Capacity rises with diameter to the power of 8/3, so one size up is nearly three times the flow, not one third more.
ℹ️Every run has to be one continuous length, so the coil or stick count is a cutting problem rather than a total divided by a coil length.
💡Stone laid before the pipe goes in. At least 2 in / 5 cm, so the pipe can be brought to a true grade.
ℹ️The bedding is what lets you set the pipe to an even fall instead of following every dip in the trench floor. It also comes off the stone available above the pipe, which is why the cover figure in the result moves when you change it.
💡Unperforated pipe from the end of the stone to the outlet. Enter 0 if there is none.
ℹ️This is what the "add 10% for the outlet" rule on other calculators is standing in for — except it is a different product, not extra of the same one. Perforated pipe outside the trench just puts the water back into the ground you are draining.
ℹ️Solid pipe is a different product from the perforated coil in the trench — you cannot cut unperforated pipe off a perforated coil — so it has its own stock size. It is most often sold in 10 ft sticks.
ℹ️A knitted filter sleeve on the pipe itself. Useful in clean sandy ground; in silty soil it is the layer that blinds first, and once it does the water cannot reach the pipe even though the stone around it is still clear.
ℹ️Every French drain needs somewhere to put the water. Where the outlet ends up is also what limits how deep the trench can be at its far end.
The pipe simply emerges on a bank or in a ditch. The cheapest and most reliable outlet there is — provided the ground actually falls away far enough for the pipe to reach it.
Drainage Stone
ℹ️Open-graded angular stone is what a French drain runs on — it holds its voids open under load. The void ratio shown drives the storage figure in the result.
The near-universal specification for a French drain envelope: large enough to hold open voids, small enough to place and rake by hand.
ℹ️Stone is quoted by weight, by loose volume or by the bag depending on the yard — the result shows all three regardless, but the bag count needs to know which bag.
💡Stone that ends up beside the trench rather than in it, plus settlement as it is raked in.
Geotextile
ℹ️Decides the width of fabric the trench actually needs. A full wrap has to cover the floor, both walls AND two flaps that fold back over the stone — roughly twice the section a bottom-and-walls lining needs.
The specification that actually keeps a French drain alive. The fabric lines the floor and both walls, the stone goes in, and the two flaps fold back over the top and overlap in the middle so silt cannot enter from any direction.
ℹ️Non-woven needle-punched fabric. 4 oz is the standard residential weight; step up to 6 oz in fine or silty soil, where a lighter fabric blinds sooner.
ℹ️If the wrapped section is wider than the roll, the fabric has to be seamed lengthwise — and every seam is somewhere silt can find its way in. The result will tell you if a wider roll would cover it in one piece.
💡12 in / 30 cm is the standard specification, for both the fold over the stone and any lengthwise seam.
Finishing the Surface
ℹ️A cap comes off the top of the stone, so it reduces the stone you buy and the cover over the pipe at the same time. A soil cap also seals the drain from surface water, which matters if the problem is water standing ON the lawn.
A soil cap so the drain disappears into the lawn. Note this seals the trench to surface water — the drain now only takes what soaks down to it.
💡Grated boxes that let surface water in directly. The usual fix for a drain capped with soil and grass.
💡Soil takes up more room out of the ground than in it — 20-30% for most soils, more for clay.
Capacity Check
Sizes the water arriving against what the pipe can carry away and what the stone can hold. This is the question a materials list on its own never answers.
💡Everything that sheds water toward the drain — roof, paving, and the slope of ground above it.
ℹ️Sets the runoff coefficient — the fraction of rain that actually runs off rather than soaking in. A roof sheds nearly all of it; sandy turf sheds almost none.
💡Your local short-duration design storm. The reference table further down the page gives typical starting figures.
Cost Estimation
Drainage Stone Needed
5.64
US tons (5.12 tonnes) — 4.32 cu yd / 3.3 cu m loose
Perforated Pipe
60
ft (18.29 m) — 1 x 100 ft coil
Excavation
5.11
cu yd (3.91 cu m) in the ground — 6.39 cu yd of spoil to move
The fall is dug into the trench, so the outlet end finishes deeper than the inlet end. That wedge is 0.67 cu yd (0.51 cu m) more dig and stone than a constant-depth estimate — 15% above the 4.44 cu yd a flat calculation would have given you.
Capacity: Comfortable — the design storm delivers 17 gpm (1.07 l/s) into a pipe that carries 46.3 gpm full bore at 1%, so it runs at 36.7% of full.
