Sizing a pump by turnover
A pool pump has one job that a calculator can size: moving your whole pool’s water through the filter in a target time. The pump size calculator works in gallons per minute: gallons divided by turnover hours times 60. A residential turnover target of 6 to 10 hours is common and 8 hours is a typical default. A pump that is too small leaves water stagnant, and one that is too big wastes electricity, so the flow you need and the pump’s actual performance at your plumbing both matter.
Sizing a heater in BTU
The heater size calculator uses the physics of heating water. A gallon of water weighs 8.34 pounds, and one BTU raises one pound of water by one degree Fahrenheit, so raising your pool’s temperature by a set number of degrees in a set time gives a BTU rate. Manufacturers’ sizing charts also account for surface heat loss, climate and covers, which a simple calculation does not, so use the result to compare options and ask the maker before you buy.
Choosing the type
Size and type are separate decisions. The pump finder and heater finder help you choose between single-speed and variable-speed pumps, and between gas, heat pump and solar heaters.
Putting size and volume together
Both calculators start from your pool volume, so work that out first with the volume calculators. A wrong volume carries through to every flow and BTU figure.