Plumbing
Pipe Size Calculator
Find the supply pipe size needed to carry a given flow without excessive velocity.
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. Nominal pipe sizes differ from true internal diameter, which varies by material and schedule. This uses nominal size as an approximation.
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About this calculator
Why does velocity matter?
High velocity causes noise, water hammer and erosion of the pipe wall, particularly in copper. Most codes cap cold water around 8 ft/s and hot water lower, since hot water erodes copper faster.
What size is a typical house main?
Three-quarter inch or one inch for most single-family homes, with half-inch branches to individual fixtures. Long runs and multiple simultaneous fixtures push the main up a size.
The formula
GPM = 2.448 × diameter² × velocity (diameter in inches, velocity in ft/s)
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