Drain pipe slope calculator
How much fall does this run need — and does what you've got pass? Total fall to the nearest 1/16", checked against code minimums. Built to use one-handed on a job site.
✓ Minimums from IPC Section 704.1 — confirm against your local codeFall at every 5 feet
Mark these on your string line or laser readings.
Minimum drain slope by pipe size (IPC 704.1)
| Pipe size | Minimum slope | Grade |
|---|---|---|
| 2-1/2" and smaller | 1/4" per foot | 2.08% |
| 3" to 6" | 1/8" per foot | 1.04% |
| 8" and larger | 1/16" per foot | 0.52% |
These are the International Plumbing Code minimums for horizontal drainage piping. Your jurisdiction may have adopted a different edition, the Uniform Plumbing Code, or local amendments — and an inspector's call is final. This calculator does the math; it isn't a substitute for the code book or the AHJ.
Quick answers
How much fall does a 4" pipe need over 40 feet?
At the 1/8" per foot minimum: 5 inches. At 1/4" per foot: 10 inches.
Is more slope always better?
Not always. Very steep horizontal runs can let water outrun solids. Many plumbers stick close to 1/4" per foot on small lines; follow your local code and inspector on maximums.
How do I turn percent grade into inches per foot?
Multiply the percent by 0.12. A 2% grade is 0.24" per foot — just under 1/4".