Pool heater size calculator
Enter your pool's surface area, the temperature you want and the average air temperature in the coldest month you'll swim. The calculator gives the heater output you need, using the sizing method published by the US Department of Energy, and estimates how long a heater takes to warm the pool.
Need the surface area or volume? The pool volume calculator gives both and can pass them to this page.
The sizing formula
Temperature rise = desired pool temperature − average air temperature in the coldest month of use
This is the method in the US Department of Energy's guide to gas pool heaters. Its factor of 12 assumes a heat-up rate of about 1 to 1¼ °F per hour and an average wind of 3½ mph across the pool surface. For faster heating, the DOE multiplies the result by 1.5 (about 1½ °F per hour) or 2 (about 2 °F per hour), which the "heat-up speed" option applies. The DOE also recommends having a trained pool professional do a full sizing analysis for your pool.
The formula is based on surface area rather than volume, because a heated pool loses most of its heat from the surface: evaporation, wind and radiation to the night sky. Two pools with the same surface but different depths lose heat at about the same rate.
Input vs. output, and efficiency
Gas pool heaters are usually sold by their input rating (for example 200,000 or 400,000 BTU/h), the gas they burn. The heat delivered to the water is the input × the heater's thermal efficiency. Standard gas pool heaters are typically rated around 82–84%, and condensing models higher. The calculator divides the needed output by the efficiency to suggest an input size. Heat pumps are rated by output at specific air and water temperatures, and that output drops in cool air, so compare their ratings at conditions close to yours.
Worked example
A 16 × 32 ft pool has 512 sq ft of surface. You want 80 °F, and the coldest month you'll swim averages 60 °F, so the rise is 20 °F.
- Output: 512 × 20 × 12 ≈ 122,900 BTU/h.
- Input at 82% efficiency: 122,900 ÷ 0.82 ≈ 150,000 BTU/h. The next common size up would be a sensible choice.
Heat-up time: the pool holds about 22,000 gallons. Warming it from 65 °F to 80 °F takes 22,000 × 8.34 × 15 ≈ 2.75 million BTU. A 200,000 BTU/h heater at 82% delivers 164,000 BTU/h, so about 17 hours, plus extra time to make up heat lost to the air while it runs.
Ways to cut heating costs
- Use a cover. Evaporation is the largest source of heat loss from an outdoor pool. A cover at night keeps much of the heat in and cuts water and chemical loss too.
- Block the wind. A fence, hedge or screen reduces evaporation.
- Lower the set point. Each degree lower saves energy, and a few degrees often makes little difference to comfort.
- Size for your real season. Heating into the colder months needs a much bigger heater. Choosing the coldest month carefully avoids paying for capacity you won't use.
Frequently asked questions
What size heater do I need for a 20,000-gallon pool?
The DOE method uses surface area, not gallons. A typical 20,000-gallon in-ground pool has 450–600 sq ft of surface. With a 20 °F rise, that's about 108,000–144,000 BTU/h of output, or roughly 130,000–175,000 BTU/h of gas input.
Is a bigger heater better?
A larger heater warms the pool faster and copes with colder weather, but costs more up front. The gas used to hold a temperature depends on heat loss, not heater size. An oversized heater mainly saves heat-up time.
Does this work for spas?
Not well. Spas are small, deep and heated to much higher temperatures for short periods. Follow the spa or heater manufacturer's sizing guide instead.