Hydraulics and pipes
Pipe Slope Calculator
Gravity drain, sewer and storm pipes need a minimum slope to keep flow moving. Enter the two invert elevations and the pipe length to get the slope in the units used on plans and in plumbing codes.
Formula
Slope = (upstream invert - downstream invert) ÷ horizontal length. Percent = slope × 100. Inches per foot = slope × 12. For example 1/4 in per foot = 0.0208 ft/ft = 2.08%.
Worked example
A storm pipe from invert 100.00 ft to 98.80 ft over 150 ft: fall = 1.20 ft, slope = 0.0080 ft/ft = 0.80%, or 0.096 in/ft.
Assumptions and limits
- Length is horizontal distance; for typical drainage slopes the difference from pipe length is negligible.
- Elevations are pipe inverts (inside bottom), not top of pipe.
Frequently asked questions
What is the minimum slope for a sewer pipe?
The Ten States Standards list minimum slopes for sanitary sewers of 0.40% for 8 in, 0.28% for 10 in, 0.22% for 12 in and 0.15% for 15 in pipe. Local utilities may require more.
What slope does the plumbing code require for drain pipes?
The International Plumbing Code (Table 704.1) requires 1/4 in per foot for drains 2.5 in and smaller, 1/8 in per foot for 3 to 6 in, and 1/16 in per foot for 8 in and larger.
Can a pipe be too steep?
Very steep sewers can produce high velocities that cause scour and, in sanitary lines, separation of liquids and solids. Designers add drop manholes or anchors where velocities exceed about 10 to 15 ft/s.