Pool Heater Size Calculator
Find out what size pool heater you need. Get the right BTU output from your pool surface area, location, and desired temperature rise.
Safety
Heater sizing is approximate — wind exposure, shade, altitude, and insulation all affect heat loss. A pool cover reduces heat loss by 50–70% and is the single most cost-effective heating upgrade.
Size a pool heater on surface area, not gallons: BTU/hr = surface area (sq ft) × temperature rise (°F) × 12, rounded up to the next size sold. A 16 × 32 ft pool needing a 20°F rise works out at 122,880 BTU/hr, so you buy a 125,000 BTU heater. A cover roughly halves that. Enter your pool above for the exact size.
Pool Heater Size Chart
Heater sizing is driven by surface area, not gallons. Almost all of a pool's heat leaves through the top, so a shallow 20 × 40 loses heat about as fast as a deep one. The rule every manufacturer sizing guide reduces to: BTU/hr = surface area (sq ft) × temperature rise (°F) × 12, then round up to the next size a manufacturer actually sells.
The chart below is that formula applied to common rectangular pool sizes, uncovered. Figures in brackets are the raw requirement; the bold number is the heater size you would buy.
| Pool size | Surface area | +10°F rise | +15°F rise | +20°F rise | +25°F rise |
|---|---|---|---|---|---|
| 12 × 24 ft | 288 sq ft | 50,000 (34,560) | 75,000 (51,840) | 75,000 (69,120) | 100,000 (86,400) |
| 14 × 28 ft | 392 sq ft | 50,000 (47,040) | 75,000 (70,560) | 100,000 (94,080) | 125,000 (117,600) |
| 16 × 32 ft | 512 sq ft | 75,000 (61,440) | 100,000 (92,160) | 125,000 (122,880) | 200,000 (153,600) |
| 18 × 36 ft | 648 sq ft | 100,000 (77,760) | 125,000 (116,640) | 200,000 (155,520) | 200,000 (194,400) |
| 20 × 40 ft | 800 sq ft | 100,000 (96,000) | 150,000 (144,000) | 200,000 (192,000) | 250,000 (240,000) |
| 25 × 45 ft | 1,125 sq ft | 150,000 (135,000) | 250,000 (202,500) | 300,000 (270,000) | 400,000 (337,500) |
Reading the chart
Temperature rise is the gap between the coldest water you want to start from and your target — not the air temperature. A pool sitting at 62°F that you want at 82°F is a 20°F rise. Pick your row, pick the rise column, and you have the BTU output to shop for. Odd-shaped pools: multiply length × width and take roughly 85% of it, or run your real number through the pool volume calculator first.
A cover roughly halves everything in this chart. Evaporation is the single largest loss on an open pool, and a solar blanket cuts total heat loss by 50–70%. Sizing with a cover in mind is the cheapest way to drop a heater size — and the calculator above has a cover toggle that applies it.
Gas heater or heat pump?
The chart gives required output, which both types are rated in — but they get there differently. A gas heater delivers its rated BTU regardless of the weather and typically raises water about 1.5°F per hour, so it suits pools used on short notice and shoulder-season swimming. A heat pump moves heat out of the air instead of burning fuel, running at a COP of 3.5–5.0 — three to five units of heat per unit of electricity — which makes it far cheaper to run, but its output falls away below roughly 50°F ambient and it heats slowly enough that it wants to be left running.
One practical consequence: heat pumps are built in a narrower and lower output range than gas heaters, so on a large pool with a big temperature rise the chart can land on a number no heat pump offers. That is the point to either add a cover, accept a smaller rise, or go gas. Running costs for both are on the pool heating cost calculator, and the pump that has to feed the heater is sized on the pool pump size calculator.