Supply pipe size vs GPM capacity
A supply pipe carries only so much flow before velocity and friction get excessive. The table maps a nominal pipe size to a planning GPM capacity at a typical velocity. These are labeled planning bands, not code — confirm with a licensed plumber and your local code. Use them with the supply pipe size and peak-demand GPM tools.
| Nominal size | Planning capacity (GPM) |
|---|---|
| 1/2 in | up to ~9 GPM |
| 3/4 in | up to ~18 GPM |
| 1 in | up to ~33 GPM |
| 1-1/4 in | up to ~53 GPM |
| 1-1/2 in | up to ~83 GPM |
Labeled planning bands at a typical design velocity — actual capacity depends on material, pressure and developed length. Confirm with a licensed plumber and your local plumbing code.