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掳F takes 1 BTU
- BTU/hr = Gallons 脳 8.34 脳 Temp Rise 梅 Hours
- Example: 15,000 gal raised 20掳F in 24 hours = 15,000 脳 8.34 脳 20 梅 24 鈮?104,000 BTU/hr
Gas heater vs heat pump
- Gas heater: heats fast regardless of air temperature 鈥?best for pools used on demand. Common sizes: 200k, 250k, 300k, 400k BTU.
- Heat pump: 3鈥?脳 more efficient (lower running cost) but works slower and less effectively below ~50掳F air temperature 鈥?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掳F in 24 hours you need roughly 100,000 BTU/hr 鈥?but because of heat loss overnight, most owners of this size pool choose a 200,000鈥?00,000 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掳F per hour 鈥?roughly 12鈥?5掳F per full day of running. Heat pumps are 3鈥?脳 slower but cheaper per degree.
Does a solar cover really help?
Yes 鈥?dramatically. Up to 70% of heat loss is evaporation. A solar cover can cut heating costs by 50鈥?0% and is the single best upgrade for any heated pool.