PEX Pipe Size Calculator
PEX pipe size for fixture flow demand.
Enter your values
ⓘ Live results
Results
- Recommended PEX size
- 3/4 in
- Minimum ID needed
- 0.639 in
- Velocity at selected size
- 7.25 ft/s
Quick answer
PEX branch size is chosen so flow velocity stays under about 8 ft/s, based on the required flow in GPM.
velocity = GPM x 0.408 / ID^2 (in), solved for pipe ID
How to use the PEX Pipe Size Calculator
- 1Estimate the design flow in GPM for the branch, based on fixture units served.
- 2Keep velocity under 8 ft/s (4-6 ft/s is quieter) to avoid noise and water hammer.
- 3Pick the smallest PEX size that meets the velocity limit.
Worked example
Using the values the calculator loads with:
Inputs
- Design flow8 GPM
- Max velocity8 ft/s
Results
- Recommended PEX size3/4 in
- Minimum ID needed0.639 in
- Velocity at selected size7.25 ft/s
Frequently asked questions
Why does velocity matter?
High velocity causes noise, water hammer and erosion at fittings over time.
Does PEX size differ from copper for the same flow?
Yes, PEX has a smaller inside diameter than copper of the same nominal size, so check manufacturer ID specs.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.