Pool Heater Sizing and Heat-Up Calculator

People ask them as one question and they have different answers, which is why heater advice so often sounds contradictory.

Surface area sets the size you need. Heat leaves a pool through the surface, overwhelmingly by evaporation, and that loss scales with area rather than with how deep the pool is. So the sizing rule is 1.5 BTU per hour per square foot, per degree the water sits above the air — a 512 square foot pool held 20 degrees above ambient needs about 15,400 BTU/hr of delivered heat just to stand still. The calculator reads your temperature rise as that gap, which is right for a pool starting at its own unheated temperature and understates the load if it is already warm.

Volume sets how long it takes. Water takes 8.34 BTU per gallon per degree, so the heat-up is a fixed amount of energy divided by whatever your heater delivers per hour.

The consequence is worth holding onto: a deep pool heats slowly but holds its heat cheaply, and a wide shallow one heats fast and costs a fortune to keep warm. A tanning ledge is lovely and it is a radiator.

Two heaters, two numbers, measured differently — and almost nobody says so on the shelf.

Gas heaters are rated by input. A 400,000 BTU heater burns 400,000 BTU of fuel an hour and turns about 84% of it into water temperature, so it delivers around 336,000.

Heat pumps are rated by output. A 140,000 BTU heat pump delivers 140,000 BTU an hour. There is no efficiency to subtract, because the rating already is the delivered heat.

So the gas heater is 2.4 times the heat pump, not 2.9. It matters when you are comparing quotes, and it matters more in the other direction: a heat pump that looks like a third of the heater is really closer to half, which changes the sizing conversation.

Same pool, same 20 degrees, three fuels. Raising 20,000 gallons by 20°F takes 3.3 million BTU delivered.

Natural gas at $1.50 a therm, through an 84% heater: about $60. A heat pump at $0.17 a kWh with a COP of 5: about $33. Propane at $2.55 a gallon: about $111.

The gas and propane prices are the calculator's defaults, US residential averages from the EIA: natural gas for 2025, and propane over the 2025–26 heating season, before tax. The 17 cents is a little under the 2026 electricity average.

That ordering holds almost everywhere, but the size of the gap is entirely your utility rates — which is why the calculator asks for your own price rather than assuming one. The heat pump advantage is largest against propane and smallest where natural gas is cheap and electricity is expensive.

None of that counts standing losses, which is the cost you actually live with. A cover is worth more than any heater upgrade because it attacks evaporation, and evaporation is most of where the heat goes.

Every time and cost above assumes a perfectly insulated pool. Yours is not one, and the gap is not small.

While the heater runs, the pool keeps losing heat to evaporation, wind and cold air — and it loses faster the warmer the water gets, because the gap between water and air is what drives the loss. Without a cover, expect a heat-up to take a quarter to a half again longer than the figure above. With one, you will land close to it.

That is also why the theoretical sizing number and real heaters are so far apart. The rule says 15,400 BTU/hr for that 20 degree rise; people buy 250,000 to 400,000. The difference is the weather.

Two things make the estimate worse than it says: wind, which drives evaporation harder than cold air does, and a heat pump in cold air, whose output collapses below about 50°F. Which heater suits which climate goes through that properly.