Pool heater size calculator: how many BTU, and how long to heat?

A pool heater has two jobs. The first is the heat-up: taking cold water up to swimming temperature at the start of the season or after a cold snap. The second is holding it there against everything the pool loses overnight, mostly to evaporation. The heater has to do both at once: while it warms the water, the pool is still losing heat. So this calculator sizes the heater for the heat-up you want plus the DOE formula’s heat-loss allowance, and shows a rough heat-up time that allows for those losses.

How the pool heater calculator works

Heat to warm the water. One BTU warms one pound of water by 1°F, and a US gallon of water weighs 8.34 lb. So the heat is gallons × 8.34 × degrees of rise. In metric, warming a litre of water by 1°C takes 4.186 kJ; divide by 3,600 for kWh.

Heat-up time. Divide that heat by what the heater actually puts into the water. A gas heater’s nameplate is usually its gas input, so multiply by its efficiency (82% is the federal minimum). A heat pump is rated by heat output, so its rating is what goes in. The result ignores losses, so treat it as the best case.

DOE sizing. The US Department of Energy sizes both gas heaters and heat pumps with one formula: surface area in square feet × temperature rise in °F × 12, where the rise is the pool temperature you want minus the average air temperature of the coldest month you plan to swim. The result is the heat output in BTU per hour. The DOE says it is based on a 1° to 1-1/4°F rise per hour with a 3-1/2 mph average wind; for a 1-1/2°F per hour rise multiply by 1.5, and for 2°F by 2.

heat (BTU) = gallons × 8.34 × °F rise
heat (kWh) = litres × 4.186 × °C rise ÷ 3,600
heat-up hours = heat ÷ (gas rating × efficiency)   or   heat ÷ heat pump output
DOE output needed (BTU/h) = surface area (sq ft) × (pool temp − coldest-month average air temp, °F) × 12
heater output needed = heat ÷ hours wanted + DOE allowance
gas input rating = output needed ÷ efficiency
1 kW = 3,412 BTU/h

Typical heater sizes

What the heat-up looks like

Most of the wait is set by volume. Warming 20,000 gallons by 10°F takes about 1.67 million BTU: roughly 8 hours of a 250,000 BTU/h gas heater at 82%, or 17 hours of a 100,000 BTU/h heat pump, before losses. Halve the volume and the time halves; double the rise and it doubles. Two worked examples go further: what size pool heater for 20,000 gallons runs the DOE sizing for a typical inground pool, and how long to heat a pool 10 degrees compares a heat pump with a gas heater on a 15,000-gallon pool.

Tips and common mistakes

• Use the coldest month you’ll actually swim, not the coldest month of the year. Heating a pool for a few weeks in spring and fall needs far less heater than keeping it at 82°F through a frosty October.
• Input vs output. A 250,000 BTU/h gas heater at 82% delivers about 205,000 BTU/h into the water. A 125,000 BTU/h heat pump delivers its full 125,000, but only in warm air.
• Cover it. The DOE calls evaporation by far the largest source of energy loss from a pool, and puts the heating savings from a cover at 50–70%. A cover during the heat-up shortens it, and with one at night a smaller heater can hold temperature. The solar cover calculator sizes one.
• Every degree costs. The DOE estimates each 1°F of extra pool temperature costs 10–30% more energy. Run the pool heating cost calculator before settling on 86°F.
• Bigger is faster, not cheaper per degree. An oversized gas heater warms the pool sooner but burns the same gas per BTU; an undersized heat pump may never catch up in cool weather.

Choosing between gas and a heat pump is mostly about climate and how often you heat — see pool heater vs heat pump, and pool running costs for the whole yearly bill. Need the gallons first? The pool volume calculator gives both volume and surface area.