Turnover: the number that sizes the pump
Turnover time is how long it takes to push the whole pool’s volume through the filter once. A shorter turnover means faster filtering and better chemical mixing, but a bigger flow rate and more energy. Industry guidance for home pools is at least one turnover every 8 to 12 hours, with many professionals preferring 6 to 8. For public pools, the CDC’s Model Aquatic Health Code requires a turnover of 6 hours or less for most pools and 30 minutes or less for spas.
The formula
Flow (gpm) = gallons ÷ (turnover hours × 60)
Flow (L/min) = litres ÷ (turnover hours × 60)
The flow then has to fit through your pipes. The CDC code keeps water no faster than 6 feet per second on the suction side of the pump and 8 feet per second on the discharge side. Faster than that raises friction, noise, wear and, on the suction side, entrapment risk. We convert those speeds into gallons per minute for schedule 40 PVC:
| Pipe size | Inside diameter | Max suction (6 ft/s) | Max return (8 ft/s) |
|---|---|---|---|
| 1.5 inch | 1.61 in | 38 gpm | 51 gpm |
| 2 inch | 2.067 in | 63 gpm | 84 gpm |
| 2.5 inch | 2.469 in | 90 gpm | 119 gpm |
| 3 inch | 3.068 in | 138 gpm | 184 gpm |
Worked example
20,000 gallons, 8-hour turnover, 2-inch pipe
- Flow = 20,000 ÷ (8 × 60) = 41.7 gpm
- 2-inch suction pipe carries up to 63 gpm at 6 ft/s, so the pipe is fine
| Pool volume | 6 h turnover | 8 h turnover | 10 h turnover |
|---|---|---|---|
| 10,000 gal | 28 gpm | 21 gpm | 17 gpm |
| 15,000 gal | 42 gpm | 31 gpm | 25 gpm |
| 20,000 gal | 56 gpm | 42 gpm | 33 gpm |
| 25,000 gal | 69 gpm | 52 gpm | 42 gpm |
| 30,000 gal | 83 gpm | 63 gpm | 50 gpm |
Minimum flow rate by pool size
From flow rate to a pump
Pumps are sold by horsepower, but what matters is the flow they deliver against your system’s resistance, called total dynamic head. Every pump has a curve showing flow at each head. Long pipe runs, small pipes, many fittings, a heater and a dirty filter all raise head and cut flow. Head varies a lot from one pool to the next, so read the flow off the manufacturer’s curve at your estimated head rather than going by horsepower. A pool pro can measure it with a pressure and vacuum gauge.
Bigger is not better. An oversized single-speed pump wastes electricity and can exceed your filter’s rating. A variable-speed pump lets you set the flow to what you need, and running slower for longer saves a lot of energy. The pump running cost calculator shows how much, and the filter size calculator checks your filter can handle the flow.
Frequently asked questions
What size pump do I need for a 20,000 gallon pool?
One that delivers about 42 gpm at your system’s head for an 8-hour turnover, or 56 gpm for 6 hours. Check the pump curve, not just the horsepower.
How many hours a day should I run my pool pump?
Long enough for at least one turnover a day. With a variable-speed pump, running longer at low speed is usually cheaper and filters just as well.
What is a good turnover rate for a home pool?
Six to eight hours is common practice. Industry guidance sets one turnover every 8 to 12 hours as the minimum.
Can a pool pump be too big?
Yes. Too much flow can exceed the filter’s rating, push water too fast through pipes, and waste electricity. Match the flow to the pool, the pipes and the filter.
What is total dynamic head?
The total resistance the pump works against: friction in pipes and fittings, the filter, heater and any height difference. More head means less flow from the same pump.
Why does pipe size matter?
Small pipes limit how much water can move at a safe speed. Above about 6 ft/s on the suction side, friction and wear rise and suction outlets become more dangerous.
Sources
- Model Aquatic Health Code, 5th edition (2024). CDC.
- The critical role of circulation and turnover in residential pool performance. Pool Magazine.
- ENERGY STAR Certified Pool Pumps. U.S. EPA, ENERGY STAR.
Reviewed November 7, 2026. We check the formulas against the sources above and re-run the worked examples whenever we update this page. Found something wrong? Tell us.