What size pool heater do I need?
Two different jobs hide inside that question. Holding temperature is about your pool's surface: an uncovered pool loses roughly 12 BTU per hour for every square foot, for every degree it sits above the air. Heating up is about your gallons: 8.34 BTU per gallon per degree, however long that takes. A heater has to win the first fight and finish the second in a time you can live with — which is why sizing by "pool gallons" alone, the way sales flyers do, misses the point. Typical results, sized to hold 20 °F above air with a cover:
| Pool | Surface | Gas heater | Heat pump |
|---|---|---|---|
| 15 ft round above-ground | 177 ft² | 125k BTU | 50k BTU |
| 15 × 30 inground | 450 ft² | 125k BTU | 65k BTU |
| 16 × 32 inground | 512 ft² | 125k BTU | 65k BTU |
| 20 × 40 inground | 800 ft² | 150k BTU | 95k BTU |
Covered maintenance sizing — an uncovered pool needs roughly 2.4× the output, and most people up-size gas 1–2 steps anyway for faster heat-up. The calculator above works both numbers for your pool.
A "400,000 BTU" gas heater burns 400k of gas and puts roughly 330k into the water — standard heaters run ~82% efficiency, condensing models up to 95%. Heat pumps are labeled the opposite way: their rating is output — but measured at about 80 °F air, and real output sags as air cools (roughly 20–40% lower at 60 °F, model-dependent). Compare like with like: output to output, at the air temperature you'll actually swim in.
Gas or heat pump? Size for how you swim
Gas heats fast and doesn't care about weather — the right tool for weekend use ("heat it Friday, swim Saturday"), spas, and shoulder-season swimming in cool climates. Size it generously: an oversized gas heater just finishes sooner. A heat pump is a marathon runner — 3–6× cheaper per BTU to run, but slower and weather-dependent, so it suits pools kept warm continuously through a long season in mild air. Undersizing a heat pump is the classic regret: it runs around the clock and still loses ground on cool nights. When in doubt on a heat pump, buy the next size up — the heating cost calculator compares the operating bills side by side.
Around 70% of a pool's heat walks out through evaporation, and a cover stops most of it — which is why the maintenance factor drops from 12 to about 5 with one on. In heater terms: a $60 solar blanket does the work of tens of thousands of BTU, every night, for free. If a heater quote scares you, price the cover first — sometimes the honest answer is a smaller heater plus a cover rather than the bigger box.
How long will the heat-up take?
The calculator shows it for your recommended size, but the shape of the answer: BTU needed = gallons × 8.34 × degrees. A 15,000-gallon pool coming up 15 °F needs about 1.9 million BTU — call it 9 hours on a 250k gas heater, or a day and a half on a 95k heat pump. That's not the heat pump failing; it's the design. It also explains the cardinal rule of pool heating: don't heat from cold for every swim. Heat-up is the expensive part; holding temperature under a cover is cheap. Pick a temperature, hold it through the season, and let the thermostat do the small corrections.