Plumbing
Drainage Pipe Size Calculator
Estimate drain pipe size from drainage fixture units and check the slope needed.
Result
Pipe and drain sizing is governed by plumbing code (IPC/UPC or local equivalent) and must be designed and installed by a licensed plumber. This is an educational estimate, not a design. Fixture unit values and sizing tables differ between the IPC and UPC and between jurisdictions. Venting, stack sizing and offsets all affect the answer and are not modelled here.
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About this calculator
What slope does a drain need?
A quarter inch per foot for pipes up to 2-1/2 inches, and an eighth of an inch per foot for 3 inch and larger. Too steep is a real problem, liquid outruns solids and the line blocks.
What are drainage fixture units?
A relative measure of a fixture's discharge load. A toilet is typically 4, a shower 2 and a basin 1. Totalling them tells you the pipe size the run needs.
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