Pipe Drop Per Foot Calculator
Find the total fall of a drain or sewer pipe from its length and slope in inches per foot — plus the equivalent percent grade.
How it works
Total drop (in) = run (ft) × slope (in/ft) · Percent grade = slope ÷ 12 × 100
Drain and sewer pipes need a steady downhill slope so solids stay suspended and flow by gravity. The plumbing rule of thumb is 1/4 inch of fall for every foot of horizontal run; too little and waste stalls, too much and liquid outruns the solids. Multiply the slope by the pipe length to get the total drop between the two ends, which is what you set with your level or laser.
Worked example
A 20-foot drain line at the standard 1/4 in/ft slopes down 20 × 0.25 = 5 inches from start to finish. That 0.25 in per 12 in of run is a 2.08% grade.
Frequently asked questions
What is the standard slope for a drain pipe?
For pipes up to about 3 inches, 1/4 inch per foot is standard. Larger pipes (4 inches and up) can run at 1/8 inch per foot. Always follow your local plumbing code.
Can a drain pipe be too steep?
Yes. If the slope is too steep the liquid runs off and leaves solids behind, causing clogs. The usual practical maximum is about 1/2 inch per foot.
How much does 1/4 inch per foot drop over a long run?
Multiply the run by 0.25 inches. Over 40 feet that is a 10-inch drop, which is why long runs need enough depth at the far end.
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This calculator is for educational purposes. Double-check important results before acting on them.