Pool Heater Sizing Calculator
Sizing a pool heater means matching its BTU output to how much water you want to heat, how many degrees you want to raise it, and how quickly. This calculator uses the water-heating energy balance to estimate the heater output in BTU per hour needed to reach your target temperature in a chosen time. For final sizing, also consider surface heat loss, wind, and cover use, which a professional load calculation includes.
Pool heater sizing formula
Net energy = gallons * 8.33 * temperature rise
Required output (BTU/hr) = net energy / heat-up hours
Required input rating = required output / (efficiency / 100)
8.33 = lb per gallon (BTU per gallon per degree F)
Heating water takes about 8.33 BTU per gallon per degree Fahrenheit. Dividing total energy by the desired heat-up time gives the output rate. The input rating accounts for the heater's thermal efficiency (gas heaters are commonly 80 to 84 percent; high-efficiency models higher). This estimate excludes ongoing surface heat loss.
Worked example
To raise a 20,000 gallon pool by 10 F in 24 hours: net energy = 20,000 times 8.33 times 10 = 1,666,000 BTU. Required output = 1,666,000 / 24 = 69,416.67 BTU per hour. At 82 percent efficiency the input rating = 69,416.67 / 0.82 = 84,654.47 BTU per hour, so a 100,000 to 125,000 BTU heater (allowing for surface loss) is a sensible choice.
Pool heater sizing notes
- This calculation covers raising the water temperature; it does not include ongoing surface heat loss, which can be large for uncovered pools.
- A pool cover dramatically reduces heat loss and is the single most effective way to keep a pool warm.
- Bigger heaters reach temperature faster and recover quicker after cold spells, but cost more.
- Heat pump pool heaters are rated differently (by COP) and perform best in warm air.
- 8.33 BTU per gallon per degree F is the fundamental water-heating constant.
Pool Heater Sizing Calculator: frequently asked questions
What size pool heater do I need?
It depends on pool volume, the temperature rise you want, and how fast. To raise 20,000 gallons by 10 degrees in 24 hours needs about 69,000 BTU per hour of output, or roughly 85,000 BTU per hour input at 82 percent efficiency, before allowing for surface heat loss.
Does this include heat loss from the pool surface?
No. This estimate sizes the heater to raise the water temperature in a set time. Uncovered pools also lose heat continuously to evaporation and wind, so add capacity or use a cover. A professional load calculation accounts for surface loss.
How does a pool cover affect heater sizing?
A cover greatly reduces evaporative and convective heat loss, which is the largest ongoing heat drain on a pool. Using a cover lets a smaller heater maintain temperature and reach the setpoint faster.
What is the 8.33 figure?
One US gallon of water weighs about 8.33 pounds, and raising one pound of water one degree Fahrenheit takes 1 BTU. So heating a gallon by one degree needs about 8.33 BTU. That constant drives the energy calculation.
Sources and methodology
- U.S. Department of Energy: Swimming Pool Heating.
- The energy balance (8.33 BTU per gallon per degree F) uses a fixed physical constant; efficiency and times are user inputs.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.