TryBuildCalcFrench Drain Pipe Size Calculator (Sized Against the Water Actually Arriving)
Check whether your pipe size and fall can carry the water arriving.
🕒 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
7.93
US tons (7.19 tonnes) — 6.07 cu yd / 4.64 cu m loose
Perforated Pipe
80
ft (24.38 m) — 1 x 100 ft coil
Excavation
7.11
cu yd (5.44 cu m) in the ground — 8.89 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 1.19 cu yd (0.91 cu m) more dig and stone than a constant-depth estimate — 20% above the 5.93 cu yd a flat calculation would have given you.
Capacity: Marginal — the design storm delivers 34 gpm (2.15 l/s) into a pipe that carries 46.3 gpm full bore at 1%, so it runs at 73.5% of full.
Drainage Stone
Washed #57 Stone (3/4 in, open-graded)
In the trench: 156.1 cu ft
Order volume (incl. 5% settlement/spillage): 6.07 cu yd
Weight: 7.93 US tons (7.19 tonnes)
Void space at 40%: 62.44 cu ft — 467 gal (1768 L) held in the stone itself
Pipe
100 mm Corrugated HDPE Drain Tile
Perforated: 80 ft (24.38 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 90 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: 7.93 ft (2.42 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: 80 ft (24.38 m) over 1 run
Excavation: 7.11 cu yd in the ground
Spoil, bulked 25%: 8.89 cu yd
Bedding under the pipe: 3 in (7.6 cm)
Cap: 0.99 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 |
|---|---|---|---|---|---|
| Main run | 80 ft | 1% — 9.6 in drop | 24 in | 33.6 in | 12.4 in → 22 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: 80 ft — 20 ft left over
Ways to Add Capacity
- Same diameter in smooth-bore pipe: 111 gpm (+140%) — no extra digging at all.
- Step up to 150 mm pipe: 136.4 gpm (+195%) — needs a wider trench.
- Double the fall to 2%: 65.4 gpm (+41%) — costs nothing in materials, but deepens the outlet end.
Before You Dig
- Because the ground is flat, the fall has to be dug into the trench: Main run starts 24 in deep and finishes 33.6 in deep, a drop of 9.6 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 1.19 cu yd (0.91 cu m) of dig and stone — 20% 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 design storm puts 34 gpm through a pipe whose full-bore capacity is 46.3 gpm at 1% — that is 73.5% of full. Storm drainage is normally sized to stay under half full so there is somewhere for a worse-than-design storm to go. It will cope with the storm you specified and little more.
- Three ways out, in rough order of cost: going up to 150 mm pipe raises full-bore capacity to 136.4 gpm (+195%); switching the same diameter to smooth-bore pipe raises it to 111 gpm (+140%) with no extra digging at all; doubling the fall to 2% raises it to 65.4 gpm (+41%), at the cost of a deeper outlet end.
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: 7.93 US tons (7.19 tonnes) + recommended tools
+−
Bill of Materials
Main material: 7.93 US tons (7.19 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)
7.93 US tons (7.19 tonnes)
6.07 cu yd / 4.64 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 7.93 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 80 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.
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete French Drain / Land Drain Materials Calculator.
Pipe size is a hydraulics question, not a habit
4 in is the right answer often enough that most people never ask the question, and that is exactly why undersized drains are common. Two things decide whether a pipe copes: how much water arrives, which comes from the area draining to it and how much of the rain that area sheds, and how much the pipe can carry, which comes from its bore, its internal roughness and its fall.
The roughness is the part that surprises people. Cheap single-wall corrugated drain tile has a Manning roughness of about 0.024 and carries roughly 46 gpm full bore at 1%. Smooth-bore pipe of exactly the same diameter, at exactly the same fall, carries about 111 gpm — nearly two and a half times as much, for a modest price step and no extra digging. And because capacity rises with diameter to the power of 8/3, going from 4 in to 6 in nearly triples the flow rather than adding half.
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 |
|---|---|---|
| Main run | 80 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 | 3,000 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 | 80 ft x 1% | 9.6 in drop |
| Depth at the inlet | as entered | 24 in |
| Depth at the outlet | 24 in + 9.6 in | 33.6 in |
| Mean depth | (24 + 33.6) / 2 | 28.8 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 80 ft x 28.8 in | 7.11 cu yd (5.44 cu m) |
| A constant-depth sum would give | 12 in x 80 ft x 24 in | 5.93 cu yd |
| Dig the fall adds | 7.11 - 5.93 cu yd | 1.19 cu yd (20% more) |
| Spoil to move | 7.11 cu yd x (1 + 25%) | 8.89 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 | 28.8 in - 4 in | 24.8 in of stone |
| Stone in the trench | stone envelope less the pipe | 156.1 cu ft |
| Order volume | 156.1 cu ft x (1 + 5%) | 6.07 cu yd (4.64 cu m) |
| Weight to order | 163.9 cu ft x 96.8 lb/cu ft | 7.93 US tons (7.19 tonnes) |
| Water held in the voids | 156.1 cu ft x 40% | 62.4 cu ft (467 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 | 33.6 in - cap - bedding - 4.6 in pipe | 22 in |
| Perforated pipe laid | 1 run | 80 ft (24.4 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 | 7.93 ft (2.42 m) |
| Strips off the roll | 7.93 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 0.0689 acres | 34 gpm (2.15 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 | 34 / 46.3 gpm | 73.5% of full — marginal |
| Storage buffer | 62.4 cu ft / 34 gpm | 13.7 minutes of the design inflow |
Therefore, this drain needs 7.11 cu yd of excavation and 7.93 US tons (7.19 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 8.89 cu yd of loose spoil to move. The pipe copes with the design storm and little more — see the ways to add capacity on the result card.
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.