Size a pool heater from surface area, not volume: BTU/hr = surface area (sq ft) × temperature rise (°F) × 12, then round up to the next size sold. A 16 × 32 ft pool needing a 20°F rise works out at 122,880 BTU/hr, so you buy a 125,000 BTU heater. A solar or thermal cover roughly halves that requirement.
Why Sizing Runs on Surface Area, Not Gallons
Almost everything a heater fights leaves through the top of the pool:
- Evaporation — the biggest single loss, because turning water to vapour costs roughly a thousand BTU per pound of water that leaves.
- Radiation and convection — the other two, and they happen at the surface as well.
- The ground, barely — the earth around a buried shell is comparatively good insulation, so a deep pool and a shallow one of the same footprint lose heat at close to the same rate.
That is why every manufacturer sizing sheet asks for length × width and not for gallons. Volume decides how long the heater runs to get you there; surface area decides how big the heater has to be to hold the temperature once it arrives.
Odd-shaped pools
For a kidney, freeform or L-shape, multiply the longest length by the widest width and take about 85% of the result. That is close enough for sizing — you are rounding up to a stock heater anyway.
The multiplier in the formula (×12) is the industry rule of thumb for an uncovered outdoor pool, and it is doing the work of a full heat-loss calculation: evaporation, wind, radiation and the night-time drop. It is deliberately conservative, which is what you want in a number you will live with for fifteen years.
Heater Size by Pool Size
The table below runs the formula for a 20°F rise — the usual ask for extending a season — on common rectangular pools, with and without a cover, and adds the rough heat-up time from cold for the uncovered size.
| Pool size | Surface area | Uncovered | With cover | Gas heat-up |
|---|---|---|---|---|
| 12 × 24 ft | 288 sq ft | 75,000 | 50,000 | ~29 hrs |
| 14 × 28 ft | 392 sq ft | 100,000 | 50,000 | ~30 hrs |
| 16 × 32 ft | 512 sq ft | 125,000 | 75,000 | ~31 hrs |
| 18 × 36 ft | 648 sq ft | 200,000 | 100,000 | ~25 hrs |
| 20 × 40 ft | 800 sq ft | 200,000 | 100,000 | ~30 hrs |
Two things stand out. A cover drops most pools a full size or two — the cheapest way to buy a smaller heater is to buy a cover first. And heat-up from cold is measured in days rather than hours on any pool of real size, which is the fact that decides how you will actually use the thing.
For a rise other than 20°F, or a pool that isn't a plain rectangle, the pool heater size calculator runs the same formula on your own numbers, with a cover toggle and metric units.
Picking Your Temperature Rise
The rise is the gap between the coldest water you expect to start from and the temperature you want to swim in — not the air temperature, and not the difference between air and water. Get this wrong and every number downstream is wrong with it.
Most pools settle a few degrees below the average air temperature of the month, so use your coldest intended swimming month as the starting point. Comfortable swim temperatures run about 78–82°F for lap swimming and general use, and 84–88°F for small children, therapy pools and anyone who prefers warm water. Every degree above 82°F costs noticeably more to hold, because losses grow with the gap between water and air.
| What you want | Typical rise | What that means for the heater |
|---|---|---|
| Take the chill off mid-summer | 5–10°F | Smallest sizes; a heat pump is comfortably enough |
| Extend the season a month either side | 15–20°F | The mainstream case — the chart above |
| Swim through spring and autumn | 25–30°F | Large gas heater, or a heat pump plus a cover |
| Warm water on demand for a weekend | 20°F+ fast | Gas only — heat pumps cannot recover quickly |
One more input the formula does not see: wind. An exposed pool on open ground loses heat far faster than a sheltered one, and a windbreak or hedge is worth more than a heater size. If your site is windy, size up rather than trimming the rise.
The Size Is the Same — The Heater Type Isn't
Gas heaters, heat pumps and electric units are all rated in BTU/hr, so the number you just worked out applies to all of them. What changes is whether that number exists in the catalogue, what it costs to run, and how it behaves in cold weather.
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Gas (natural gas or propane)
Delivers its rated output whatever the weather and runs at about 82% efficiency, so it is the only realistic choice for heat on demand or a large rise. Available up to 500,000 BTU and beyond. Highest running cost per hour, lowest purchase price for the output.
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Heat pump
Moves heat out of the air rather than making it, at a COP of roughly 3.5–5.0 — three to five units of heat per unit of electricity, which is why running costs are a fraction of gas. The catch is that output falls away below about 50°F ambient, the units top out at a lower BTU rating than gas, and they heat slowly enough that they want to be left running rather than switched on for a weekend.
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Electric resistance
Nearly 100% efficient and still the most expensive way to heat a pool, because it buys every BTU at the price of electricity instead of multiplying it like a heat pump. Reasonable for a hot tub or a very small plunge pool; punishing on anything above about 5,000 gallons.
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Solar
Sized by collector area rather than BTU — the usual target is panel area equal to 50–100% of the pool's surface area. Free to run, but it produces heat when the sun decides, not when you want to swim, so it is best paired with a cover or used to shrink the load on a smaller conventional heater.
The practical consequence of the BTU ceiling: on a large pool with a big rise, the formula can land on a number no heat pump on the market offers. That is the point to add a cover, accept a smaller rise, or go gas. To compare what each type would cost you per month at your own fuel prices, run the size through the pool heating cost calculator; the heat-pump case in detail is in cost to run a pool heat pump.
How Long a Heater Takes to Get There
Heat-up time is straight physics: one BTU raises one pound of water by one degree, and water weighs 8.34 lb per gallon. So
hours = gallons × 8.34 × rise ÷ (BTU/hr × efficiency)
A 125,000 BTU gas heater on a 19,000-gallon pool at 82% efficiency needs about 31 hours to lift the water 20°F — and that is a floor, because the pool keeps losing heat the whole time. In practice, plan on a day and a half for a season-opening warm-up, and less every time after that, since you are topping up a few degrees rather than starting cold.
Two rules of thumb fall out of it:
- About 1–2°F per hour. That is what a correctly sized gas heater moves an average pool; a heat pump on the same pool manages a fraction of it.
- Oversizing buys speed, not efficiency. A bigger heater reaches temperature sooner but burns roughly the same fuel getting there, because the energy required is set by the water, not by the burner.
The heater also needs the water delivered to it. Every unit has a minimum flow rate, and an undersized or throttled pump will trip it on high limit. If you are replacing equipment as a set, check the pump against the pool pump size calculator and confirm your plumbing carries that flow.
Where People Go Wrong
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Sizing on gallons
The commonest error, and it usually undersizes a shallow pool badly — a 20 × 40 splash pool loses heat like a 20 × 40 diving pool despite holding half the water. Measure the surface, not the shell. If you need the volume for dosing or heat-up time, the pool volume calculator handles the shapes.
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Buying the smallest heater that fits the formula
Round up, always. Undersizing shows up as a heater that runs continuously and never quite reaches target on the cold evenings you bought it for, and the running cost is not lower — it is the same fuel spread over more hours.
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Sizing for a cover you never use
The cover multiplier is real but conditional. If the cover only goes on twice a summer, size the heater as uncovered — otherwise you have bought half a heater for a pool that behaves like an open one.
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Ignoring the gas line or the electrical supply
A 400,000 BTU gas heater needs a gas line that can feed it, and a large heat pump needs its own breaker and a spot with real airflow around it. Both are frequently the expensive surprise in the install quote, so check them before you settle on a size.