Plumbing
Pipe Slope Calculator
Fall for a drain line, in inches per foot, total drop and percentage grade.
Result
Pipe sizing, venting and drainage are set by the plumbing code adopted where you are, and the IPC and UPC do not agree with each other. Treat this as a planning figure and size the finished design from the code table your inspector works to. A quarter inch per foot is the usual minimum for pipe under 3 inches. Too little slope leaves solids behind; too much lets the water run away and strand them, which is why steeper is not automatically better.
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About this calculator
Can a drain be too steep?
Yes. Above about half an inch per foot the liquid outruns the solids and the pipe blocks. The classic case is a drain dropped sharply to clear an obstacle.
What slope does a 4 inch line need?
An eighth of an inch per foot is usually permitted for 4 inch and larger, because the larger bore carries solids at a lower grade. Local code decides.
The formula
total fall = slope per foot x run length
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