Pool Heater Size Calculator
Heaters are rated in BTU/hour. Undersized heaters take forever to warm a pool; oversized ones cost more upfront but heat faster. Here's the number that fits your pool and patience.
How the math works
- One gallon of water weighs 8.34 lbs; raising 1 lb of water 1°F takes 1 BTU
- BTU/hr = Gallons × 8.34 × Temp Rise ÷ Hours
- Example: 15,000 gal raised 20°F in 24 hours = 15,000 × 8.34 × 20 ÷ 24 ≈ 104,000 BTU/hr
Gas heater vs heat pump
- Gas heater: heats fast regardless of air temperature — best for pools used on demand. Common sizes: 200k, 250k, 300k, 400k BTU.
- Heat pump: 3–5× more efficient (lower running cost) but works slower and less effectively below ~50°F air temperature — best for keeping a pool warm all season.
- Rule of thumb: if you heat occasionally and on demand, go gas and size up. If you maintain a temperature all season, a heat pump usually wins on cost.
Frequently asked questions
What size heater for a 15,000 gallon pool?
To raise the water 20°F in 24 hours you need roughly 100,000 BTU/hr — but because of heat loss overnight, most owners of this size pool choose a 200,000–300,000 BTU gas heater.
How fast does a pool heater heat?
A 300,000 BTU gas heater raises a 15,000 gallon pool about 0.6°F per hour — roughly 12–15°F per full day of running. Heat pumps are 3–5× slower but cheaper per degree.
Does a solar cover really help?
Yes — dramatically. Up to 70% of heat loss is evaporation. A solar cover can cut heating costs by 50–70% and is the single best upgrade for any heated pool.