Drainage Stone
Washed #57 Stone (3/4 in, open-graded)
In the trench: 111.08 cu ft
Order volume (incl. 5% settlement/spillage): 4.32 cu yd
Weight: 5.64 US tons (5.12 tonnes)
Void space at 40%: 44.43 cu ft — 332 gal (1258 L) held in the stone itself
Pipe
100 mm Corrugated HDPE Drain Tile
Perforated: 60 ft (18.29 m)
Solid to the outlet: 10 ft (3.05 m)
Coils to buy: 1 x 100 ft coil, plus 1 x 10 ft stick (rigid) of solid pipe
That is 110 ft of pipe bought for 70 ft laid — whole coils and sticks, so the cost below is based on what you buy.
Roughness n = 0.024 — full-bore capacity 46.3 gpm at 1%
Geotextile
4oz non-woven, full wrap (burrito)
Section to cover: 7.53 ft (2.3 m) across
Strips: 1 off a 12.5 ft roll, no seams
Rolls to buy: 1 x 12.5 ft x 360 ft full roll — standard drainage roll
Trench & Finish
Total run: 60 ft (18.29 m) over 1 run
Excavation: 5.11 cu yd in the ground
Spoil, bulked 25%: 6.39 cu yd
Bedding under the pipe: 3 in (7.6 cm)
Cap: 0.74 cu yd of topsoil and grass over the stone, 4 in deep
Fittings: 3 — 1 cap, 1 coupling, 1 outlet grate
Depth Along Each Run
| Run | Length | Fall | Depth at inlet | Depth at outlet | Stone over pipe |
|---|---|---|---|---|---|
| Run 1 | 60 ft | 1% — 7.2 in drop | 24 in | 31.2 in | 12.4 in → 19.6 in |
Depths are measured from the finished surface. In a dig-to-fall trench the stone cover over the pipe grows toward the outlet, so it is the inlet end that has to satisfy the minimum.
Which Runs Come Off Which Coil
- Coil 1: 60 ft — 40 ft left over
Before You Dig
- Because the ground is flat, the fall has to be dug into the trench: Run 1 starts 24 in deep and finishes 31.2 in deep, a drop of 7.2 in over its length. That deep end is what decides whether the outlet can daylight at all — check it against the ground you are discharging onto before you start digging, not after.
- That wedge of extra depth is 0.67 cu yd (0.51 cu m) of dig and stone — 15% more than a constant-depth estimate would have told you to order. It is the single commonest way a French drain takeoff comes up short.
- A topsoil-and-grass cap makes the drain disappear into the lawn, and it also seals it off from surface water. The drain will now only take what soaks down through the soil. If the problem you are solving is water standing ON the lawn rather than in it, leave the stone open to the surface or add a channel or catch basin that feeds the trench directly.
- The pipe carries the design storm at 36.7% of full bore, and the stone voids hold a further 44.43 cu ft (332 gal) — roughly 19.5 minutes of the design inflow even if the outlet were blocked entirely. This drain has real margin in it.
Assumptions Used
Straight, rectangular-section trenches with vertical walls. The ground surface is level, so the trench floor falls away from it and the excavation is a trapezoidal wedge — depth, stone and wrapped fabric are all integrated along the run rather than taken at the shallow end. Stone quantity is the trench section less the cap layer and less the volume the pipe itself displaces, at 96.8 lb/cu ft loose — published densities for washed open-graded stone range from roughly 89 to 104 lb/cu ft, so treat the tonnage as a band rather than an exact figure. Geotextile is planned as real strips off a 12.5 ft roll, with the widest (deepest) section governing, and the rolls are packed rather than divided. Every run is one continuous length of pipe, so coils and sticks are packed rather than divided; runs longer than the stock length are counted as jointed. Inflow uses the rational method (Q = C x i x A) at a runoff coefficient of 0.55 and 2 in/hr; pipe capacity uses Manning's equation running full at the flattest run's grade, reported against the standard practice of staying under 50% of full bore during the design storm. This calculator sizes materials and checks pipe conveyance — it does not verify that water can enter the perforations fast enough, model how quickly the surrounding soil releases water into the trench, check soakaway infiltration, or replace a drainage design where structures, retaining walls or property boundaries are involved.
Bill of Materials
Main material: 5.64 US tons (5.12 tonnes) + recommended tools
+−
Bill of Materials
Main material: 5.64 US tons (5.12 tonnes) + recommended tools
Computed quantities come from your own runs, trench dimensions and material settings above; the consumables below are general recommendations, filtered to this job — actual needs vary by product, ground conditions and site access.
