To estimate gas or propane pool heater output, multiply pool surface area by the desired temperature rise and by 12 BTU/h per ft² per °F, reducing by 30% if the pool is covered.
Pool Heater Sizing Calculator
BTU/h output needed from a gas or propane pool heater, from pool surface area, desired temperature rise and whether the pool is covered.
Sizing a Gas or Propane Pool Heater
Pool heater sizing depends on surface area (where most heat loss occurs, through evaporation), how much temperature rise above ambient you want, and whether a pool cover is used, since covers significantly reduce evaporative heat loss. Gas and propane heaters run at a higher BTU/h-per-square-foot rate than heat pumps, since they can deliver more output on demand rather than running continuously.
This covers related searches such as pool heater sizing calculator, how many BTU do I need to heat my pool, and gas pool heater size by surface area.
How the Pool Heater Sizing Calculator Works
Recommended output = Pool surface area × Desired temperature rise × 12 BTU/h per ft²/°F × (0.7 if covered, 1 if uncovered)
Worked Example
A 450 ft² uncovered pool with a desired 20°F temperature rise: Recommended output = 450 × 20 × 12 = 108,000 BTU/h.
Frequently Asked Questions
Why does a pool cover reduce the required heater size so much?
Most pool heat loss occurs through evaporation at the water’s surface, and a cover significantly reduces that evaporative loss, which is why covered pools need meaningfully less heater output to reach and maintain the same temperature.
What temperature rise should I choose?
Temperature rise is the difference between your pool’s typical unheated temperature and your desired swimming temperature — check your local average water temperature and your comfort preference to estimate this figure.
Is gas pool heating faster than heat pump heating?
Gas and propane heaters generally heat pools faster since they can deliver a high BTU/h output on demand, while heat pumps run at a lower rate continuously — see the swimming pool heat pump sizing calculator for that alternative approach.
Does wind exposure affect pool heater sizing?
Yes — pools exposed to significant wind lose heat faster through increased evaporation, so exposed pools may need a heater sized toward the higher end of the estimate, or benefit more from a cover.
Should I size for the coldest month I plan to swim in?
Yes — size your heater for the largest temperature rise you’ll need during your intended swimming season, since a heater sized only for mild conditions may struggle during cooler months.
Does pool volume matter for heater sizing?
Surface area, not volume, is the primary driver of heat loss and therefore heater sizing, since heat escapes mainly at the water’s surface rather than throughout its depth.
Can a spa or hot tub attached to the pool change the sizing?
An attached spa adds additional surface area and heating demand, so include the spa’s surface area in your total calculation if it shares the same heater system.
Does altitude affect gas pool heater performance?
Gas appliances, including pool heaters, can lose some rated output at higher altitudes due to lower air density, so check your specific heater’s manufacturer guidance for any altitude derating that applies in your area.
How long does it typically take a gas heater to raise pool temperature?
Heat-up time depends on the heater’s actual output relative to the pool’s volume and the desired temperature rise, so a properly sized heater following this calculator’s recommendation should reach target temperature within a reasonable timeframe — ask your installer for a specific estimate.
