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Calculators
Quick answer: PoolHeatMath has five free planning calculators. Start with pool volume, then size a gas or propane heater or a heat pump, check how long a heat-up will take, and estimate monthly running cost with your own prices. Each tool shows its formula and a worked example so you can check the result yourself.
Which calculator to use
- Pool volume calculator: gallons and litres from shape and depths.
- Pool heater size calculator: input rating for gas or propane heaters from heat-up and holding needs.
- Pool heat pump size calculator: output needed at your coolest swim-month air temperature.
- Pool heating time calculator: hours to reach your target from a known heater rating.
- Pool heater operating cost calculator: therms, propane gallons or kWh and dollars per month.
The physics behind them
All five rely on a few constants. A US gallon of water weighs about 8.34 lb, and one BTU raises one pound of water by one degree Fahrenheit. A therm of natural gas is 100,000 BTU, a gallon of propane holds about 91,500 BTU, and one kWh equals about 3,412 BTU. Heat pumps multiply that by their COP. Surface loss uses a planning rule of roughly 12 BTU per hour per square foot per degree in light wind, adjusted for exposure and covers. These are rules of thumb that professionals also use for first passes, which is why each page tells you to confirm with a licensed installer.
Worked example: running all five
Figures below are illustrative examples (2026-10-29), not quotes or guarantees.
Assumptions: 32 × 16 ft pool, 3.5 to 6 ft deep, so about 18,200 gallons and 512 sq ft. Target 82°F from 65°F with 60°F nights. Size: about 135,000 BTU/h holding output uncovered, so a gas input near 160,000 BTU/h at 84%. Time: a 250,000 BTU/h heater with a 20% loss allowance needs roughly 15 hours. Cost: 100,000 BTU/h delivered for 120 hours is about 143 therms at 84% efficiency.
Tips for better inputs
Measure water depth rather than wall height, use the average night air temperature for the month you care about, and take prices from a recent utility bill including delivery charges. If you plan to use a cover, choose that setting and keep the cover habit; otherwise the estimates will be optimistic.
FAQ
Are the results quotes?
No. They are planning estimates; a licensed pro confirms size and cost.
Do you store my inputs?
The calculators run in your browser and do not send your numbers anywhere.
Where are the guides?
In the pool heater FAQ hub.