For Your Job
Washed #57 Stone (3/4 in, open-graded)
5.64 US tons (5.12 tonnes)
4.32 cu yd / 3.3 cu m loose, including 5% settlement and spillage
Find suppliers100 mm perforated drain pipe
100 mm solid (unperforated) outlet pipe
4oz non-woven geotextile
1 x 12.5 ft x 360 ft full roll — standard drainage roll
1 strip at 7.53 ft across the wrapped section
Find suppliers100 mm fittings
Topsoil and grass over the stone
Excavated spoil to remove
General Tools & Consumables
Trenching spade / mattock
String line, pegs and a long spirit level
Laser level or water level (long runs)
Wheelbarrow and a stone rake
Garden hose (for the flow test)
Marker paint and utility locate reference
Pipework
Pipe cutter or fine-toothed saw
Rodent guard / outlet grate
Rodding point or clean-out (long or silty runs)
Geotextile
Landscape staples / fabric pins
Sharp scissors or a hooked utility knife
Capping & Making Good
Topsoil screen or landscape rake
Grass seed or turf offcuts
Safety
Gloves, eye protection and steel-toe boots
Worth Checking
Catch basin or channel drain
French Drain Diagram
Long section drawn from your own run: the surface stays level and the floor falls away from it, so the outlet end is genuinely deeper. Vertical scale is exaggerated against horizontal so the fall is visible — a 60 ft trench drawn true to scale would be a hairline. Cross-section layers are proportional to their real depths, with a minimum thickness so thin layers stay legible.
What Is a French Drain Calculator?
A French drain calculator works out how much stone, pipe and geotextile a drainage trench needs, and turns that into an order you can take to a supplier. It solves a problem that looks like simple volume arithmetic and is not: a French drain only works because it falls, and the fall changes the shape of the thing you are digging.
This calculator is built for anyone actually buying the materials — homeowners pricing a DIY drain, landscapers quoting a wet garden, builders sizing a footing drain, smallholders laying land drain across a field. You enter the runs, the trench section and the fall; it returns the excavation, the stone by volume and by weight, the pipe as real coils or sticks, the geotextile as real strips off a roll, the fittings, and the spoil to cart away.
It also answers a question no other French drain calculator asks: whether the drain is big enough. A materials list tells you what to buy. It does not tell you that a 3 in pipe at half a percent under a 4,000 sq ft roof will surcharge in the first real storm. This one sizes the water arriving against what the pipe can carry and what the stone can hold, and says which of the three is the binding constraint.
The one thing to take away. On flat ground the surface stays level and the trench floor drops away from it, so the outlet end of the trench is deeper than the inlet end by the length times the slope. A 60 ft run at the standard 1% ends 7.2 in deeper than it starts. Every mainstream French drain calculator takes the depth you type as the depth of the whole trench, which understates the dig and the stone by about 15% on that run — and the error grows with every extra foot of length.
French Drain Formula
Six steps, in the order the calculator works them out. Every one of them is a step where the common shortcut gives a different answer from the real geometry.
Step 1 — Depth along the run
Depth at the outlet = depth at the inlet + (length x slope)
Mean depth = depth at the inlet + (length x slope) / 2
On ground that already falls, both ends are the same and the mean is simply that depth. On flat ground the fall has to be dug in, and this is where every constant-depth estimate goes wrong.
Step 2 — Excavation (a wedge, not a box)
Trench volume = width x length x mean depth
Spoil to move = trench volume x (1 + bulking %)
The trench is a trapezoidal prism whenever the fall is dug in, so the mean depth — not the depth you typed — is what multiplies out. Soil also takes up more room out of the ground than in it.
Step 3 — Drainage stone
Stone envelope = width x length x (mean depth - cap thickness)
Pipe displacement = pi / 4 x (pipe outside diameter)^2 x length
Stone volume = stone envelope - pipe displacement
Order volume = stone volume x (1 + settlement %)
Weight = order volume x loose density
Void storage = stone volume x void ratio
The pipe occupies space the stone would otherwise fill, and any cap layer comes off the top. Void storage is what the trench itself holds during the peak of a storm.
Step 4 — Stone cover over the pipe (derived, not assumed)
Cover at the inlet = depth at the inlet - cap - bedding - pipe outside diameter
Cover at the outlet = depth at the outlet - cap - bedding - pipe outside diameter
Cover is not an input — you know how deep you are digging, and the cover is whatever is left. It grows toward the outlet, so the inlet end is the one that has to clear the 3 in minimum.
