Pool Heater Size Calculator
Find the heater output in BTU per hour you need to raise your pool to temperature in a target time, from your pool volume and the temperature rise.
Calculator
To heat 15,000 gallons by 10 °F in 4.0 h you need about 381,402 BTU/hr — pick a 400,000 BTU heater.
The right pool heater is the one that reaches your comfort temperature in a time you can live with. Too small and it never quite catches up on a cool weekend; oversized and you pay more than you need to. Sizing is straightforward once you know how much water you are heating and how fast you want it warm: the energy to warm a pool depends only on the weight of the water and the number of degrees you are adding, and the output you need is that energy spread over your target time. Because a US gallon of water weighs 8.34 pounds and it takes exactly one BTU to raise one pound of water one degree Fahrenheit, the math is completely deterministic — no product database and nothing that goes out of date.
Enter your pool volume in gallons, the temperature rise you want (the difference between the current water temperature and your target), and how many hours you are willing to wait. The tool returns the required output in BTU per hour and rounds up to the next common heater size, since heaters are sold in fixed steps (for example 200,000, 250,000, 333,000 and 400,000 BTU).
Formula
The energy to warm the water and the output to deliver it in your target time:
BTU = gallons × 8.34 × ΔT
Required output (BTU/hr) = (gallons × 8.34 × ΔT) ÷ hours ÷ efficiency
- gallons — pool volume.
- 8.34 — pounds per US gallon of water.
- ΔT — temperature rise in °F (target − current).
- hours — how quickly you want to reach temperature.
- efficiency — the fraction of fuel energy that reaches the water (~0.82 for gas).
A quick sanity check for a fully exposed pool is the surface-loss rule of thumb, BTU/hr ≈ surface_ft² × ΔT × 12: if your sized heater is far below that, an unshielded pool will struggle to hold temperature on a windy evening — a cover closes most of that gap.
Worked example
A 15,000-gallon pool, warming 10 °F, in 4 hours, with an 82% efficient gas heater:
15,000 × 8.34 × 10 = 1,251,000 BTU of energy needed.
1,251,000 ÷ 4 ÷ 0.82 = 381,402 BTU/hr of output.
Round up to a 400,000 BTU heater. Give yourself more time — say 8 hours — and the same pool only needs about 190,700 BTU/hr, so a 200,000 BTU unit would do.
How to read the result
The output figure is the heat delivered to the water. Gas heaters are usually labeled by input rating (the fuel they burn); at 82% efficiency a 400,000 BTU input model delivers about 328,000 BTU to the pool, which is why the tool already divides by efficiency — you compare its answer directly against a heater's input BTU rating.
Two practical points. First, this is a pure heat-up calculation: it assumes the heater's only job is to raise the water, and ignores the heat a bare pool loses to the air and by evaporation while it runs. On a calm, mild day that loss is small; on a cool, breezy evening it can be substantial, and a solar or bubble cover is the single most effective thing you can add — it can cut heat loss by half or more and keeps your sized heater comfortably ahead. Second, faster is not free: halving the target time doubles the required output, so if you can heat overnight a smaller heater is often the smarter buy. Many owners size for an 8-to-12-hour recovery and simply plan ahead rather than paying for a unit that reheats the pool in an hour.
The result is an estimate. Measure your pool to get an accurate volume, and treat the number as a floor rather than a ceiling — rounding up to the next standard size gives you a margin for wind, cool nights and the days you forget to put the cover on.
Reference table
Required heater output to raise the water 10 °F in 4 hours at 82% efficiency, and the nearest common heater size to buy:
| Pool volume | Required output | Nearest heater |
|---|---|---|
| 10,000 gal | 254,268 BTU/hr | 300,000 BTU |
| 15,000 gal | 381,402 BTU/hr | 400,000 BTU |
| 20,000 gal | 508,537 BTU/hr | 550,000 BTU |
| 25,000 gal | 635,671 BTU/hr | 650,000 BTU |
| 30,000 gal | 762,805 BTU/hr | 800,000 BTU |
Halve the time (2 hours) and you roughly double the output; double the time (8 hours) and you roughly halve it.
Frequently asked questions
What size heater do I need for my pool?
Work out the energy as gallons × 8.34 × ΔT, then divide by the hours you are willing to wait and by the heater efficiency. A 15,000-gallon pool warming 10 °F in 4 hours needs roughly 381,000 BTU/hr, so a 400,000 BTU heater. Enter your own numbers above for an exact figure.
Is a bigger heater always better?
A bigger heater heats faster but costs more up front and burns fuel at a higher rate. The total energy to reach a temperature is the same either way; you are only paying for speed. If you can heat overnight, a mid-size heater plus a cover is usually the better value.
Does the calculator include heat lost to the air?
No — it sizes the heater for the heat-up itself. A bare pool also loses heat to wind and evaporation while the heater runs. Use the surface-loss rule of thumb (surface_ft² × ΔT × 12) as a check, and fit a cover to close most of that gap.
Input rating or output rating — which do I compare against?
The tool already divides by efficiency, so its answer is what the water actually needs. Compare it directly against a gas heater's advertised input BTU rating and pick the next size up.
Why does the answer round up?
Heaters are sold in fixed sizes (200,000, 250,000, 333,000, 400,000 BTU and so on). Rounding up to the next standard size gives you a small margin for cool nights and wind rather than leaving you exactly on the line.
How do I find my pool volume?
Use the pool volume calculator: it turns your shape and dimensions into gallons. That figure is the single most important input here — an accurate volume makes every heating estimate reliable.