What size heater for a 15,000-gallon pool?

To raise it 10 °F in 4 hours at 82% efficiency you need about 381,000 BTU/hr — round up to a 400,000 BTU heater. Slower targets or smaller temperature rises need less.

How is pool heater BTU calculated?

BTU/hr = (gallons × 8.34 × ΔT) ÷ hours ÷ efficiency. Water is 8.34 lb/gal and takes 1 BTU per pound per °F, so this is just mass × specific heat × temperature rise, spread over your target time and adjusted for heater efficiency.

Gas heater or heat pump — which should I get?

Gas heats fast and suits occasional or spa use; heat pumps are much cheaper to run but heat slowly, suiting pools held at a steady temperature all season. Size a heat pump with the same formula but a longer target time.

Should I oversize my pool heater?

A little. The sizing formula ignores surface heat loss during heat-up, so rounding up to the next standard size keeps real recovery times close to plan — and a pool cover reduces the loss the formula omits.

What size pool heater do I need? BTU sizing explained

Heater sizing comes down to how much water you have, how far you want to raise its temperature, and how fast. Here is the BTU math.

By Francesco Zinghinì

A pool heater is rated in BTU per hour — how much heat it delivers. Sizing one means balancing three things: the volume of water, the temperature rise (ΔT) you want, and how quickly you want to get there. The physics is simple and timeless, resting on the specific heat of water: it takes 1 BTU to raise 1 pound of water by 1 °F.

The formula

Water weighs 8.34 lb per gallon, so the energy to heat the whole pool by ΔT is gallons × 8.34 × ΔT (in BTU). Spread that over your target hours and divide by the heater’s efficiency:

BTU/hr = (gallons × 8.34 × ΔT) ÷ hours ÷ efficiency

Efficiency for a gas heater is typically around 0.82 (82%). A second, quicker cross-check for sizing against ongoing surface heat loss is the surface rule of thumb: BTU/hr ≈ surface ft² × ΔT × 12.

Worked example

15,000-gallon pool, raise 10 °F in 4 hours, efficiency 0.82:

(15,000 × 8.34 × 10) ÷ 4 ÷ 0.82 = 381,402 BTU/hr.

Heaters come in standard sizes, so you would round up to a 400,000 BTU unit. Rounding up is the right instinct: the formula ignores heat lost from the surface while you heat, so a little headroom keeps real-world recovery times close to plan.

Gas heater vs heat pump

Gas heaters deliver a lot of BTU quickly, which makes them ideal for on-demand heating and spas — you can raise temperature in hours. Heat pumps move heat from the air rather than burning fuel, so they are far more energy-efficient to run but deliver fewer BTU per hour (often ~100,000–140,000), meaning slower recovery measured in many hours. If you keep a pool at a steady temperature all season, a heat pump’s running economy usually wins; if you heat occasionally and want it fast, gas wins. Size a heat pump with the same energy formula but a longer target time — for example, 15,000 gallons and +10 °F over 12 hours needs about 127,134 BTU/hr.

Things that change your answer

Wind and exposure: an open, windy site loses heat fast — lean toward a larger heater or a cover.
A pool cover is the single biggest efficiency upgrade, cutting evaporative loss (the dominant heat drain) dramatically and letting a smaller heater keep up.
Target time: halving the hours doubles the BTU/hr you need, so be realistic about how quickly you truly need heat.
Climate and season: a bigger ΔT (cold water to warm target) scales the requirement directly.

Because the formula uses only the water you enter and stable physical constants — never a fuel price or product model — it stays correct indefinitely. Running cost is a separate calculation that uses the energy rate you enter.

Fuel type and matching the heater to your plumbing

Gas heaters run on either natural gas or propane, and the two are not interchangeable — a heater is built for one or the other, and propane packs more energy per unit volume, so your gas supply dictates the model. Beyond fuel, a heater has to match your circulation system: every gas heater specifies a minimum and maximum flow in GPM, and running it outside that window either fails to fire or risks overheating. That is one more reason to size your pump and plumbing sensibly, since the heater sits in the same loop.

Why 400,000 BTU is such a common size

Residential gas heaters cluster around a handful of standard outputs — commonly 250,000, 300,000, 350,000 and 400,000 BTU/hr — so real sizing means calculating your requirement and rounding up to the next size. A typical mid-size in-ground pool wanting a reasonably quick warm-up lands near the 400,000 BTU mark, which is why it is the default many owners end up with. If you also heat a spillover spa, lean larger still: a spa needs to reach a high temperature fast, and the heater that serves both should be sized for the spa’s quick-recovery demand rather than the pool’s. Remember that the sizing formula ignores surface heat loss, so headroom is protection, not waste — and a pool cover cuts that loss enough to let the heater you chose keep up comfortably.

The bottom line

Heater sizing is a single, timeless formula: BTU per hour equals gallons times 8.34 times your temperature rise, divided by your target hours and by efficiency. Decide how fast you truly need heat, because halving the hours doubles the heater. Gas delivers big BTU for quick, on-demand heating and spas; a heat pump sips energy but heats slowly, suiting a pool held warm all season. Round up to the next standard size — often 400,000 BTU for a mid-size pool — since the formula ignores the surface losses a real pool fights while heating. The single best companion to any heater is a cover, which cuts the evaporation that dominates those losses. Running cost is a separate calculation using the energy rate you enter, so this sizing figure never goes stale.

Size your heater on the heater size calculator, compare a heat pump, and estimate running cost with the heating cost calculator.