Step 5 — Geotextile, as strips off a roll
Full wrap section = (2 x trench width) + (2 x stone depth) + overlap
Lining only = trench width + (2 x stone depth) + (2 x margin)
Strips = 1 + ceil((section - roll width) / (roll width - overlap))
Rolls = the strip lengths packed into the roll length
The leading 1 matters: the first strip has nothing to lap onto. A full wrap needs roughly twice the width a bottom-and-walls lining does, because of the two flaps that fold back over the stone.
Step 6 — Capacity check
Inflow (cfs) = C x i x A (rational method, A in acres, i in in/hr)
Pipe capacity (cfs) = (1.486 / n) x A x R^(2/3) x S^(1/2) (Manning, full bore)
Usable capacity = pipe capacity x 0.5
Buffer = void storage / inflow
Three separate questions with three separate answers: how much water arrives, how much the pipe carries away, and how much the stone holds while that happens.
Why pipe and roll counts are packed, not divided. Every run has to be one continuous length of pipe, and every fabric strip one continuous piece. Three 60 ft runs off 100 ft coils need three coils, not the two that 180 ÷ 100 suggests — no coil can hold two of them. This calculator packs the lengths into stock rather than dividing, and tells you which runs come off which coil so the count can be checked on site.
Real-World French Drain Calculation Example
This example uses the values you have entered above and follows the same six steps as the formula section. Each table shows the value used, the formula applied, and the result produced.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Run 1 | 60 ft long, 12 in wide, 24 in deep at the inlet, falling 1% | Sets the excavation, the stone envelope and the fabric section for this run |
| Ground profile | Flat — the fall is dug into the trench | Decides whether the trench is a wedge or a constant-depth box |
| Drainage stone | Washed #57 Stone (3/4 in, open-graded) at 96.8 lb/cu ft, 40% void | Converts stone volume into weight, and gives the trench its water storage |
| Pipe | 100 mm Corrugated HDPE Drain Tile, roughness n = 0.024 | Displaces stone, and sets how much water can be carried away |
| Bedding under the pipe | 3 in | Lets the pipe be set to a true grade, and comes off the cover above it |
| Geotextile | 4oz non-woven, full wrap (burrito), 12 in overlap | Sets the fabric width the trench section actually needs |
| Cap layer | Topsoil and grass over the stone, 4 in | Comes off the top of the stone, reducing both stone and cover |
| Allowances | 5% stone settlement, 25% spoil bulking | Covers spillage into the order, and sizing the spoil to be carted away |
| Design storm | 1,500 sq ft at C = 0.55, 2 in/hr | The water the drain has to cope with |
Step 1 — Depth along the run
The fall is added to the inlet depth to find how deep the trench finishes, and the mean of the two ends is what the excavation multiplies out by.
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Fall over the run | 60 ft x 1% | 7.2 in drop |
| Depth at the inlet | as entered | 24 in |
| Depth at the outlet | 24 in + 7.2 in | 31.2 in |
| Mean depth | (24 + 31.2) / 2 | 27.6 in |
Step 2 — Excavation
The trench volume uses the mean depth, not the depth entered — which is the difference between this calculator and a constant-depth estimate.
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Trench volume | 12 in x 60 ft x 27.6 in | 5.11 cu yd (3.91 cu m) |
| A constant-depth sum would give | 12 in x 60 ft x 24 in | 4.44 cu yd |
| Dig the fall adds | 5.11 - 4.44 cu yd | 0.67 cu yd (15% more) |
| Spoil to move | 5.11 cu yd x (1 + 25%) | 6.39 cu yd loose |
Step 3 — Drainage stone
The cap layer comes off the top of the trench and the pipe displaces stone from within it; what is left is the stone to order.
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Cap taken off the top | 27.6 in - 4 in | 23.6 in of stone |
| Stone in the trench | stone envelope less the pipe | 111.1 cu ft |
| Order volume | 111.1 cu ft x (1 + 5%) | 4.32 cu yd (3.3 cu m) |
| Weight to order | 116.6 cu ft x 96.8 lb/cu ft | 5.64 US tons (5.12 tonnes) |
| Water held in the voids | 111.1 cu ft x 40% | 44.4 cu ft (332 gal) |
Step 4 — Pipe, cover and fittings
Cover over the pipe is what is left after the cap and the bedding, and it grows toward the outlet. Pipe is packed into whole coils or sticks, never divided.
