Heater size calculator
Undersize a heater and it never catches up; oversize it and you buy capacity you burn money idling. This works the sizing from surface area and the rise you want, then prices the heat-up on both fuels.
Sizing is surface area, temperature rise and how long you are prepared to wait — plus the heat the surface gives back to the sky while you do it.
Heater output needed
105kBTU/h
- Total heat for this rise
- 2,512 kBTU
- Fuel cost for the heat-up
- $76.29
- Priced at
- $2.55/therm
Energy prices: US EIA, July 2026 residential price, read 2026-10-18. Your utility rate is the one that matters — overwrite these in your head.
The three inputs that decide it
Heating a pool is one equation repeated: a BTU raises one pound of water one degree Fahrenheit. A 512 ft² pool holds around 32,000 pounds of water per foot of average depth, so a fifteen-degree rise is a large number before you have lost a single BTU to the sky. The other two inputs are how fast you want it and how exposed the surface is.
How to use it
- Enter the water surface area in square feet — length × width on a rectangle, or take it from the pool volume calculator. The tool assumes a 5 ft average depth.
- Enter the rise you want (target temperature minus the water today) and the hours you are prepared to wait for it.
- Choose the exposure honestly. “Sheltered” means a cover goes on; if it will not, pick average or windy — the cover comparison shows what one changes.
- Pick gas or heat pump and enter the efficiency or COP from the unit’s own spec sheet, then read the output in kBTU per hour and the fuel cost of the heat-up.
Worked example
A 400 ft² pool, 10 °F to gain, average yard, 84% gas heater or a heat pump at COP 5. Every figure below is what the calculator returns for those inputs:
| Step | Working | Result |
|---|---|---|
| Water to heat | 400 ft² × 5 ft × 7.48 gal × 8.345 lb | 124,850 lb |
| Heat-up | 124,850 lb × 10 °F | 1,248 kBTU |
| Surface loss, average | 400 × 10 × 12 × 1.25 | 60 kBTU |
| Total heat | 1,248 + 60 | 1,308 kBTU |
| Output for a 24 h heat-up | 1,308 ÷ 24 | 55 kBTU/h |
| Output for a 12 h heat-up | 1,308 ÷ 12 | 109 kBTU/h |
| Gas at 84%, $2.55/therm | 1,308 kBTU ÷ 0.84 = 15.6 therms | $39.73 |
| Heat pump at COP 5, 17.30¢/kWh | 1,308 kBTU ÷ 5 = 76.7 kWh | $13.27 |
Halving the wait doubles the heater size but, in this tool’s model, not the fuel bill: the window mainly decides how big a heater you buy.
Where the heat actually goes
| Loss path | What drives it | What stops it |
|---|---|---|
| Evaporation | Wind and dry air | A cover — by far the biggest single lever |
| Radiation to sky | Clear cold nights | A cover, again |
| Conduction to ground | Shell type and soil | Little you can do after the build |
| Splash-out and top-up | Bather load | Refill from a warm line where possible |
The exposure setting in the calculator is a multiplier on the first two rows. It is an assumption, stated plainly, not a measurement of your yard.
Sizing to the fuel bill, not the brochure
A gas heater sized for a four-hour heat-up will do it — and burn the fuel to match. A heat pump sized for a twenty-four-hour rise costs a fraction to run and asks you to plan a day ahead. Decide which of those you actually are before you shop.
Prices, and why they are shown
The running cost above uses the US average residential rates published by the EIA and read on 2026-10-18: 17.30 cents per kilowatt-hour for electricity and $26.45 per thousand cubic feet for natural gas, which works out near $2.55 a therm. Your own tariff is the only one that decides the answer — the heater comparison shows how the ranking flips when power is cheap and gas is dear.