Drain Pipe Slope Calculator
Work out the slope and total fall of a drain pipe. Enter the run length in feet and the slope in inches per foot to get the percent grade and the total drop in inches. Leave the slope blank to use the code default for your pipe size.
How to calculate drain slope
- Enter the horizontal run length of the drain in feet.
- Enter the slope in inches of fall per foot, or leave it blank for the default.
- Optionally enter the pipe diameter so larger pipes default to 1/8 in per foot.
Examples
20 ft run at the standard slope
20 ft long at 1/4 inch per foot
2.08 percent grade with 5 inches of total fall
Frequently asked questions
What slope should a drain pipe have?
The standard is 1/4 inch of fall per foot for pipes 2.5 inches and smaller. Larger pipes can use 1/8 inch per foot so water does not outrun the solids.
What is 1/4 inch per foot as a percent?
It is about 2.08 percent. Divide the slope in inches per foot by 12 and multiply by 100 to convert any slope to a percent grade.
Can a drain be too steep?
Yes. Very steep drains let water race ahead and leave solids behind, which can cause clogs. Most codes cap the slope, often around 1/2 inch per foot for small pipe.
How do I find the total fall over a run?
Multiply the slope in inches per foot by the run length in feet. A 20 foot run at 1/4 inch per foot drops 5 inches end to end.
Does this work for sewer and condensate lines?
Yes. The math is the same for any gravity drain. Always confirm the required minimum slope with your local plumbing code.
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