PIPE SLOPE

Enter the pipe run length in feet (the horizontal distance from start to end). Pick the slope per foot — code minimums depend on pipe size: 1/4" per ft for drains 2.5" or smaller (sinks, lavs, tubs, washing machines); 1/8" per ft for 3" or larger (toilets, building drains, sewer lines).

The calculator returns total drop in inches, drop in feet, and grade as a percentage. Use the drop number to plan elevation differences when designing a drain layout — your highest fixture's drain outlet must be at least the drop value above the discharge point at the building drain or sewer.

For a 20-ft horizontal drain at 1/4" per foot slope:

Drop: 20 × 0.25 = 5 inches. In feet: 0.42 ft. Grade: 0.42 ÷ 20 × 100 = 2.08%.

A 5-inch drop over 20 feet means the upstream end of the drain is 5 inches higher than the downstream end. For an under-sink lav drain: trap arm exits at 18" above floor → drain enters wall at 13" above floor where it joins the stack 20 ft away.

For a 50-ft sewer line at 1/8" per ft (3"+ pipe): drop = 50 × 0.125 = 6.25 inches. The outdoor cleanup is 6.25" above the connection point at the city sewer.

For a 100-ft branch at 1/4"/ft: drop = 25 inches. Over long runs, the slope adds up — your starting elevation must accommodate it. Long horizontal runs in basements often need belly-up branches plus a sewage ejector pump because gravity doesn't allow enough drop.

Going too steep. Above 1/2" per foot, water outruns the solids. They drop out and clog the line within months. Code minimums work because they keep solids suspended in the flow.

Going too shallow. Below 1/4" per ft on small pipe (or 1/8" on large), solids settle out at low spots in the flow. Same clog result.

Forgetting that fittings have an effective length. A 90° elbow eats roughly 1 foot of equivalent run for slope purposes. Long runs with multiple turns may need to start at a steeper drop than calculated.

Mixing pipe sizes incorrectly. A 1.5" branch joining a 3" main can use the 1.5" pipe at 1/4"/ft, but the 3" main only needs 1/8"/ft. Using 1/4"/ft on the 3" line wastes drop you may need elsewhere.

IPC/UPC minimum slope: 1/4" per ft for ≤2.5" pipe, 1/8" per ft for 3"+ pipe.

Maximum slope: don't exceed 1/2" per ft — water outruns solids, causes clogs.

Drop = run × slope. A 20-ft run at 1/4"/ft drops 5 inches.

Grade %: 1/4"/ft = 2.08%. 1/8"/ft = 1.04%. 1/16"/ft = 0.52%.

For long branches in tight basements, plan the slope BEFORE finalizing fixture locations — a 30-ft run at 1/4"/ft needs 7.5" of drop, which can force ceiling-mounted drains or a sewage ejector pump.

IPC also limits maximum total fall in a single drain to 1 vertical pipe diameter per fitting — beyond that, additional vents may be required.