What the animation shows
The animation and the step-by-step video above use your numbers. Here is the same walkthrough written out for the default example.
What you enter
- Flow rate: 10 gpm.
- Maximum velocity: 8 ft/s. Common guidance: 8 ft/s cold, 5 ft/s hot in copper.
- Pipe type: Copper type L.
What happens, step by step
- ½ in: water would race at 13.8 ft/s, over the 8 ft/s limit.
- ¾ in has a 0.785 in bore, so water slows to 6.63 ft/s.
- Use ¾ in type L copper, the smallest size that stays under 8 ft/s.
The answer: smallest nominal size is ¾ in.
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
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.