Pool Heater BTU & Heat-Up Time Calculator

Find the energy needed to raise your pool temperature, how long your heater will take, and what it costs in gas.

Tip: calculate your exact volume on the pool volume calculator first, then bring that gallons number here.

How pool heating is calculated

One BTU raises one pound of water one degree Fahrenheit. Water weighs 8.34 pounds per gallon, so the energy to heat a pool is volume × 8.34 × temperature rise. Real pools lose heat to the air and ground, so we add roughly 20% for losses — more if your pool is uncovered or windy. A pool cover dramatically cuts both heat-up time and running cost.

How heater size and heat-up time are calculated

Find the water weight. Pool gallons × 8.34 = pounds of water. It takes 1 BTU to raise 1 pound of water by 1°F.
Calculate the BTU to raise the temperature. BTU required = pounds of water × desired temperature rise (°F).
Add a loss factor. Surface evaporation and conduction lose heat as you warm the pool, so multiply by a loss factor (around 1.25) for a realistic figure.
Divide by heater output for time. Heat-up time (hours) = total BTU ÷ the heater's BTU-per-hour output.

Worked example

Raising 20,000 gallons by 10°F: 20,000 × 8.34 = 166,800 lb, × 10°F = 1.67 million BTU, × 1.25 for losses ≈ 2.1 million BTU. A 400,000 BTU/hr heater takes about 5 hours.

Questions

This tool, explained

It depends on volume, the temperature rise you want, and heater output. Enter all three and the calculator gives an estimate including typical losses.

Yes — most pool heat is lost from the surface to evaporation. A cover can cut heat loss by half or more, shortening heat-up time and lowering cost.