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Cover at the inlet | 24 in - cap - bedding - 4.6 in pipe | 12.4 in |
| Cover at the outlet | 31.2 in - cap - bedding - 4.6 in pipe | 19.6 in |
| Perforated pipe laid | 1 run | 60 ft (18.3 m) |
| Pipe to buy | packed into 100 ft coil | 1 coil |
| Solid outlet pipe | 10 ft off 10 ft stick (rigid) | 1 x 10 ft stick (rigid) |
| Fittings | 1 cap, 0 tees, 1 coupling | 3 pieces |
Step 5 — Geotextile
The wrapped section is what the fabric has to cover across the trench, measured at the deepest end. If it exceeds the roll width, the fabric is seamed lengthwise.
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Section to cover | (2 x 12 in) + (2 x stone depth) + 12 in overlap | 7.53 ft (2.3 m) |
| Strips off the roll | 7.53 ft fits inside the 12.5 ft roll — no lengthwise seam | 1 strip, 0 seams |
| Rolls to buy | strip lengths packed into 360 ft rolls | 1 x 12.5 ft x 360 ft full roll — standard drainage roll |
Step 6 — Capacity check
The water arriving is sized against what the pipe can carry away at the flattest run's grade, and against what the stone can hold while that happens.
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Design inflow | 0.55 x 2 in/hr x 0.0344 acres | 17 gpm (1.07 l/s) |
| Pipe capacity, full bore | Manning at n = 0.024, 1% | 46.3 gpm |
| Usable capacity | 46.3 gpm x 50% | 23.1 gpm |
| Pipe running at | 17 / 46.3 gpm | 36.7% of full — comfortable |
| Storage buffer | 44.4 cu ft / 17 gpm | 19.5 minutes of the design inflow |
Therefore, this drain needs 5.11 cu yd of excavation and 5.64 US tons (5.12 tonnes) of washed #57 stone (3/4 in, open-graded), 1 x 100 ft coil of 100 mm perforated pipe, 1 x 12.5 ft x 360 ft full roll — standard drainage roll of geotextile and 3 fittings, with 6.39 cu yd of loose spoil to move. The pipe carries the design storm at 36.7% of full bore, with room to spare.
Essential Checklist+−
Complete these critical checks before approving the work or proceeding to the next construction stage.
✓Survey, Outlet & Utilities+-
- A real outlet has been identified, and it is lower than the deep end of the trench
- Buried services located and marked before any digging
- The source of the water is understood, not assumed
✓Setting Out the Fall+-
- The fall set at 1% or steeper, and checked along the whole run, not just end to end
- The depth at the OUTLET end calculated and confirmed as achievable
✓The Trench & Geotextile+-
- Trench width leaves stone all round the pipe, not just above it
- Geotextile specified, and it is non-woven filter fabric rather than woven weed membrane
- The fabric wraps the stone completely, with the flaps folded over and overlapped
- Trench stability assessed, and anything over 4-5 ft deep properly supported
✓Pipe & Stone Envelope+-
- Pipe laid with the perforations facing DOWN
- Clean, washed, angular open-graded stone — not pea gravel and not crusher run
- At least 2-3 in of bedding stone placed and levelled before the pipe goes in
- At least 3 in of stone over the crown of the pipe before any cap goes on
- The run from the stone to the outlet is SOLID pipe, not perforated
✓Outlet, Finishing & Testing+-
- The outlet is protected against rodents, blockage and being mown over
- The drain flow-tested with a hose before the trench is closed
- If the drain is capped with soil, surface water has a deliberate way in
Full QC Checklist+−
Verification checklist for a French drain or land drain — covering where the water is coming from and where it is going, setting out the fall, the trench and its geotextile, the pipe and the stone envelope, and the outlet, testing and aftercare. Use the Essential Checklist for the critical checks before you dig or order, and expand to the Full QC Checklist for complete verification.
