PEX Pipe Size Calculator
PEX pipe size for fixture flow demand.
Enter your values
Enter the design flow in GPM.
Enter the max velocity in ft/s.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | PEX pipe size for fixture flow demand. |
|---|---|
| Category | Home & DIY |
| Inputs needed | Design flow, Max velocity |
| Main output | Recommended PEX size |
| Formula | velocity = GPM x 0.408 / ID^2 (in), solved for pipe ID |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Design flow(GPM)
- Enter the design flow in GPM.
- Max velocity(ft/s)
- Enter the max velocity in ft/s.
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.
What do I need to enter into the PEX Pipe Size Calculator?
Just 2 values: design flow, max velocity. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the PEX Pipe Size Calculator work out the answer?
PEX branch size is chosen so flow velocity stays under about 8 ft/s, based on the required flow in GPM. It applies the formula velocity = GPM x 0.408 / ID^2 (in), solved for pipe ID and shows the working so you can check each step by hand.
Is the PEX Pipe Size Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.