Pool Heater Size Calculator
What size heater does your pool need? Get the BTU output your pool needs and the gas heater size to buy, with the math shown, then let us confirm it on site.
Pool heater sizing calculator
Surface area × temperature rise × 12 = required heater output. Surface area drives it — evaporation is where pools lose heat.
84–88°F is the family sweet spot.
South Florida unheated pools sit in the low-to-mid 70s most winters and can dip into the 60s after a hard cold front; size for the cold weeks.
Gas heater: 200k BTU nameplate
A 19°F rise on 392 sq ft needs ≈ 89,000 BTU/hr of output (≈ 109k input at ~82% efficiency). Oversizing a gas heater is fine — it just reaches temperature faster. Undersizing means it runs forever on cold weekends. Heat pumps are rated differently (typically 110k–140k BTU output) and heat slower but far cheaper per degree — the right call for set-and-hold temperature in South Florida; gas wins for fast weekend heat-ups.
Reference sizing for an uncovered residential pool with typical wind exposure, per manufacturer surface-area charts. A solar cover cuts the real load dramatically. Final sizing and gas-line capacity are the installer's responsibility.
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How the sizing works
The industry rule of thumb, printed in manufacturer sizing guides, is surface area × temperature rise × 12 = the heat output your pool needs, in BTU per hour. Surface area drives it, not gallons, because evaporation off the surface is where a pool loses most of its heat. The rise is your target temperature minus how cold the water gets in the coldest month you plan to swim. In South Florida, unheated pools sit in the low-to-mid 70s most winters and can dip into the 60s after a hard cold front, so size for the cold weeks.
Heat pumps are sold by output, so that number is the size to look for, and they heat slowly and cheaply: the right call for holding a temperature here. Gas heaters are sold by input, the fuel they burn, so the calculator divides by their efficiency (about 82%) to give the nameplate size. They heat fast, which is why they win for weekend heat-ups and spas. A solar cover cuts the real load sharply, which can let a smaller heater keep up.
Not sure which type fits your pool? Read heat pump vs gas vs solar in Florida. Have an existing gas heater? Check your gas line size before you upsize. Ready for a new unit? See pool heater installation and replacement.
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Heater sizing FAQs
For a heat pump, multiply your pool's surface area in square feet by the temperature rise you need, then by 12. The rise is your desired pool temperature minus the average air temperature in the coldest month you want to swim. A 16 by 32 foot pool (512 square feet) held at 84°F when the water would otherwise drop to 65°F needs about 117,000 BTU per hour of output. Gas heaters are sold by input, so the calculator converts that output to the nameplate size to buy.
Both start from the same number: the heat output your pool needs to hold its temperature, which comes from surface area because evaporation off the surface is where a pool loses most of its heat. Gas heaters are sold by input, the fuel they burn, so the calculator divides by their efficiency (about 82%) to give the nameplate size. Heat pumps are sold by output and heat more slowly, so they're the set-and-hold choice, and gas is the fast heat-up choice.
A little, yes, if the pool is windy, shaded, or uncovered. Wind and evaporation are the biggest heat losses, and a pool cover can cut heat loss sharply, which lets a smaller heater keep up. What you don't want is a heater oversized for show: it costs more to buy and run, and heater performance depends on the flow your pump and plumbing can deliver. That's why we confirm the size on site.
For most pools here, a heat pump. The air is warm enough most of the year for a heat pump to be the cheapest way to hold a steady temperature. Gas heats faster and works in any weather, so it's the better fit for spas, pools heated only now and then, or warm water during a cold front. Some properties run both.
Most new pool heaters, heat pump or gas, run $5,000–$7,000 installed in South Florida, including removal of the old unit. A new gas line or electrical circuit is quoted separately when the site needs one. We give you one written price after a free on-site sizing visit.
Want the exact size, installed?
Free on-site sizing visit: we measure the pool, check your pump, plumbing, gas, and panel, and give you one written price.