Trade ToolkitTools › Plumbing
Drain slope calculator
Fall over a run, and what the level shows.
Open the Drain Slope toolFree. Runs in your browser, no account, and keeps working offline.
What it does
Enter the length of the run and the slope in inches per foot, and the calculator gives the total fall, the grade in percent, and what your level shows — how far the low end of a 2′ or 4′ level sits off level.
It checks the slope against the IPC 2021 704.1 minimum for the pipe size: 1/4″ per foot for 2 1/2″ and smaller, 1/8″ for 3″ to 6″. Check your local code; UPC areas and local amendments differ.
Already installed? Enter the run and the drop you measured to check the slope. Add a hanger spacing and a starting height to get a mark at every hanger.
Worked example
A 2″ drain running 20 ft at 1/4″ per foot, checked with a 4′ level.
- Total fall
- 5″
- Grade
- 2.08%
- Under a 4′ level
- 1″ off level
- Meets the IPC minimum for 2″
- Yes (1/4″ per foot)
IPC 2021 704.1 minimum; check your local code.
Questions
How much fall does a drain need per foot?
Under IPC 2021 704.1, 1/4″ per foot for pipe 2 1/2″ and smaller, 1/8″ for 3″ to 6″, and 1/16″ for 8″ and up. Check your local code.
How much does a 20 foot drain drop at 1/4″ per foot?
5″ over the run. The worked example below shows what a 4′ level reads on it.
Can a drain have too much slope?
The IPC table sets a minimum; this tool only checks the minimum. Ask your inspector about any maximum your jurisdiction applies.