Pool Heater Calculator
Size your pool heater correctly and you save thousands over the life of the unit. Undersize and it runs 24/7 without ever catching up. This pool heater calculator uses the industry-standard formula (surface area x temperature rise x 12) that Hayward, Pentair, and Raypak use in their sizing guides. Works for gas heaters, heat pumps, and electric resistance heaters. Covers above ground pools and in-ground pools of any shape.
Pool Heater Size Formula (Gas)
The industry-standard pool heater size calculator formula used by Hayward, Pentair, and Raypak is:
- BTU/hr needed = Pool Surface Area (sq ft) x Temperature Rise (F) x 12
- With a solar cover, multiply by 6 instead of 12 (cover cuts evaporative loss roughly in half).
Example: a 16 x 32 ft rectangular pool has 512 sq ft of surface area. To raise water 20 F in a reasonable time: 512 x 20 x 12 = 122,880 BTU/hr, so you'd pick a 150,000 BTU heater from the gas heater range.
Pool Heater BTU Chart
This pool heater btu chart shows the typical heater size (BTU/hr) needed for common pool surface areas and a 20 F temperature rise with no cover:
| Pool Size | Surface Area | BTU/hr (no cover) | Typical Model |
|---|---|---|---|
| 12 x 24 ft rect | 288 sq ft | 70,000 | 100K BTU (Hayward H100) |
| 14 x 28 ft rect | 392 sq ft | 95,000 | 100K BTU |
| 16 x 32 ft rect | 512 sq ft | 123,000 | 150K BTU (Hayward H150) |
| 18 x 36 ft rect | 648 sq ft | 155,000 | 200K BTU (Hayward H200) |
| 20 x 40 ft rect | 800 sq ft | 192,000 | 200K BTU |
| 24 ft round | 452 sq ft | 108,000 | 150K BTU |
| 15 x 30 ft oval | 353 sq ft | 85,000 | 100K BTU |
| 25 x 50 ft rect | 1,250 sq ft | 300,000 | 300K or 333K BTU |
Pool Heater Size Chart by Heater Type
The same temperature rise needs different heater sizes depending on the technology:
- Natural gas / propane heater: BTU/hr = Surface Area x Temp Rise x 12. Fastest heat-up (24-48 hr), highest operating cost.
- Heat pump: BTU/hr = Volume (gal) x 8.33 x Temp Rise / Desired Hours. Slower heat-up (2-5 days), lowest operating cost.
- Electric resistance heater: kW = BTU/hr / 3412. Works only for small spas and above ground pools under 5,000 gallons. Very expensive to run on full-size pools.
Hayward Pool Heater Size Calculator Notes
The hayward pool heater size calculator (and pdf sizing chart) uses the same surface-area-times-rise-times-12 formula above, with an adjustment for cover use. A hayward pool heater size calculator pdf download from Hayward's technical literature matches the numbers this tool produces to within rounding. Raypak pool heater sizing calculator and pentair pool heater calculator use identical math because they all implement the same ASHRAE pool heat loss model.
Above Ground Pool Heater Calculator
An above ground pool heater calculator uses the same formula, but above ground pools are usually round or oval and sit fully exposed to wind, so heat loss runs slightly higher than in-ground. For above ground pool heater size calculator results, consider adding 15-20% to the gas BTU size to handle extra wind-driven evaporation. Small above ground pools under 10,000 gallons often use a small gas heater (100K BTU Hayward H100), an electric heat pump (65K-90K BTU), or even a 5.5-11 kW electric heater for quick heat.
Electric Pool Heater Size Calculator
An electric pool heater size calculator gives kW, not BTU. Conversion: 1 kW = 3,412 BTU/hr. A 20,000 gallon pool needing a 20 F rise in 24 hours needs about 20,000 x 8.33 x 20 / 24 = 139,000 BTU/hr, or about 41 kW. That is a very large electric heater and realistically you'd pick a heat pump instead. Small above ground pools (under 5,000 gallons) can use 5.5-11 kW electric heaters, which run about $1-2 per hour of use at $0.15/kWh.
Pentair Pool Heater Calculator & Raypak Pool Heater Sizing Calculator
A pentair pool heater calculator result will match a raypak pool heater sizing calculator result for the same pool because all three major manufacturers (Hayward, Pentair, Raypak) sell heaters in the same standard BTU sizes: 100K, 150K, 200K, 250K, 300K, 333K, 400K, and 500K. Pick the next size up from what the formula returns. If you are between a 150K and 200K, go with the 200K because it heats faster and runs less often, which means longer lifespan.