Pool Pump Size Calculator Size It by Turnover, Not by Horsepower
Enter the pool dimensions and how long one full turnover should take. You get the flow rate the pump has to sustain.
Flow Rate, Not Horsepower
Pumps are chosen by the flow they deliver against your plumbing. Horsepower is a motor rating, not a promise.
Bigger Is Not Better
An oversized pump costs more to run, strains the filter and can outpace what the pipes can carry.
What size pool pump do I need?
At a Glance
Pool pumps are sized by turnover: the whole water volume should pass through the filter once per turnover period, and eight hours is the residential norm. Divide the pool's volume by the turnover time and you have the flow rate to shop for — m³ per hour in Europe, gallons per minute in the US. An 8 × 4 m pool averaging 1.5 m deep holds 48 m³ and needs 6 m³/h for an eight-hour turnover.
Filtration only works on water that reaches the filter. The turnover rate is how long the system takes to move a volume equal to the entire pool through it — not how long until every molecule has been filtered, which takes considerably longer because clean and dirty water mix as they circulate.
That is why the standard target is conservative. Public-pool codes in the US set turnover requirements by pool type, typically six hours or less for a general-use pool and much shorter for spas and wading pools. Residential pools are not held to those codes, and eight hours is the practical default.
| Pool | Volume | 8 h turnover | 6 h turnover |
|---|---|---|---|
| 6 × 3 × 1.3 m | 23.4 m³ | 2.9 m³/h | 3.9 m³/h |
| 8 × 4 × 1.5 m | 48.0 m³ | 6.0 m³/h | 8.0 m³/h |
| 10 × 5 × 1.5 m | 75.0 m³ | 9.4 m³/h | 12.5 m³/h |
| 16 × 32 × 5 ft | 19,150 gal | 39.9 GPM | 53.2 GPM |
| 20 × 40 × 5.5 ft | 32,914 gal | 68.6 GPM | 91.4 GPM |
flow = (length × width × average depth) ÷ turnover hoursIn imperial the volume comes out in cubic feet, so it is multiplied by 7.48 to get US gallons and then divided by minutes rather than hours — that is what turns it into GPM.
These are the values the calculator loads by default.
Find the average depth
A floor sloping from 1.0 m to 2.0 m averages 1.5 m. Use the average, not the deep end.
Work out the volume
8 × 4 × 1.5 = 48 m³, which is 48,000 litres.
Divide by the turnover time
48 ÷ 8 = 6 m³/h.
Read it against the pump curve
A pump rated "8 m³/h" delivers that at zero resistance. At a realistic 8–10 m of head it might give 6 — which is exactly what this pool needs. The curve on the data sheet is the honest figure.
The flow rate is the requirement. Three things stand between it and the pump you buy.
Head loss eats into the rating. Every metre of pipe, every elbow, the skimmer, the filter and the heater all resist flow. A pump's headline rating is measured at low resistance; the pump curve shows what it actually delivers at the head your plumbing imposes. Size from the curve, at roughly 8–12 m of total head for a typical domestic installation.
The filter has a maximum flow, too. Sand filters are rated in m³/h per square metre of filter area, and pushing more water through than the filter is rated for reduces filtration quality rather than improving it. Check the filter's rating against your figure before buying a bigger pump.
Variable-speed pumps change the arithmetic. Running a variable-speed pump at half speed roughly halves the flow but cuts power consumption by far more, because power scales with roughly the cube of speed. The usual strategy is a longer run at a lower speed: the same turnover, a fraction of the electricity.
For the pipework side of the same question, see the Volumetric Flow Rate Calculator.
A rectangular pool, and flow before losses
The volume here is length × width × average depth, so it assumes a broadly rectangular pool. For a round pool, multiply the result by about 0.79 (π ÷ 4); for a kidney or freeform shape, measure the surface area properly and treat length × width as an approximation that runs high.
The output is the flow the system must achieve, not a pump specification. It does not account for head loss, filter capacity, pipe diameter or suction-line limits, all of which decide which pump actually delivers it. Public and commercial pools are subject to local health-code turnover requirements that override any rule of thumb — check the applicable regulation rather than this calculator.