Pool & spa
Pool Pump Size Calculator
Sizes a pump from turnover rather than from horsepower, which is how it should be done.
Result
Flow is only half the sizing question. The pump also has to overcome the resistance of the plumbing, filter and heater, and an oversized pump on undersized pipe wastes energy and can damage the filter. Many jurisdictions now require variable-speed pumps, which run longer at lower flow and cost far less to operate.
Get a Free Business AuditHow the calculation works
Required flow is the pool volume divided by the turnover time in hours, divided by sixty to get gallons per minute. A turnover is one complete circulation of the pool's water, and the conventional target is turning the volume over in a reasonable period rather than continuously. From that flow rate, the pump is selected against the system's total dynamic head, meaning the resistance of the plumbing, filter and fittings, because a pump's output depends on what it is pushing against rather than on its horsepower label.
What moves the number
- Pool volume and the turnover period targeted.
- Total dynamic head of the plumbing, which determines actual flow.
- Pipe diameter, since undersized pipe limits flow regardless of the pump.
- Whether the pump is single speed or variable speed.
Where this one goes wrong
Horsepower is not flow. A larger pump on undersized plumbing produces more noise, more energy use and more wear while moving little extra water, because the pipe is the restriction. Sizing from required flow and the system's head is the correct method, and it frequently selects a smaller pump than an owner expects. Oversized pumps also push water through the filter too fast for it to filter properly and can exceed the plumbing's safe flow velocity. Variable speed pumps change the economics entirely, because running longer at lower speed moves the same water for far less energy, which is why they usually repay their cost.
Before you quote
- Size from required flow and system head, not horsepower.
- Check pipe diameter can carry the flow at a safe velocity.
- Compare a variable speed option, which usually repays itself.
Where the rules come from
Questions
About this calculator
What size pool pump do I need?
Work out the flow needed to turn the pool volume over in your target period, then select against the system's total dynamic head. Horsepower is a poor guide, because actual flow depends on what the pump is pushing against. Doing it properly often selects a smaller pump than expected.
Is a bigger pump better?
No. On undersized plumbing a larger pump makes more noise, uses more energy and wears faster while moving little extra water, because the pipe is the restriction. It can also push water through the filter too fast to filter properly and exceed safe flow velocity in the pipe.
Are variable speed pumps worth it?
Usually, because power rises with roughly the cube of speed while flow rises in proportion to it. Running longer at half speed moves the same water for a fraction of the energy. On a pump that runs daily for years that difference normally repays the higher purchase price.
The formula
GPM = gallons / turnover hours / 60
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How to use this calculator
- Pool volume (gallons). Enter your own figure; the page starts at 20,000.
- Turnover time (hours). Enter your own figure; the page starts at 8.
- Hours the pump runs per day. Enter your own figure; the page starts at 8.
- Electricity per kWh ($). Enter your own figure; the page starts at $0.16.
- Pump power draw (kW). Enter your own figure; the page starts at 1.1.
The result updates as you type, so you can try a figure, see what it does to flow needed, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Pool volume (gallons) | 20,000 |
| Turnover time (hours) | 8 |
| Hours the pump runs per day | 8 |
| Electricity per kWh | $0.16 |
| Pump power draw (kW) | 1.1 |
What comes out
| Result | Value |
|---|---|
| Flow needed main result | 41.7 |
| Per hour | 2,500 |
| Turnovers per day at that run time | 1.0 |
| Running cost per month | $42.24 |
You need 41.7 GPM to turn the water over in 8 hours. Running 8 hours a day gives 1.0 turnovers and costs about $42.24 a month in electricity.
How sensitive the answer is
Holding everything else steady and changing only electricity per kwh, this is what happens to flow needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Electricity per kWh ($) | Flow needed |
|---|---|
| $0.08 | 41.7 |
| $0.12 | 41.7 |
| $0.16 example above | 41.7 |
| $0.24 | 41.7 |
| $0.32 | 41.7 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Pool volume (gallons)
- A number, between 0 and 2000000, starting at 20,000.
- Turnover time (hours)
- A number, between 1 and 48, starting at 8.
- Hours the pump runs per day
- A number, between 0 and 24, starting at 8.
- Electricity per kWh ($)
- A money amount, starting at $0.16.
- Pump power draw (kW)
- A number, between 0 and 20, starting at 1.1.
What the results tell you
| Result | What it is |
|---|---|
| Flow needed | The headline result. A decimal figure. |
| Per hour | A number. |
| Turnovers per day at that run time | A decimal figure. |
| Running cost per month | A money figure. |
Keep going
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