Pipe Slope Calculator
Work out drainage pipe slope (total fall over a run, as inches, percent, and angle) and see the recommended slope for the pipe size. Calculates in your browser.
Total fall
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Show the math
Fall is the run length times the slope per foot:
fall = run × slope per foot
slope% = (slope per foot ÷ 12) × 100
The International Plumbing Code (2021, Table 704.1) sets the minimum slope at 1/4 inch per foot for pipes 2½ inches or less, 1/8 inch per foot for 3 to 6 inches, and 1/16 inch per foot for 8 inches or larger. Check your local code. All math runs locally in your browser.
What this does
A pipe slope calculator works out drainage slope (total fall over a run, as inches, percent, and angle) and shows the recommended slope for the pipe size.
How to use it
- Enter the run length.
- Enter the slope per foot.
- Or pick the pipe size for a recommendation.
- Read the fall, percent, and angle.
How it works
The International Plumbing Code (2021, Table 704.1) sets the minimum at 1/4 inch per foot for pipe 2½ inches or less, 1/8 inch per foot for 3 to 6 inches, and 1/16 inch per foot for 8 inches or larger. Local codes may differ.
fall = run length × slope per foot
Understanding your result
The recommended slope is a balance: too little and waste water sits and stalls, too much and the liquid races ahead of the solids and leaves them behind to clog. Aim for a steady, consistent fall along the whole run rather than dips and humps, and treat the minimum as code-driven, since drainage rules are strict for good reason.
Example
A 20-ft run at 1/4 inch per foot drops 5 inches.
Last updated .
Frequently asked questions
What slope should a drain pipe have?
The International Plumbing Code minimum is 1/4 inch per foot for pipes 2½ inches or less, 1/8 inch per foot for 3 to 6 inches, and 1/16 inch per foot for 8 inches or larger. The tool shows the minimum for the size you enter; check your local code.
Why not steeper is better?
Too steep can let water outrun solids and cause clogs; the standard slopes balance flow velocity and solid transport.
How is fall calculated?
Fall = run length × slope per foot. The tool also shows the slope as a percentage and an angle.
Is my data saved?
No. Everything is calculated locally in your browser.
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