Quick answer
Size a pool heater by working backward from how fast you want the water to warm up: multiply gallons x 8.34 x desired temperature rise, then divide by the hours you’re willing to wait — that BTU/hr figure is your minimum heater output. In practice, a 20,000-gallon pool needs roughly a 300,000-400,000 BTU gas heater or a 120,000-140,000 BTU heat pump, and a correctly sized gas heater raises temperature about 1-2.5°F per hour versus 0.5-1.5°F per hour for a heat pump. Use the chart below to skip the math for common pool sizes.
The BTU calculator formula
The underlying physics is simple: it takes 1 BTU to raise 1 pound of water 1°F, and a gallon of water weighs 8.34 pounds. So the total heat energy needed is:
Total BTU = Gallons x 8.34 x Temperature rise (°F)
Divide that total by how many hours you want the heat-up to take, and you get the heater output (BTU/hr) you need:
Heater BTU/hr = Total BTU ÷ Desired hours
Worked example: a 20,000-gallon pool going from 60°F to 80°F (a 20-degree rise) in 24 hours needs (20,000 x 8.34 x 20) ÷ 24 = 139,000 BTU/hr of continuous, sustained output — which is why a heat pump in the 120,000-140,000 BTU range is the common recommendation for that size pool. Want the same rise in 6 hours instead of 24? You’d need roughly 556,000 BTU/hr, well beyond what a residential gas heater delivers — this is why gas heaters are sized to the temperature-rise-per-hour they can sustain (below) rather than a single-day target.
Pool heater sizing chart by gallons
| Pool size (gallons) | Gas heater BTU | Heat pump BTU | Example products |
|---|---|---|---|
| Up to 10,000 | 125,000-200,000 | 75,000-110,000 | Hayward Universal H150 / Gulfstream HE110-RA |
| 10,000-15,000 | 150,000-266,000 | 110,000-125,000 | Pentair MasterTemp 250 / Gulfstream HE125-RA |
| 15,000-20,000 | 250,000-300,000 | 120,000-140,000 | Raypak AVIA 264K / Raypak Crosswind V 8550 |
| 20,000-25,000 | 300,000-400,000 | 140,000+ | Pentair MasterTemp 400 / AquaCal SQ225 |
| 25,000+ or commercial | 400,000+ | Multiple units | See our commercial pool heater picks |
These ranges match the products in our pool heater picks and inground pool heater guide — when your pool size lands between tiers, size up rather than down, since an oversized heater just heats faster while an undersized one runs constantly and never quite closes the gap on a cold night.
Pool thermometer with wireless temperature display
- A floating or wireless pool thermometer gives you the real starting number for the formula above instead of guessing.
- Useful year-round for tracking how fast your specific heater actually raises temperature versus the estimate.
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How long it actually takes to heat a pool
Sizing charts tell you the heater, but “how long” depends on the temperature-rise rate each heater type sustains once it’s running:
| Heater type | Typical rise rate | 20°F rise (e.g. 60°F to 80°F) |
|---|---|---|
| Gas (correctly sized) | 1-2.5°F per hour | ~8-20 hours |
| Heat pump (correctly sized) | 0.5-1.5°F per hour | ~13-40 hours |
| Solar | Varies with sun exposure | Days, weather-dependent |
Gas heaters win on speed because they burn fuel on demand at high output; a correctly sized unit can have a pool swimmable the same evening. Heat pumps move heat rather than generate it, so they’re more efficient per dollar but slower — most owners run them continuously and let the pool reach temperature over a day or two rather than timing a single heating session. A pool cover cuts heat-up time meaningfully in both cases since it stops the evaporative heat loss that fights the heater the whole time it’s running. For what each option costs to run once you’re at temperature, see our pool heating cost breakdown.
The bottom line
Work out your BTU/hr need with gallons x 8.34 x desired rise ÷ hours, cross-check it against the sizing chart above, and round up to the next tier if you land between two. Budget roughly 8-20 hours for a correctly sized gas heater to hit a 20-degree rise, and closer to a full day or two for a heat pump doing the same job continuously. Once you’ve settled on a BTU tier, double-check the unit will physically fit your equipment pad too — our pool heater dimensions guide covers real cabinet sizes and clearance requirements, since a correctly sized heat pump can still be too big for a pad built around an old gas heater’s footprint. For picks matched to your exact pool size, see our best pool heater and inground pool heater guides, or our best pool heater for a 10,000-gallon pool, best pool heater for a 15,000-gallon pool, and best pool heater for a 20,000-gallon pool picks if any is close to your volume; for installation and running-cost numbers once you’ve settled on a size, see our installation cost and pool heating cost guides.
Sources: Calculator Academy — Pool Heat Time Calculator, AQUA Magazine — How to Size a Pool Heater, FunCenter Pools — How Long Does It Take to Heat a Pool?