Main Sewer Line Slope Calculator (6" Main at 1/8" per ft)

Check fall and code minimum slope for a 6 inch main sewer line.

Check fall and code-minimum slope for a 6 inch main sewer line running 100 feet at 1/8 inch per foot, plus its DFU capacity from IPC Table 710.1(1).

🕒 Last updated: July 31, 2026

Line Role & Units

Line Role

▾

ℹ️Sets which code table and slope rules apply — Stacks are vertical, so no slope requirement applies.

Code Basis

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ℹ️Capacities always use IPC tables — UPC / Local Code just adds a reminder to confirm against your local table (see FAQ).

Units

▾

ℹ️Switching units converts your existing values too, so their real-world meaning stays the same.

Slope & Fall

Calculation Mode

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ℹ️Choose which quantity is unknown — the slope you want to hold, or the slope you get from a fixed available fall.

Run Length

ℹ️Horizontal developed length of this straight run, not a straight-line site distance.

Slope

ℹ️Inches of fall per foot of horizontal run — e.g. 0.25 for the common 1/4 inch per foot.

Code Check & Drainage Capacity

Pipe Diameter

▾

ℹ️Sets the code-minimum slope and the DFU capacity table row used below.

Capacity Slope

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ℹ️The DFU capacity table (IPC 710.1(1)) only publishes values at these four standard slopes — this is a separate lookup from your actual slope above, so pick the slope you're actually designing to.

DFU Load

Add your fixture list, or enter a known total DFU load directly, to compare against this line's capacity.

DFU Input Mode

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Fixture 1

Fixture Type

Quantity

Fixture 2

Fixture Type

Quantity

Fixture 3

Fixture Type

Quantity

Up to 30 fixture rows total.

✅ Slope meets code minimum for a 6" pipe

Mode: Calculate Fall from Slope · Line Role: Building Drain / Building Sewer

Slope & Fall

Run Length: 100 ft

Fall Over This Run: 12.5 in (1.042 ft)

Slope: 0.125 in/ft (1.04%)

Slope Ratio: 1 : 96

Rise/fall of 1 unit over this many units of horizontal run

Code Minimum Slope Check

Pipe Diameter: 6"

Code Minimum Slope: 0.125 in/ft

Per IPC 704.1 / UPC 708.1

Your Slope: 0.125 in/ft

Pass — meets or exceeds code minimum

Drainage Capacity

Line Role: Building Drain / Building Sewer

Pipe Diameter: 6"

Hypothetical Capacity (at 1/8" per ft): 700 DFU

Achieved Capacity (at your actual slope): 700 DFU

DFU Load & Utilization

Water Closet, Private (1.6 gpf) (x1): 3 DFU

Lavatory (x1): 1 DFU

Bathtub / Combo Bath-Shower (x1): 2 DFU

Total DFU Load: 6 DFU

Utilization: 6 DFU / 700 DFU = 0.9%

Trench / Stake Checkpoints

Cumulative fall at each point along the run, for staking or checking trench depth on site.

Checkpoint	Distance	Cumulative Fall
25%	25 ft	3.13 in
50%	50 ft	6.25 in
75%	75 ft	9.38 in
100%	100 ft	12.5 in

Show every 10 ft checkpoint

Distance	Cumulative Fall
10 ft	1.25 in
20 ft	2.5 in
30 ft	3.75 in
40 ft	5 in
50 ft	6.25 in
60 ft	7.5 in
70 ft	8.75 in
80 ft	10 in
90 ft	11.25 in
100 ft (end)	12.5 in

Design Notes

Building Drain/Sewer capacity uses IPC Table 710.1(1) (slope-dependent). Horizontal Fixture Branch and Stack capacity use IPC Table 710.1(2) (fixed by diameter, and by branch-interval count for stacks — not slope-dependent). These are different tables answering different sizing questions for different pipe segments.

The minimum size of any building drain serving a water closet is 3 inches, regardless of DFU load. Confirm the exact table and slope requirements adopted by your local jurisdiction (IPC, UPC, or a local amendment) before finalizing a design.

Approximate results for planning only. Verify with a professional.

Pipe Sizing by Fixture Units CalculatorSize the supply-side pipe feeding these same fixtures.Septic Tank Size CalculatorSize the septic system this drain run connects to.Pipe Volume CalculatorCheck volume once a drain pipe size is confirmed.

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Looking for the verification checklist, reference tables, or tips and mistakes?See the complete Drain Pipe Slope Calculator.

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100 ft, 6 inch main sewer line example

This page is set up for a 6 inch main sewer line running 100 feet at 1/8 inch per foot — the code-minimum slope for this diameter. That gives a total fall of 12.50 inches (1.04 ft) over the run, a substantial drop that matters for trench depth and invert elevation planning on a longer main line.