✓Survey, Outlet & Utilities+-
- A real outlet has been identified, and it is lower than the deep end of the trench
- Buried services located and marked before any digging
- The source of the water is understood, not assumed
- The contributing area measured, including roof and hard surfaces that discharge to it
- Permissions and discharge restrictions checked
✓Setting Out the Fall+-
- The fall set at 1% or steeper, and checked along the whole run, not just end to end
- The depth at the OUTLET end calculated and confirmed as achievable
- Whether the ground already falls has been established, not assumed
✓The Trench & Geotextile+-
- Trench width leaves stone all round the pipe, not just above it
- Geotextile specified, and it is non-woven filter fabric rather than woven weed membrane
- The fabric wraps the stone completely, with the flaps folded over and overlapped
- The fabric roll is wide enough for the wrapped section, or the seams are planned
- Trench stability assessed, and anything over 4-5 ft deep properly supported
- Spoil disposal planned on the bulked volume, not the trench volume
✓Pipe & Stone Envelope+-
- Pipe laid with the perforations facing DOWN
- Clean, washed, angular open-graded stone — not pea gravel and not crusher run
- At least 2-3 in of bedding stone placed and levelled before the pipe goes in
- At least 3 in of stone over the crown of the pipe before any cap goes on
- The run from the stone to the outlet is SOLID pipe, not perforated
- Grade re-checked across every coupling and along the pipe after backfilling stone
- Filter sock use decided deliberately, not by default
✓Outlet, Finishing & Testing+-
- The outlet is protected against rodents, blockage and being mown over
- The drain flow-tested with a hose before the trench is closed
- If the drain is capped with soil, surface water has a deliberate way in
- The line of the drain recorded before it disappears
- A maintenance expectation set — a French drain is not fit-and-forget
French Drain Reference Tables
The figures the calculator works from, in full: what each stone weighs and how much void it holds, how far each pipe can carry water at a given fall, how much fall a slope actually gives you, and how wide a roll of fabric has to be to wrap a trench in one piece.
Drainage stone — density and void ratio
| Stone | Loose density (lb/cu ft) | Void ratio | Water held per cu yd | Notes |
|---|---|---|---|---|
| Washed #57 Stone (3/4 in, open-graded) | 97 | 40% | 81 gal | Angular — interlocks |
| Washed #67 Stone (3/4 in to No. 4) | 99 | 38% | 77 gal | Angular — interlocks |
| Washed #4 / #3 Ballast (1.5-2 in) | 94 | 44% | 89 gal | Angular — interlocks |
| Clean Angular 20-40 mm Stone | 95 | 40% | 81 gal | Angular — interlocks |
| Pea Gravel / Rounded River Shingle | 95 | 38% | 77 gal | Rounded — migrates under load |
| Clean Crushed Concrete (recycled) | 91 | 38% | 77 gal | Angular — interlocks |
Published densities for washed open-graded stone genuinely vary, from about 89 to 104 lb/cu ft depending on the rock, so treat any tonnage as a band rather than an exact figure and check the conversion your own supplier uses.
Pipe capacity, full bore (gpm)
| Pipe | Roughness n | 3 in at 1% | 4 in at 1% | 4 in at 2% | 6 in at 1% |
|---|---|---|---|---|---|
| Corrugated HDPE Drain Tile | 0.024 | 21 | 46 | 65 | 136 |
| Corrugated HDPE | 0.012 | 43 | 93 | 131 | 273 |
| Rigid Perforated PVC (SDR 35 / Schedule 40) | 0.01 | 52 | 111 | 157 | 327 |
Standard practice is to keep the design storm under half of these figures, leaving the rest as freeboard. Note that the pipe material matters more than one size step does: 4 in smooth-bore pipe carries more than 6 in single-wall corrugated at the same fall.
Slope, fall and depth at the outlet
| Slope | Drop per foot | Drop per metre | Fall over 30 ft | Fall over 60 ft | Fall over 100 ft |
|---|---|---|---|---|---|
| 0.5% (1 in 200) | 1/16 in | 5 mm | 1.8 in | 3.6 in | 6 in |
| 1% (1 in 100) — standard minimum | 1/8 in | 10 mm | 3.6 in | 7.2 in | 12 in |
| 1.5% (1 in 67) | 3/16 in | 15 mm | 5.4 in | 10.8 in | 18 in |
| 2% (1 in 50) | 1/4 in | 20 mm | 7.2 in | 14.4 in | 24 in |
| 3% (1 in 33) | 3/8 in | 30 mm | 10.8 in | 21.6 in | 36 in |
On flat ground every figure in the last three columns is added to the depth at the outlet end. That is the number to check against the ground you are discharging onto, before you start digging.
