Pool Pump RPM Calculator
Last updated 15 October 2026
Enter a measured flow and pump speeds to get a corresponding-flow scenario.
Your result will appear here
Enter your measurements, then choose Calculate.
Use the corresponding flow to compare the entered speed settings.
Still unknown or unconfirmed
Formula, sources and blank printable worksheet
RPM-1: r=N₂/N₁; Q₂=Q₁r; H₂=H₁r²; shaft P₂=P₁r³, constant-efficiency corresponding points only.
Calculation version 1.0 uses historical engineering references for arithmetic, not current residential law.
- DOE / Hydraulic Institute, Improving Pumping System Performance, second edition https://www.energy.gov/sites/default/files/2014/05/f16/pump.pdf
| Quantity / unit | Reading | Source and date |
|---|---|---|
| Baseline RPM | ________________ | ________________ |
| Baseline US GPM | ________________ | ________________ |
| Scenario input | ________________ | ________________ |
| New RPM | ________________ | ________________ |
| Target US GPM | ________________ | ________________ |
| Same pump and impeller confirmed | ________________ | ________________ |
| Measurement or curve reference | ________________ | ________________ |
Mark missing measurements Unknown. Keep exact model, source revision and assumptions with this worksheet. Report a calculation or source correction with reproducible inputs.
This pool pump RPM calculator needs a baseline speed and flow for the same pump and impeller. It shows an affinity scenario. It cannot supply a universal RPM-to-GPM chart for a different circuit.
Divide new RPM by baseline RPM to get the speed ratio. Flow scales with that ratio. Head scales with its square. Shaft power scales with its cube if efficiency stays fixed.
These rules describe the pump. They do not give exact input watts or flow after a circuit change. Check the result against the right model curve and device limits.
On this page9
Pool pump RPM calculator baseline
Enter baseline RPM and measured flow or a clearly identified curve point. Add baseline head and shaft power only when known. Record the model, impeller, source curve and voltage condition. Confirm that the comparison uses the same hardware.
An electrical-watt reading is not shaft power. The motor and drive use energy too. Do not put measured input watts into a shaft-power field and label the cubic result a bill estimate. The kWh tool takes measured electrical power directly.
RPM calculator relationships
Let r be new speed divided by old speed. Then the corresponding-point estimates are:
| Quantity | Relationship |
|---|---|
| Flow | New flow = baseline flow × r |
| Head | New head = baseline head × r² |
| Shaft power | New power = baseline power × r³ |
The DOE pumping sourcebook explains these rules and their limits. The DOE pool-pump test-procedure proposal, footnote 51 also states the affinity laws. We use that old proposal as a source for the maths. It does not establish current law.
Worked half-speed scenario
Start at 3,000 RPM, 60 GPM, 40 feet of head and 1 kW of shaft power. At 1,500 RPM the ratio is 0.5. Corresponding estimates are 30 GPM, 10 feet and 0.125 kW of shaft power.
The last figure is not 125 measured electrical watts. The pump's efficiency and the drive's losses can differ. These artificial baseline figures show the arithmetic; they do not describe a tested pump.
Where actual flow can differ
A system with static head, a changed valve or a newly active heater may not follow the assumed relationship. A low speed may also fail an accessory's minimum flow. The pump and circuit still need a common operating point.
Use actual-flow guidance for measurement. Use the pool pump size calculator for required circulation flow and equipment checks. Do not use the speed scenario alone to approve a setting.
Work backward from a target-flow ratio
Keep the same pump-point assumption for this reverse calculation. New RPM equals baseline RPM × target flow ÷ baseline flow. A target flow half the baseline gives half the speed. Check that proposed speed against the real circuit. The meter may show a different flow.
Save both the original and target figures. A baseline without a model or source can make a neat result that cannot be reproduced. The worksheet leaves missing head and shaft power unknown while still showing a flow-ratio scenario.
Keep the estimate separate from a meter reading
Save the baseline source with the result. A speed display can confirm RPM. It cannot, on its own, confirm GPM or input watts. If you later measure those quantities, add the reading date and circuit state. Keep the measured record apart from this calculated scenario so the two are not confused.
FAQ
Can this choose a speed from gallons alone?
No. Gallons and a selected circulation time establish required flow. They do not identify a baseline pump point or circuit resistance. Start with the sizing tool, then use a suitable meter or the exact model curve to establish a baseline for this scenario.
Can I compare two different pump models?
The simple speed-ratio method assumes the same pump and impeller. Different models need their own curves at the required duty. Even equal speed labels do not make their flow and head equal. Keep each model's curve and rating conditions with its comparison.
Related calculations and owner guides
Verdict on pump RPM
Use the pool pump RPM calculator for a labelled same-pump scenario. Confirm the real operating point before changing a working schedule. Formula version 1.0 uses the stated same-pump assumptions. Send reproducible issues through contact.