Drain Slope Calculator
Calculate the correct drain slope and total drop for horizontal drain pipes per plumbing code.
Results
Visualization
How It Works
Drain pipes must be sloped to maintain self-cleaning velocity. Too little slope causes clogs; too much slope causes water to outrun solids, also causing clogs. Code specifies minimum slopes based on pipe size.
The Formula
Total Drop = Slope per Foot x Run Length
Variables
- Slope — 1/4" per foot for pipes 3" and smaller; 1/8" per foot for 4" and larger
- Run — Horizontal distance in feet
Worked Example
A 20 ft run of 3" drain pipe needs: 0.25 in/ft x 20 ft = 5 inches of total drop from start to end.
Practical Tips
- Never exceed 1/2 inch per foot slope — water outruns solids
- 1/4 inch per foot = approximately 2% grade
- Use a torpedo level and mark the pipe every 4 feet
- Building sewers typically require 1/8" to 1/4" per foot