Geotextile — roll width needed for a full wrap
| Trench | Stone depth | Full wrap section | Fits a 3 ft roll? | Fits a 6 ft roll? | Fits a 12.5 ft roll? |
|---|---|---|---|---|---|
| 6 in wide | 18 in | 5.0 ft | No — 2 strips | Yes | Yes |
| 12 in wide | 18 in | 6.0 ft | No — 3 strips | Yes | Yes |
| 12 in wide | 24 in | 7.0 ft | No — 3 strips | No — 2 strips | Yes |
| 12 in wide | 36 in | 9.0 ft | No — 4 strips | No — 2 strips | Yes |
| 18 in wide | 36 in | 10.0 ft | No — 5 strips | No — 3 strips | Yes |
| 24 in wide | 48 in | 13.0 ft | No — 6 strips | No — 3 strips | No — 2 strips |
Section = (2 × trench width) + (2 × stone depth) + a 12 in overlap where the flaps meet. Strip counts assume a 12 in lap between strips. This is why the 12.5 ft roll is the standard drainage roll despite looking like overkill beside a 12 in trench.
Runoff coefficients and design rainfall
| Surface | Runoff coefficient C | Region | Typical short-duration design storm |
|---|---|---|---|
| Roof or paving | 0.90 | UK / Ireland, temperate maritime | 0.8 in/hr (20 mm/hr) |
| Compacted gravel or bare soil | 0.70 | US Pacific Northwest | 1 in/hr (25 mm/hr) |
| Mixed yard | 0.55 | US Midwest / Northeast | 2 in/hr (51 mm/hr) |
| Lawn over heavy clay | 0.45 | US Southeast / Gulf Coast | 3.5 in/hr (89 mm/hr) |
| Lawn over loam | 0.30 | Tropical / monsoon | 4 in/hr (102 mm/hr) |
| Lawn over sandy soil | 0.15 | — | Use your own local figure |
Design rainfall varies enormously with location and return period — the figures above are a starting ladder, not a lookup. Your local authority, met service or published intensity-duration- frequency curves will give the figure that actually applies to your site.
Typical trench sections by application
| Application | Trench width | Depth | Pipe | Notes |
|---|---|---|---|---|
| Lawn low spot / surface water | 8-12 in | 18-24 in | 4 in | Leave the stone open to grade, or add catch basins if capping with soil |
| Intercepting a slope | 12 in | 24-36 in | 4 in | Run across the slope, above the wet area, not through it |
| Footing / foundation drain | 12 in | At the top of the footing | 4 in | Outside the footing for new build, under the slab for a retrofit |
| Behind a retaining wall | 12 in | Full wall height | 4 in | Wrap fully; the wall depends on the drain not blinding |
| Driveway edge | 12-18 in | 18-24 in | 4-6 in | Use smooth-bore or rigid pipe where anything may drive over it |
| Field / land drain lateral | 10-12 in | 24-36 in | 80-100 mm | Laterals into a larger main; check the main can carry the total |
How to Use This Calculator
- Establish where the water goes first. Before entering anything, confirm there is somewhere lower than the deep end of your trench to discharge to. Everything else is academic if there is not.
- Say whether the ground already falls. This is the single input that changes the answer most. If the surface is level, the fall is dug into the trench and the excavation is a wedge; if the ground already falls at or steeper than the drain needs, the trench follows it at a constant depth.
- Add a row per straight run. A main with laterals is the main plus one row per lateral. Each gets its own length, width, depth at the shallow end, and fall — because each has its own geometry and its own share of the water.
- Pick the pipe on capacity, not habit. Switch the capacity check on, enter the area that drains to the trench and your local design rainfall, and let the result tell you whether 4 in single-wall is enough. If it comes out marginal, the smooth-bore option of the same diameter is usually the cheapest fix.
- Check the geotextile roll width against the section. If the result says three strips and two seams, a wider roll covers it in one piece — and each seam deleted is one fewer place silt can get in.
- Read the depth table before you order. The outlet-end depth is the figure that decides whether the drain can daylight, whether you need shoring, and whether you are about to find a service.
- Use Compare to price the trade-offs. A 6 in trench against a 12 in one, single-wall against smooth-bore, 1% against 2% — set up two options side by side and see the quantities and the capacity move together.
The result is a procurement document, not a design certificate. Take it to the supplier for the stone tonnage and the pipe and fabric counts; take the capacity verdict as a sanity check on the design, and involve an engineer wherever the drain is protecting a structure, a retaining wall or a boundary.
Site Tips
- Dig from the outlet back. Starting at the low end and working uphill means you always know the grade you have to hold to, and any water in the trench drains away from you as you go rather than collecting at your feet.
- Set the string line before the first spade goes in. A line at the intended grade, checked at intervals as you dig, is what stops a belly forming in the middle of an otherwise correct run. Past about 30-40 ft, string sags enough to matter — use a laser or a water level.
