| Pipe size | Velocity | Max flow | Verdict |
|---|---|---|---|
| Fill in the fields above. | |||
How the calculation works
Velocity is what matters, not the pipe label. Velocity (ft/s) = GPM × 0.4085 ÷ internal diameter², using Schedule 40 internal diameters rather than the nominal size printed on the pipe.
The accepted ceilings are 6 ft/s on suction and 8 ft/s on returns. Suction is stricter because that is where low pressure turns into cavitation — the pump pulling harder than the water can follow, which is both noisy and destructive.
Worked example
At 42 GPM, 1½ inch pipe runs at about 6.8 ft/s. That is fine on a return line but over the limit on suction, which is why a pool moving that much water usually wants 2 inch suction plumbing.
Assumptions
- Schedule 40 PVC internal diameters — the standard for residential pool plumbing.
- Velocity limits of 6 ft/s on suction and 8 ft/s on returns.
- The whole flow passes through a single pipe of this size.
Limitations
- Velocity is only part of the story. Long runs and many elbows add friction that this does not account for.
- Two suction lines sharing the flow each carry roughly half, which changes the answer.
- Flexible pipe and corrugated hose have different internal diameters and higher friction than rigid PVC.
Questions people ask
Does bigger pipe save energy?
Yes, and more than people expect. Friction rises steeply with velocity, so the pump works noticeably less hard through 2 inch pipe than 1½ inch at the same flow — every hour it runs.
What does cavitation sound like?
Like gravel rattling through the pump. It happens when water cannot reach the impeller fast enough and vapour bubbles form and collapse — undersized or blocked suction plumbing is the usual cause, and it damages the impeller over time.
Can I just fit a smaller pump instead?
Often the better answer, yes. Running a variable-speed pump at a lower speed for longer keeps velocity down and costs less in electricity than re-plumbing the pool.