Plumbing
Plumbing Pipe Size Calculator
Size a water supply pipe from the flow it has to carry and the velocity you want to hold it to.
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. Velocity sizing ignores friction loss along the run and through fittings, which on a long or convoluted run is what actually limits flow. Hot water is held to a lower velocity because erosion corrosion attacks copper faster at temperature.
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About this calculator
Why is hot water limited to a lower velocity?
Erosion corrosion. Hot water moving fast through copper scours the inside of the pipe, and pinhole leaks follow. Five feet per second is the usual ceiling, lower again for recirculating loops.
Does PEX size the same as copper?
Not quite. PEX of a given nominal size has a smaller bore than copper, so the same nominal size carries less flow. Fittings that insert into the pipe restrict it further.
Why does my shower drop when a tap opens?
Undersized branch or too much friction loss, not usually pressure at the main. Adding one size to the shared run fixes more of these than a pressure booster does.
The formula
pipe area = GPM / (448.8 x velocity in ft/s); diameter = sqrt(4 x area / pi)
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