Drain slope calculator
Pick the pipe and the run to get the code-minimum slope and the total fall as a fraction you can mark on the level.
Result
Meets the 4 in minimum of 1/8 in/ft.
How is this calculated?
Using the tool
Every horizontal drain needs pitch — enough for the water to carry the solids, not so much that it leaves them behind. Pick the pipe and the run: you get the code-minimum slope for that diameter, the total fall as a fraction you can mark on the level, and an honest verdict when the number drifts out of the working range.
The code
Drain slope is a compromise the code settles for you: too flat and the water can't carry the waste, too steep on a horizontal run and the liquid races ahead leaving solids stranded. The minimums go by diameter — ¼ in/ft up to 2½", ⅛ in/ft for 3"–6", 1/16 in/ft at 8" and larger— because bigger pipes keep carrying velocity at gentler pitch. The math is just multiplication: fall = slope × run, so a 40 ft run of 4" pipe at ⅛ in/ft drops 5 inches from end to end. Mark the fall on a level or laser and the pitch takes care of itself.
New to the math? Learn the concept: Plumbing fundamentals