Size your pool heater in 7 modes: BTU sizing, heat-up time, heat-loss estimate, gas vs heat-pump running cost, solar-cover savings, sizing charts, and gas/electrical install guidance. 100% client-side — your numbers never leave this page.
The 20% allowance covers normal wind and evaporation. Use 50% for windy, uncovered pools, or shoulder-season heating.
Ideal time, ignoring losses. Rule of thumb: real-world heat-up takes 20–50% longer on a windy day or without a cover.
Uses the industry 12 BTU/hr per sq ft per °F rule (assumes ~3.5 mph wind), adjusted by wind and cover factors. Evaporation alone causes about 70% of a pool's heat loss — covering the pool is the single biggest lever.
Rates default to US averages for 2025. Replace them with your bill for a real answer. Equipment purchase and install costs are not included.
Assumes the cover cuts total heat loss to 35% of uncovered and evaporation by 95% (a 2016 Cal Poly study measured ~95%). A cover preserves heat more than it creates it — pair it with a right-sized heater rather than replacing one.
| Pool volume | BTU/hr required | Gas heater | Heat pump |
|---|---|---|---|
| 5,000 gal | 62,475 | 100,000 BTU | 75,000 BTU |
| 7,500 gal | 93,712 | 100,000 BTU | 100,000 BTU |
| 10,000 gal | 124,950 | 150,000 BTU | 125,000 BTU |
| 15,000 gal | 187,425 | 200,000 BTU | — |
| 20,000 gal | 249,900 | 250,000 BTU | — |
| 25,000 gal | 312,375 | 400,000 BTU | — |
| 30,000 gal | 374,850 | 400,000 BTU | — |
| 40,000 gal | 499,800 | — BTU | — |
| Surface area | Heat loss (BTU/hr) | Gas heater |
|---|---|---|
| 200 sq ft | 48,000 | 100,000 BTU |
| 400 sq ft | 96,000 | 100,000 BTU |
| 600 sq ft | 144,000 | 150,000 BTU |
| 800 sq ft | 192,000 | 200,000 BTU |
| 1,000 sq ft | 240,000 | 250,000 BTU |
| 1,500 sq ft | 360,000 | 400,000 BTU |
| Pool volume | BTU/hr required | Gas heater | Heat pump |
|---|---|---|---|
| 18,927 L | 62,475 | 100,000 BTU | 75,000 BTU |
| 28,391 L | 93,712 | 100,000 BTU | 100,000 BTU |
| 37,854 L | 124,950 | 150,000 BTU | 125,000 BTU |
| 56,781 L | 187,425 | 200,000 BTU | — |
| 75,708 L | 249,900 | 250,000 BTU | — |
| 94,635 L | 312,375 | 400,000 BTU | — |
| 113,562 L | 374,850 | 400,000 BTU | — |
| 151,416 L | 499,800 | — BTU | — |
| Surface area | Heat loss (BTU/hr) | Gas heater |
|---|---|---|
| 18.6 sq m | 48,000 | 100,000 BTU |
| 37.2 sq m | 96,000 | 100,000 BTU |
| 55.7 sq m | 144,000 | 150,000 BTU |
| 74.3 sq m | 192,000 | 200,000 BTU |
| 92.9 sq m | 240,000 | 250,000 BTU |
| 139.4 sq m | 360,000 | 400,000 BTU |
Volume table: gas sizes are the next standard unit at or above the required BTU; heat-pump column shows "—" where the need exceeds standard residential heat-pump sizes (above 166,000 BTU/hr). Charts are starting points — run mode 1 with your own rise time and allowance for a tailored answer.
Gas derate: about 4% per 1,000 ft above 2,000 ft (manufacturer tables are rated to 2,000 ft). Breaker guidance applies the 125% continuous-load rule.
Use BTU per hour = gallons x 8.33 x desired temperature rise in degrees F, divided by your target heat-up hours, then add a heat-loss allowance. Example: a 20,000-gallon pool heated 10 degrees F in 8 hours needs 208,250 BTU per hour before losses; with a 20 percent allowance that is 249,900, so the next standard size is a 250,000 BTU gas heater.
Hours = gallons x 8.33 x degrees F of rise, divided by the heater output in BTU per hour. Example: a 20,000-gallon pool raised 10 degrees F with a 200,000 BTU heater takes 8.33 hours, ignoring heat loss. Wind and an uncovered surface make the real time longer.
For the same 100,000 BTU per hour output, 8 hours a day, 150 days: a heat pump at COP 5.0 uses about 7,034 kWh, which is $1,195.78 at $0.17 per kWh; gas at 82 percent efficiency burns about 1,463.41 therms, which is $1,975.61 at $1.35 per therm; electric resistance uses about 35,169.99 kWh, which is $5,978.90. The heat pump wins on running cost in most cases.
Yes, mainly by stopping evaporation, which causes about 70 percent of a pool's heat loss. For a 16 by 32 ft pool at a 20 degree F water-to-air difference, a bubble cover cuts losses by about 79,872 BTU per hour, roughly $955.09 per 150-day season at heat-pump rates, and saves about 11,370 gallons of water. These are rule-of-thumb estimates; real savings depend on wind and climate.
Heat escapes through the surface, not through the bulk of the water. Gallons only decide how long heat-up takes. The industry rule is about 12 BTU per hour per square foot per degree F of difference between water and air. Check: an 800 sq ft pool at a 30 degree F difference loses about 288,000 BTU per hour, which is why the manufacturer's own sizing table points a pool that size at a 300,000 BTU heater.
Using a 10 degree F rise in 8 hours with a 20 percent loss allowance: 15,000 gallons needs 187,425 BTU per hour, so a 200,000 BTU gas unit; 20,000 gallons needs 249,900, so a 250,000 BTU unit. For steady-state heat-loss sizing instead, see the surface-area table in mode 6.
COP, the coefficient of performance, is heat output divided by electricity input. COP 5.0 means 5 units of heat for every 1 unit of electricity. This calculator defaults to 5.0, the typical rating for modern pool heat pumps, and lets you test values from 4.0 to 6.0.
A 400,000 BTU heater burns 4.0 therms per hour at full fire, so the gas line and meter must be sized for that flow plus the rest of the house. Have a licensed installer size it. Above 2,000 ft elevation output derates about 4 percent per 1,000 ft, so at 5,000 ft a 400,000 BTU unit effectively delivers about 352,000 BTU per hour.
A 120,000 BTU heat pump at COP 5.0 draws about 7.03 kW, which is 30.58 amps at 230 V, so a 40-amp circuit is the minimum after the 125 percent continuous-load rule. A 50,000 BTU electric resistance heater draws about 14.65 kW, or 63.71 amps at 230 V, needing an 80-amp circuit. Confirm with an electrician and your local code.
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