Pool heater calculator
How big a heater you need to warm the pool in the time you want, and what one heat-up costs.
In short: Heating 1 lb of water by 1 °F takes 1 BTU, and a gallon weighs 8.34 lb. Warming 20,000 gallons by 10 °F takes 1.67 million BTU: a 100,000 BTU gas heater does it in about a day.
Formulas reviewed November 1, 2026 · How we calculate
Using your saved pool: . My pool
100k BTU gas heater
Raising 15,000 gallons by 12 °F in 24 hours takes 63k BTU an hour of heat, or a 74k BTU input heater at 84% efficiency.
- Total heat
- 1.5M BTU
- Gas for one heat-up (17.9 therms)
- $26.81
- With a 110k BTU heat pump
- 13.6 hours
What to add
What would change your result
- Aiming for 80 °F instead of 82 °F cuts the energy for this heat-up by 17%.
- The US Department of Energy estimates a pool cover can cut heating costs by 50–70%, because most heat leaves through the surface.
This counts only the heat to warm the water. A pool loses heat through its surface while heating, much faster without a cover and in wind, so allow extra.
Pool heater size chart
Gas heater size (input BTU, 84% efficiency) to warm the pool by 10 °F in the time shown. Surface losses not included.
| Pool size | In 48 hours | In 24 hours | In 12 hours |
|---|---|---|---|
| 10,000 gal | 100k BTU | 100k BTU | 100k BTU |
| 15,000 gal | 100k BTU | 100k BTU | 150k BTU |
| 20,000 gal | 100k BTU | 100k BTU | 200k BTU |
| 25,000 gal | 100k BTU | 150k BTU | 250k BTU |
| 30,000 gal | 100k BTU | 150k BTU | 250k BTU |
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Size for the heat-up you want. A bigger heater doesn't heat more water, it heats it faster. If you heat only on weekends, size for a quick warm-up; if you keep it warm all season, a smaller heater can keep up.
Gas or heat pump? Gas heats fast in any weather. A heat pump costs less to run but is slower and loses output as the air cools. In warm states, heat pumps often win.
Use a cover. Most heat leaves through the surface by evaporation. Covering the pool overnight keeps much of what you paid to add.
Every degree counts. Each degree of target temperature adds to every heat-up and to heat lost each night.
Key facts
- One BTU raises one pound of water by 1 °F, and a gallon of water weighs 8.34 lb.
- Gas heater ratings are input BTU; at about 84% efficiency, a 400k heater puts about 336k BTU an hour into the water.
- Heat pumps are cheaper to run but slower, and their output drops as the air gets cold.
How it's calculated
Total heat = gallons × 8.34 × temperature rise in °F, in BTU.
Heater size = total heat ÷ heat-up hours ÷ 0.84, rounded up to a standard gas heater size.
Gas cost = total heat ÷ 0.84 ÷ 100,000 BTU per therm × your price per therm. The heat pump time assumes 110,000 BTU an hour of output.
Questions people ask
What size heater do I need for a 20,000-gallon pool?
To raise it 10 °F in 24 hours, about 100,000 BTU input. To do the same in 12 hours, about 200,000 BTU. Most owners of pools this size choose 250k to 400k for faster heat-ups.
How long does it take to heat a pool?
Total heat ÷ heater output. A 400k BTU gas heater warms 20,000 gallons by 10 °F in about 5 hours, plus whatever the surface loses meanwhile.
How much does it cost to heat a pool?
For one heat-up, total BTU ÷ 0.84 ÷ 100,000 × your gas price per therm. 20,000 gallons by 10 °F is about 20 therms, or $30 at $1.50.
Gas heater or heat pump?
Gas is fast and works in any weather. A heat pump costs less per degree but heats slowly and struggles below about 50 °F.
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