Enter fixture units and run length — get the recommended pipe diameter for water supply lines based on IPC/UPC fixture unit method. Works for residential and light commercial.
Free PDF: IPC fixture unit table, supply main sizing chart (by FU + run length), drain stack sizing, and common residential rough-in dimensions — laminate it and keep it in the truck.
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Pipe sizing by guesswork — "it's a 3-bedroom house so we use ¾ inch" — produces low-pressure complaints on the second floor when two fixtures run simultaneously. The fixture unit method (IPC Table 604.3) gives you a demand-based load that accounts for fixture type, usage probability, and flow rate.
The longer the run, the more friction loss accumulates. A ¾-inch main that works fine at 40 feet may produce noticeably lower pressure at 100 feet with the same fixture load. This calculator adjusts the recommendation when run length exceeds thresholds where friction loss becomes significant at typical municipal pressure.
Always upsize one size when: (1) the customer has a well with a pressure tank rather than municipal service, (2) you're roughing in for future expansion and the customer mentions adding a bathroom, or (3) the run includes multiple 90-degree elbows or other fittings that add equivalent length. Copper and PEX have different friction characteristics — this calculator assumes copper sizing; PEX can sometimes drop one size on shorter runs.
Pipe sizing reference, flat rate price book, water heater proposal template, and the business forms to run a plumbing operation that wins jobs and actually makes money on them.
Undersized pipe is a common cause of low pressure complaints and failed inspections. This calculator determines the correct pipe diameter for water supply lines based on flow rate, fixture units served, and run length. Use it during design and takeoff to spec the right pipe size the first time and avoid callback pressure complaints after a new installation.