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Drain slope by pipe size
IPC 2021 704.1 — check your local code
The minimum slope for a horizontal drain by pipe size from IPC 2021 704.1, as inches per foot, fall over 10 feet and percent grade.
Steeper is fine for the minimum; UPC areas and local amendments differ, so check your local code. The drain slope calculator works out the fall for your run.
Verify with your local code and AHJ. Reference only, no warranty.
| Per foot | Per 10′ | % | |
|---|---|---|---|
| 1 1/4″ | 1/4″ | 2 1/2″ | 2.08 |
| 1 1/2″ | 1/4″ | 2 1/2″ | 2.08 |
| 2″ | 1/4″ | 2 1/2″ | 2.08 |
| 2 1/2″ | 1/4″ | 2 1/2″ | 2.08 |
| 3″ | 1/8″ | 1 1/4″ | 1.04 |
| 4″ | 1/8″ | 1 1/4″ | 1.04 |
| 6″ | 1/8″ | 1 1/4″ | 1.04 |
| 8″ | 1/16″ | 5/8″ | 0.52 |
| 10″ | 1/16″ | 5/8″ | 0.52 |
| 12″ | 1/16″ | 5/8″ | 0.52 |
Minimum fall for a horizontal drain. Steeper is fine.
Open the Drain Slope calculatorOpen this sheet in the toolkit to search it, pin it and use it offline.
Questions
What is the minimum slope for a 3 inch drain?
1/8″ per foot (1 1/4″ per 10′) under IPC 2021 704.1. Check your local code.
What slope does a 2 inch drain need?
At least 1/4″ per foot under IPC 2021 704.1 — 2 1/2″ over 10 feet.
What is 1/4 inch per foot in percent?
About 2.08% grade: a quarter inch of fall over 12 inches of run.