Pool Heater Size Calculator

Pool heaters are sized by one gap: how far above the outside air you want the water, in the coldest month you plan to swim. A pool loses almost all of its heat from the surface, so the surface area and that gap set the size, and everything else is detail. That is why two pools holding the same water can need very different heaters, and why the same pool needs a bigger heater in April than in July.

The kind of heater changes the answer. A gas heater's label is the fuel it burns, not the heat it delivers, so it has to be labelled higher than the load. A heat pump's label is the heat it delivers on a warm day, and it delivers less as the air cools, so it is chosen to hold a temperature rather than to warm a cold pool overnight. The calculator handles both and rounds each up to the sizes actually sold, so the answer is a heater you can buy, not a number you have to interpret.

What moves your number most is the air temperature you enter. Size for the coldest month you genuinely swim, not the coldest month of the year, and not a single cold night. Wind is the second lever: an open yard needs a quarter more heat, and a windy site needs double.

Start with the surface. Length times width gives the surface area, which is where evaporation and wind carry heat away. Depth does not change the heater size, only how long a cold pool takes to warm up.

Then the rise. Subtract the average air temperature of your coldest swim month from the water temperature you want. That is the temperature rise the heater has to hold against the weather. The Department of Energy's sizing method multiplies surface area by that rise by 12 to get the heat the heater must deliver every hour, and that figure assumes a light 3.5 mph breeze and about 1°F of warming per hour. If your pool is more exposed than that, the calculator scales the load: an open yard adds 25 percent, and a windy site doubles it, which is the adjustment Hayward's own sizing tool applies.

Convert to a label you can buy. Gas heaters are labelled by the gas they burn each hour, and federal rules require them to turn at least 82 percent of that into heat in the water. So the label you need is the heat output divided by 0.82, rounded up to the next standard size: 150,000, 200,000, 250,000, 300,000, 350,000 or 400,000 BTU. Heat pumps are labelled by heat delivered, so the output figure is rounded up directly to the next size sold: 50,000, 70,000, 90,000, 110,000 or 140,000 BTU. If the load is bigger than the largest unit, the calculator says how many units it takes rather than pretending one will do.

Heat needed = surface area × temperature rise × 12 × wind factor. For a 15 by 30 ft pool that is 450 square feet. Wanting 82°F water when the coldest swim month averages 60°F is a 22°F rise. 450 × 22 × 12 = 118,800 BTU per hour for a sheltered pool. In an open yard multiply by 1.25; on a windy site multiply by 2.

Gas label = heat needed ÷ 0.82, rounded up. 118,800 ÷ 0.82 = 144,878, so a 150,000 BTU gas heater is the smallest standard size that covers it. At the federal minimum efficiency it delivers about 123,000 BTU per hour of heat, comfortably above the 118,800 needed.

Heat pump label = heat needed, rounded up. The same pool needs a 140,000 BTU heat pump, the largest common residential size, and because that label is measured on an 80°F day the unit will deliver less on the 60°F days it was sized for.

Sanity check against a manufacturer table. Hayward publishes recommended gas heater sizes by pool surface area: 450 sq ft gets the 150,000 BTU model, 600 gets 200,000, 750 gets 250,000, 900 gets 300,000, 1,050 gets 350,000 and 1,200 gets 400,000. At a 20°F rise on a sheltered pool, this calculator lands on every one of those six rows.

📎Sources:U.S. Department of Energy, Energy Saver — Gas and Heat Pump Swimming Pool Heaters (sizing method; archived copy),Hayward — Pool Heaters FAQ (recommended H-Series heater by pool surface area),Hayward — Heater Size Calculator (wind adjustments),eCFR — 10 CFR 430.32(k) Pool heaters (82% minimum thermal efficiency)