Pool Heater Sizing Calculator
Heating a pool comes down to energy: one BTU raises one pound of water by one degree Fahrenheit, and pool water weighs about 8.34 pounds per gallon. The total energy for a heat-up is volume times 8.34 times the temperature rise. Divide by the hours you want it to take to get the heater output in BTU per hour. Real pools also lose heat from the surface, so this tool lets you add a loss margin. Use it to compare against heater nameplate ratings; a pool cover is the cheapest way to cut the load.
Pool heater formula
Energy (BTU) = volume (gal) * 8.34 * temperature rise (F)
Base output (BTU/hr) = energy / hours
Sized output = base output * (1 + loss% / 100)
Electric kW = sized output / 3,412.14
One BTU raises one pound of water one degree Fahrenheit; pool water is about 8.34 pounds per gallon. The loss margin accounts for surface heat loss the heater must also overcome. One kilowatt-hour is 3,412.14 BTU.
Pool heating context
- One BTU raises one pound of water by one degree Fahrenheit; a gallon is about 8.34 pounds.
- Heater size is energy per hour, so heat-up time directly sets the required BTU per hour.
- Evaporation is the largest pool heat loss, per the U.S. Department of Energy.
- A pool cover is the single most effective way to cut evaporative heat loss.
- Add a loss margin for surface losses before comparing to heater nameplate ratings.
Pool heater sizing: frequently asked questions
What size pool heater do I need?
Start from the energy to heat the water: one BTU raises one pound of water by one degree Fahrenheit. Pool water weighs about 8.34 pounds per gallon, so the total energy is volume times 8.34 times the temperature rise. Divide by the hours you want the heat-up to take to get the required heater output in BTU per hour, before adding surface heat losses.
Why does heat-up time matter?
The same temperature rise can be reached with a small heater over a long time or a large heater quickly, because heater size is energy per hour. Dividing the total energy by your target heat-up time gives the BTU per hour the heater must deliver, which is what you compare against heater nameplate ratings.
Does this include heat loss from the surface?
The core calculation is the energy to warm the water mass. Real pools also lose heat from the surface by evaporation, radiation, and convection, which a heater must overcome to hold temperature. The U.S. Department of Energy notes evaporation is the largest loss, and a pool cover greatly reduces it. Add a loss margin to the result for sizing.
What is a BTU?
A British thermal unit is the energy needed to raise one pound of water by one degree Fahrenheit. Pool and gas heaters are rated in BTU per hour of output. This calculator works in BTUs because the relationship between water mass, temperature, and energy is exact in these units.
How can I cut pool heating cost?
The Department of Energy recommends a pool cover as the single most effective step, since it slashes evaporative heat loss. Lowering the set temperature, using a timer, and sheltering the pool from wind also reduce the energy a heater must supply, which can let you choose a smaller unit.
Official sources
- U.S. Department of Energy: Energy Saver Swimming Pool Heating.
- U.S. National Institute of Standards and Technology: NIST Unit Conversion (energy units).
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.