Drain Pipe Slope Calculator
Enter a drain pipe's horizontal run and its slope to see the total vertical fall along with the equivalent percent grade. Leave the slope blank and the calculator applies the plumbing-code default of 1/4 inch per foot, dropping to 1/8 inch per foot when you enter a pipe diameter above 2.5 inches. It is handy for laying out a bathroom branch, a condensate line, or a sewer lateral before you start digging.
How to calculate drain slope
- Enter the horizontal run length of the drain in the 'Run length (ft)' box.
- Type a slope in the 'Slope (in/ft)' box, or leave it blank to use the default.
- Optionally set 'Diameter (in)' so pipes over 2.5 inches default to 1/8 inch per foot.
- Read the headline total fall, plus the slope, percent grade, and total fall cards.
- Press 'Copy result' to copy the slope, percent grade, and total fall as a sentence.
Examples
20 ft run at the standard slope
Run length 20 ft, slope 0.25 in/ft
Slope 0.25 in/ft, 2.08 percent grade, 5 in of total fall
Larger pipe using the code default
Run length 40 ft, slope left blank, diameter 4 in
Slope 0.125 in/ft, 1.04 percent grade, 5 in of total fall
Short branch at a steeper grade
Run length 8 ft, slope 0.5 in/ft
Slope 0.5 in/ft, 4.17 percent grade, 4 in of total fall
Frequently asked questions
What slope should a drain pipe have?
The common standard is 1/4 inch of fall per foot for pipes 2.5 inches and smaller. Larger pipes are often run at 1/8 inch per foot so the water does not outrun the solids. This tool uses those two values as its defaults when you leave the slope blank.
What is 1/4 inch per foot as a percent?
It is about 2.08 percent. The tool converts any slope with the formula slope in inches per foot divided by 12, times 100, so 1/4 inch per foot becomes 2.08 percent and 1/8 inch per foot becomes 1.04 percent.
How does the diameter box change the result?
The diameter only matters when the slope box is blank. If the diameter is greater than 2.5 inches the default drops to 1/8 inch per foot; at 2.5 inches or below it stays at 1/4 inch per foot. Typing an explicit slope overrides the diameter entirely.
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, which the tool shows as the headline number and in the total fall card.
Can a drain be too steep?
Yes. On a very steep run the water can race ahead and leave solids behind, which invites clogs. Many codes cap the slope, often near 1/2 inch per foot for small pipe, so more fall is not always better.
What units does the calculator use?
Run length is in feet, slope is in inches of fall per foot, diameter is in inches, and total fall comes out in inches. The tool does not convert to metric, so keep every entry in those imperial units.
Does it work for sewer, condensate, and AC lines?
Yes. The geometry is identical for any gravity drain, whether it is a sewer lateral, an air-conditioner condensate line, or a sink branch. Always confirm the minimum required slope with your local plumbing code before building.
How are the numbers rounded?
The display rounds slope to three decimals, percent grade to two decimals, and total fall to two decimals. Because of that, 1/4 inch per foot shows cleanly as 2.08 percent rather than a long repeating decimal.
Is anything sent to a server?
No. The calculation runs entirely in your browser, so your measurements stay on your device. A JSON API is also available if you want to compute slope and fall from a script using the same engine.
Learn more
- Drain slope basics: the quarter inch per foot rule
What 1/4 inch per foot means, how to convert slope to a percent grade, and how to plan the total fall for a drain run that clears solids.
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