- Intercept above the problem, not in it. A drain run across a slope above a wet area catches the water before it arrives. A drain through the middle of the wet area is dealing with water that has already got there, and usually needs to be deeper and longer to do the same job.
- Bed the pipe, then check it, then cover it. Rake the bedding to grade, lay the pipe, check the fall with a level, and only then bring the stone up around it. Once the stone is in, a correction means taking it back out.
- Pin the fabric to the trench walls. Landscape staples hold the geotextile flat against the sides while the stone goes in. Without them the fabric slumps into the middle and ends up buried in the stone, doing nothing.
- Flow-test before you close the trench. Five minutes with a hose at the inlet end, watching the outlet, catches a reversed fall, a belly, a blocked coupling and a too-high outlet all at once. After backfilling, each of those is a day with a spade.
- Photograph the open trench with a tape in shot. Mark the line on a site plan from two fixed points. In five years, when someone wants to plant a tree or lay a patio, that record is worth considerably more than it cost.
- Order the stone slightly high, not exactly. Running short with an open trench, a hired excavator and a supplier that delivers on Tuesdays is a far worse problem than a small heap left over — which will always find a use.
Common Mistakes
- Laying the pipe with the holes facing up. The most common installation error there is. Water arrives from below, so holes up means the trench has to fill to the crown of the pipe before a drop gets in — the drain only starts working once it has already surcharged.
- Using crusher run, MOT Type 1 or road base as the fill. These are engineered to compact into a solid mass with no voids. Putting one in a drainage trench builds a buried dam. Ask specifically for clean, washed, open-graded stone.
- Wrapping in woven weed membrane instead of non-woven geotextile. They look similar on the roll and behave completely differently. Weed membrane is designed to block light and passes very little water; wrapped round a drain it is a barrier, not a filter.
- Leaving the top of the stone open when the fabric was meant to wrap it. Lining the floor and walls but not folding the flaps over lets silt wash in from above — which is exactly what the fabric was bought to prevent.
- Taking the starting depth as the depth of the whole trench. On flat ground the trench deepens toward the outlet. Order stone on the shallow-end depth and you will be roughly 15% short on a 60 ft run, and 30% short on a 120 ft one.
- Running perforated pipe all the way to the outlet. Outside the stone envelope the pipe must be solid, or it discharges straight back into the ground you are draining — frequently right beside the house.
- Ending the drain in a hole. A soakaway only works if the ground beneath it takes water. Dig a test pit, fill it, and time the drop before committing — a soakaway in clay is a cistern and will back the drain up.
- Capping with topsoil when the problem is surface water. A soil cap makes the drain invisible and seals it from water standing on the lawn. If that is the problem you are solving, leave the stone open or fit catch basins that feed the trench.
- Sizing the trench and never sizing the water. A correctly built drain with an undersized pipe still floods. Work out what is arriving before deciding that 4 in is enough.
- Digging before the utility locate is done. A French drain sits at precisely the depth shallow gas, water, telecoms and electrical services live at. The locate is free and the notice period is days; the alternative is neither.
Limitations
What this calculator does not do, stated plainly so the results are used for what they are worth.
- It sizes the barrel, not the perforations. Manning's equation describes flow along the pipe. A perforated pipe also has to let water in through its slots fast enough, which depends on the open area per foot and the head in the stone. For ordinary residential work the barrel is the binding constraint, and sizing it is what every published design procedure does — but it is not the whole of the hydraulics.
- It does not model the soil. How quickly the surrounding ground releases water into the trench depends on its hydraulic conductivity, which varies by orders of magnitude between sand and clay and cannot be inferred from anything you can type into a form. The rational method sizes surface runoff arriving at the drain, not subsurface seepage through it.
- It does not check infiltration at a soakaway. If your outlet is a dry well, whether it works is a percolation question about the ground beneath it, and it needs a test pit, not a calculator.
- It assumes straight trenches of rectangular section with vertical walls. Battered or benched trenches, curved runs, and trenches that change width along their length all need adjusting by hand or entering as several rows.
- Stone density is a band, not a figure. Published loose densities for washed open-graded stone range from about 89 to 104 lb/cu ft depending on the rock, a 17% spread on the tonnage. Check the conversion your own supplier uses.
- It costs materials only. Labour, excavation plant, spoil disposal, delivery and permits are genuinely site-specific and are deliberately excluded rather than guessed at.
- It is not a drainage design. Where the drain protects a structure, a retaining wall, a basement or a property boundary, or where discharge is regulated, the design belongs with an engineer. This calculator quantifies and sanity-checks; it does not certify.
Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.