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Drain slope calculator

Drainage runs on gravity and on nothing else. Too little fall and the solids stop while the water carries on; too much and the water leaves the solids behind. The code sets a minimum by pipe size and leaves the rest to you. Enter the size, the slope and the run, and this returns the total fall you have to build into the hangers.

Drain fall over a runIPC 704.1

Pre-filled with a 24 ft 3" branch at the code minimum. Change any field.

ft
3.0inches

Total fall over the run

Code minimum for this size
1/8"per ft
Slope you entered
1/8"per ft
Fall per 10 ft
1.25in
Invert drop
0.25ft
SizeMinimum slopeFall over your run
1-1/4"1/4" / ft6.0"
1-1/2"1/4" / ft6.0"
2"1/4" / ft6.0"
2-1/2"1/4" / ft6.0"
3"1/8" / ft3.0"
4"1/8" / ft3.0"
6"1/8" / ft3.0"
8"1/16" / ft1.5"
10"1/16" / ft1.5"

How it sizes

Slope is stated as a rise over a foot, so the arithmetic is trivial: fall equals slope times run. The judgement is in the number you choose. IPC Table 704.1 sets 1/4" per foot for anything 2-1/2" and under, 1/8" for 3" through 6", and 1/16" at 8" and above. Larger pipe gets less slope because it carries a deeper stream at the same flow, and depth is what moves solids.

The one exception the code writes down is grease. Anything upstream of a grease interceptor runs at not less than 1/4" per foot regardless of size, because congealing grease needs velocity that a shallow grade cannot give it.

Worked example

A 3" branch runs 24 ft from a bathroom group to the stack. At the 1/8" minimum:

QuantityValue
Run24 ft
Slope1/8" / ft
Total fall3.0 in
Invert drop0.25 ft

Three inches over 24 ft. That is the number that has to fit between the joists and the ceiling below, and it is the number that decides whether the run clears a beam. Work it out before the holes are drilled, not after.

Uniform, not average

The code says uniform alignment at uniform slope. A run that averages 1/8" but bellies in the middle holds water and is a callback waiting to happen.

Code notes

IPC 704.1 sets the minimum by size. UPC 708.1 does not: it sets 1/4" per foot for all horizontal drainage and allows 1/8" only on a building drain of 4" or larger, where the steeper grade is impractical and the authority having jurisdiction (AHJ) approves it first. A 3" branch at 1/8" passes under the IPC and fails under the UPC. Which one applies is set by your AHJ — see the IPC vs UPC comparison.

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