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Calculation Machine

PEX Pipe Size Calculator

PEX pipe size for fixture flow demand.

Reviewed by James Whitfield · How we check our maths

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.

The formula

velocity = GPM x 0.408 / ID^2 (in), solved for pipe ID

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Design flow8 GPM
  • Max velocity8 ft/s

Result

  • Recommended PEX size3/4 in
  • Minimum ID needed0.639 in
  • Velocity at selected size7.25 ft/s

How to use the PEX Pipe Size Calculator

  1. 1Estimate the design flow in GPM for the branch, based on fixture units served.
  2. 2Keep velocity under 8 ft/s (4-6 ft/s is quieter) to avoid noise and water hammer.
  3. 3Pick the smallest PEX size that meets the velocity limit.

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