Drain slope calculator with code minimums

Slope is how far the pipe drops for every foot it runs. Fall is the total drop from one end to the other.

Fall (in) = run (ft) × slope (in per ft)

Percent grade is the drop over the run as a percentage: slope in inches per foot ÷ 12 × 100. Angle is the arctangent of slope ÷ 12. So 1/4 in per ft is 0.25 ÷ 12 = 2.08%, and an angle of 1.19°.

To set a run with a level and a tape, measure the fall over a known length. On a 4 ft level, 1/4 in per ft is a 1 in drop from one end to the other, and 1/8 in per ft is 1/2 in.

If you know the depth of the pipe at the high end, add the fall to get the depth at the low end. The calculator does this when you fill in the starting depth, which helps when the drain has to meet a sewer stub or a septic inlet at a fixed height.

The International Plumbing Code sets the minimum slope of horizontal drainage pipe by size in Table 704.1. ICC's own code essentials book shows the same table:

The same book notes that most horizontal drains in a building are simply run at 1/4 in per ft, and the lower slopes for 3 in and bigger pipe are there for long runs and tight depths. One exception it points out: drains into a grease interceptor stay at 1/4 in per ft or more.

Uniform Plumbing Code. Places on the UPC, much of the West, have a simpler rule in Section 708: horizontal drainage at no less than 1/4 in per ft (2%), and 1/8 in per ft (1%) allowed on pipe 4 in and larger only where 1/4 is impractical and the authority having jurisdiction approves it first. A 3 in drain at 1/8 in per ft can pass in an IPC town and fail in a UPC one.

Slope also sets how much a drain can carry. The IPC's Table 710.1(1) gives the maximum drainage fixture units (DFU) for building drains and building sewers, including branches of the building drain, at four slopes:

Values as printed in the 2012 IPC. A note to the table sets the smallest building drain that serves a water closet at 3 in. Horizontal fixture branches and stacks inside the building use a different table, Table 710.1(2), with lower numbers, so do not use this one for a branch under a bathroom floor.

The calculator gives a slope between two rows the lower row's capacity. A 3 in drain at 3/16 in per ft gets the 1/8 in figure, 36 DFU. Fill in the DFU on the drain from your code's fixture unit table, Table 709.1 in the IPC, and the calculator says whether it fits.

A 3 in building drain across a basement. 40 ft at the 1/8 in minimum: 40 × 0.125 = 5 in of fall, a 1.04% grade. At that slope it carries 36 DFU. If the house has more than that on it, run it at 1/4 in per ft (10 in of fall, 42 DFU) or go to 4 in.

A 4 in sewer to the street. 60 ft at 1/4 in per ft: 60 × 0.25 = 15 in, which is 1 ft 3 in, a 2.08% grade at 1.19°. If the pipe leaves the foundation 18 in down, it reaches the street end at 18 + 15 = 33 in. It carries 216 DFU at this slope.

A 2 in laundry drain. 12 ft at the 1/4 in minimum: 3 in of fall. At 1/8 in per ft it would fall only 1.5 in and fail the minimum for its size.

Checking a run on site. The fall measured over 20 ft is 2-1/2 in. Slope = 2.5 ÷ 20 = 0.125 in per ft, which is the minimum for 3 to 6 in pipe and too flat for 2 in. Enter it as your own slope to see the percent and the capacity.

The answer at the top is the total fall over your run, in inches and, past a foot, in feet and inches. Under it:

• Code minimum: the IPC Table 704.1 slope for the pipe size you picked. If your slope is under it, the note says so.
• Percent grade and angle: the same slope in the units a laser level, a grade rod or an engineer's plan uses.
• Building drain capacity: the drainage fixture units the pipe carries at your slope from Table 710.1(1). If you entered the DFU on the drain, the step says whether it fits.

The table under the answer shows the same pipe at all four code slopes side by side: the percent, the fall over your run, the capacity, and whether each slope meets the minimum for the size. It is the quickest way to see what you gain by going from 1/8 to 1/4 in per ft, and what that costs in depth at the far end.

If the depth at the far end comes out deeper than the sewer stub or septic inlet you have to meet, the choices are a flatter slope (down to the minimum), a bigger pipe that is allowed a flatter slope, starting the run higher, or a pump. Work it out on paper before the trench is dug.

Too flat is the usual failure. Water drains, but solids are left behind, and the pipe clogs. A run that sags between hangers or settles in poor backfill can hold water in a low spot, a belly, even if the two ends are at the right heights. Support the pipe on the spacing the code and the pipe maker set, and bed buried pipe on firm, even material.

Too steep comes up less often. The capacity table stops at 1/2 in per ft, and the model codes do not list steeper slopes for building drains. When a run has to drop a lot over a short distance, plumbers usually keep the horizontal part at a normal slope and make the drop with fittings. Once the pipe is at 45° or more from horizontal, the IPC treats it as vertical pipe, with its own rules.

A few other rules sit next to slope in Section 704: horizontal drainage is installed in uniform alignment at a uniform slope, pipe does not get smaller in the direction of flow, and a horizontal branch connects at least 10 pipe diameters downstream of the base of a stack (30 in for a 3 in stack, 40 in for a 4 in).

When the run is laid out, put the estimate together in Koira with the pipe, fittings, digging and labor on their own lines. The plumbing cost estimator and the plumbing labor rate calculator help with the prices.