A 6 inch pipe's minimum slope is 1/8 inch per foot, so this run's slope passes right at minimum. At 1/8 inch per foot, IPC Table 710.1(1) rates a 6 inch building drain/sewer to carry up to 700 DFU — enough for a large multi-unit building or a small commercial building's combined drainage load.

100 ft run at 1/8" per ft → 12.50 in (1.04 ft) total fall.
6" pipe minimum slope: 1/8" per ft — this run passes exactly at minimum.
DFU capacity at 1/8" per ft: 700 DFU.

How Drain Pipe Slope Is Calculated

The calculation happens in four parts — slope/fall geometry, a code-minimum slope check, a DFU capacity lookup for the selected line role, and utilization against your actual DFU load.

Step 1 — Slope ↔ Fall Conversion

Fall (in) = Slope (in/ft) × Run Length (ft)
Slope (in/ft) = Available Fall (in) ÷ Run Length (ft)

Both directions are pure geometry — this section accepts any slope or fall value, not just the four code-tabulated slopes used in Step 3. In metric mode the same math runs in mm/m and meters, converted at the boundary (1/4 in/ft ≈ 20.8 mm/m). This step doesn't apply to a Stack, which is vertical and has no slope.

Step 2 — Code Minimum Slope Check

Pass if Slope (in/ft) ≥ Minimum Slope for the selected pipe diameter

Per IPC 704.1 / UPC 708.1: pipes 2-1/2 inches and smaller need at least 1/4 inch per foot, pipes 3 through 6 inches need at least 1/8 inch per foot, and pipes 8 inches and larger need at least 1/16 inch per foot. This applies to the Building Drain/Sewer and Horizontal Branch line roles; a Stack has no slope requirement.

Step 3 — DFU Capacity Lookup

Building Drain/Sewer: IPC Table 710.1(1) lookup at [Pipe Diameter, Capacity Slope]
Horizontal Branch / Stack: IPC Table 710.1(2) lookup at [Pipe Diameter, Line Role, Branch Intervals]

Both are direct table lookups, not formulas. Table 710.1(1) is slope-dependent (four tabulated slopes); Table 710.1(2) is a flat number per diameter/role, unaffected by slope. Some diameter/slope/role combinations have no entry at all — the calculator flags those as "not permitted" or "not tabulated" rather than inventing an interpolated number the code doesn't provide.

Step 4 — DFU Load & Utilization

Total DFU Load = Σ(Fixture DFU × Quantity), or a manually entered total
Utilization % = Total DFU Load ÷ Achieved Capacity × 100

Utilization is only computed against the achievedcapacity (the figure your actual slope/design honestly delivers per Step 3), never against a hypothetical capacity your design doesn't reach — if the achieved capacity is null (not achieved), utilization is shown as not applicable rather than a misleading percentage.

Worked Example

This example uses the active inputs above and follows the same steps as the Formula section.

Input Values Used

Input	Value
Line Role	Building Drain / Building Sewer
Mode	Calculate Fall from Slope
Run Length	100 ft
Slope	0.125 in/ft
Pipe Diameter	6"
Total DFU Load	6 DFU

Step 1 — Slope ↔ Fall

Calculation	Substitution	Result
Fall	0.125 in/ft × 100 ft	12.5 in (1.042 ft)

Step 2 — Code Minimum Slope Check

Calculation	Substitution	Result
Compare	0.125 in/ft vs 0.125 in/ft minimum	Pass

Step 3 — DFU Capacity & Step 4 — Utilization

Calculation	Substitution	Result
Achieved DFU Capacity	6", Building Drain / Building Sewer	700 DFU
Utilization	6 DFU ÷ 700 DFU	0.9%

Therefore, this 6" building drain / building sewer carries 700 DFU, and your 6 DFU load uses 0.9% of that capacity.

This does not include venting, cleanouts, or fittings along the run. Cross-check against the Reference Tables in the complete Drain Pipe Slope Calculator.

This page keeps things focused on the calculation above. For the full construction guide — verification checklist, reference tables, usage steps, tips, common mistakes, and limitations.See the complete Drain Pipe Slope Calculator.

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FAQ

Over a long run, even a modest per-foot slope adds up to a large total fall — this example's 12.50 inches means the low end of the trench sits over a foot deeper than the high end just from slope, before accounting for the pipe's own bury depth and frost-line requirements. On long mains, this cumulative depth can determine whether gravity flow is achievable at all to the connection point.

Yes — an 8 inch pipe's minimum slope drops to 1/16 inch per foot (half of this 6 inch example's 1/8 inch per foot), which would roughly halve the total fall needed over the same 100 ft run, at the cost of a larger, more expensive pipe. This tradeoff is common where available fall is tight on a long main line.

No — 700 DFU is a large but finite capacity. A multi-building or multi-unit development still needs its combined DFU load tallied from its actual fixture count and compared against this figure; a sufficiently large development could still exceed 700 DFU and require a larger main or an additional line.

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