PoolOwl

Equipment & energy

Pool Heat Pump Sizing Calculator

Estimate the heater output (BTU/h) your pool needs from its surface area and climate, using the Department of Energy's sizing rule, plus how long and how much it costs to heat up.

What you'll need (3)
  • Your pool's surface area in square feet
  • The water temperature you want
  • The average air temperature of the coolest month you'll swim
Your pool
Not sure? Use the surface area calculator. Example: 512.
78–84 °F is typical.
Your climate
The average (not the low) for the coolest month you want to swim. Example: 62.
Wind at the pool
Heat-up time and cost

Optional: how long and how much to bring the pool up to temperature.

From the spec sheet. DOE cites typical COPs of 3.0–7.0; it falls as the air gets colder.

Your result

Estimate

Your result will appear here

Enter your pool's surface area, target temperature and coldest average air temperature.

The DOE sizing rule

The US Department of Energy gives a simple rule for sizing outdoor pool heaters:

BTU/h = surface area (sq ft) × temperature rise (°F) × 12

The temperature rise is the gap between the water temperature you want and the average air temperature of the coolest month you plan to swim. The rule assumes about 3.5 mph of wind across the pool. DOE says to multiply by 1.25 for about 5 mph, and by 2.0 for about 10 mph.

Pool area 15 °F rise 20 °F rise 25 °F rise
300 sq ft 54,000 72,000 90,000
450 sq ft 81,000 108,000 135,000
600 sq ft 108,000 144,000 180,000

BTU/h at sheltered (≈3.5 mph) conditions. The calculator also shows the next common heat pump size above your result.

Heat pump ratings vs. real weather

Heat pumps move heat from the air into the water, so their output depends on air temperature. DOE notes they work most efficiently above about 45–50 °F. Manufacturers usually publish output at warm test conditions (80 °F air) and often at 50 °F too. If you’ll swim in spring and fall, compare models on their cooler-air rating.

Heat-up time and cost

To heat the whole pool: BTU = gallons × 8.33 × degrees of rise. A 20,000-gallon pool warming 10 °F needs about 1.7 million BTU. Divide by heater output for the minimum hours. Divide by (COP × 3,412) for the kWh a heat pump uses. These are best-case figures: heat keeps escaping while you heat, especially with the cover off.

Assumptions and limitations

  • Uses the US Department of Energy rule of thumb for outdoor pools. It is not a site-specific heat-loss calculation.
  • Heat pump output drops in cooler air; headline ratings are measured in warm conditions.
  • Heat-up time and cost ignore heat lost while heating, so they're a best case (cover on, calm weather).

Key terms

Common questions

What size heat pump do I need for a 15,000-gallon pool?

Heater size depends on surface area and climate, not gallons. A 15,000-gallon pool might be 400–500 sq ft. At 400 sq ft and a 20 °F rise (82 °F water, 62 °F air), DOE's rule gives 96,000 BTU/h in sheltered conditions.

Is a bigger heat pump better?

A larger unit heats faster and keeps up in cooler weather, and running costs per degree are similar. The trade-off is purchase price and electrical capacity. Undersizing is the more common regret.

Does a solar cover really help?

Yes. Most heat is lost from the water surface through evaporation. Covering the pool when it's not in use is one of the biggest savings available for heated pools.

Sources and methodology references

  1. Gas swimming pool heaters (sizing), US Department of Energy, Energy SaverBtu/h = pool area × temperature rise × 12; based on 1–1.25 °F/h and 3.5 mph wind; ×1.25 for 5 mph, ×2.0 for 10 mph.
  2. Heat pump swimming pool heaters, US Department of Energy, Energy SaverCOP typically 3.0–7.0; heat pumps work most efficiently with outdoor air above about 45–50 °F.

Published October 2, 2026. Results are estimates. See how our calculators work.

Spotted a wrong result or an outdated source? Report a problem with this calculator.

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