Water Pipe Size Calculator (Velocity Method)

Velocity limits are one sizing check. Pressure available and friction loss over the run are the other — confirm with the pressure drop calculator.

Important: Plumbing sizing is governed by the plumbing code adopted in your area (for example the IPC or UPC) and local amendments. This result is an engineering estimate, not a code determination.

Tapeline — Water Pipe Size Calculator (Velocity Method)

Printed September 29, 2026

Flow rate10 gpm
Maximum velocity8 ft/s
Pipe typeCopper type L

Results update as you type.

gpm
ft/s

Common guidance: 8 ft/s cold, 5 ft/s hot in copper.

How it’s calculated

  • Minimum ID (in) = √(0.4085 × GPM ÷ max velocity)
  • Choose the smallest standard size with ID ≥ minimum

Worked example

10 gpm at no more than 8 ft/s in type L copper.

Inputs

Flow rate
10 gpm
Maximum velocity
8 ft/s
Pipe type
Copper type L

Results

Smallest nominal size
¾ in
Minimum inside diameter
0.715 in
Inside diameter of that size
0.785 in
Velocity in that size
6.63 ft/s

Load this example into the calculator

Assumptions and limits

  • Copper IDs per ASTM B88 type L; PEX IDs computed from SDR 9 minimum wall; PVC Schedule 40 nominal IDs.

Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.

Questions

What size pipe for 10 GPM?

At 8 ft/s, ¾ in type L copper just works (about 6.6 ft/s). PEX has a smaller bore, so ¾ in PEX is too small at that limit and 1 in is needed.

Why does PEX need a larger size than copper?

PEX walls are thicker, so the inside diameter of the same nominal size is smaller.