Pool Heater Calculator — Heat-Up Time & Cost
Plan pool heat-up time, the heater rating needed for a deadline, and optional gas or heat-pump cost using your measured water volume and cooldown.
Heat-up planning · US gallons · °F
About 6.0 hours to raise this pool by 10°F
A no-loss lower bound for 15,000 US gal and a 250,000 BTU/h gas heater at 84% thermal efficiency. Add your own cooldown observation to explore a second scenario.
Plan with your measurementsWater and deadline
Use the water currently in the pool. Choose when you hope to reach your target temperature.
Your heater's rated value
Gas labels commonly state fuel input; heat pumps state heating output. Compare the correct side of the heater.
Observed cooling · optional
Measure a period with the heater off. Keep cover, pump and weather conditions comparable to the period you want to plan.
This holds your observed average loss rate constant while warming. Actual loss changes as water warms and weather changes; use it as a scenario, not a guarantee.
1.25 million BTU into the water ÷ 210,000 BTU/h delivered.
To raise 10°F in 24 hours without heat loss. Rounded up to the next 1 kBTU/h; not a product recommendation.
Heat-pump output and COP can fall in cooler air. Pool losses also vary with cover, wind, evaporation and water temperature. These times and costs exclude pump electricity, standby, installation and other site loads.
Show the heat balance and units
For water, use approximately 8.34 lb per US gallon and 1 BTU per pound per °F. Required heat = water volume × 8.34 × temperature rise. Gas heat to water = rated fuel input × thermal efficiency. Heat-pump heat to water = rated heating output; COP converts that output to electric input. No-loss time = required heat ÷ heat to water per hour.
If you add a cooldown observation, estimated hourly loss = gallons × 8.34 × observed °F drop ÷ observed hours. The scenario subtracts that rate from heater output. If the loss equals or exceeds output, the chosen heater cannot raise the temperature under that repeated loss rate. Deadline sizing adds the same loss rate to the heat needed per hour. 1 therm = 100,000 BTU fuel; 1 kWh ≈ 3,412.142 BTU. Pool water's density and heat capacity vary modestly with temperature; 8.34 is a planning approximation.
Reference: Pentair engineering heat-up formula · DOE heat-pump rating conditions · Pentair performance limits. Check your own equipment's rating conditions and installation requirements.
