How the calculation works
Water takes 1 BTU to raise one pound by one degree Fahrenheit, and a US gallon weighs 8.34 lbs. So total BTU = gallons × 8.34 × temperature rise, and the heater size is that divided by the hours you are willing to wait.
Then comes the part that catches people out: a pool is losing heat the whole time it is being heated, mostly through evaporation from the surface. An uncovered pool needs roughly a third more heater than the raw arithmetic suggests. A cover cuts that loss dramatically, which is why it is the cheapest heating upgrade there is.
Worked example
A 20,000 gallon pool going from 68°F to 82°F is a 14 degree rise. That is 20,000 × 8.34 × 14 = about 2.33 million BTU. Over 24 hours, and with a 35% allowance for an uncovered pool, the heater needs to deliver around 131,000 BTU per hour — so a 150,000 BTU unit, the next common size up.
Assumptions
- Water weighs 8.34 lbs per US gallon and takes 1 BTU per pound per degree Fahrenheit.
- A 35% allowance for heat loss on an uncovered pool, 5% with a cover.
- The BTU figure is output, not input — check which one a heater's label quotes.
Limitations
- Heat loss varies enormously with air temperature, wind and humidity. A windy night can cost more than a cold one.
- This sizes the heater for a heat-up. Holding a temperature through the season is a different and usually smaller load.
- Heat pumps lose output as the air gets colder, so a heat pump sized for summer will disappoint in spring.
Questions people ask
What size heater do I need for a 20,000 gallon pool?
For a 24-hour heat-up of around 14 degrees, roughly 150,000 BTU. Most residential pools of that size are fitted with 250,000 to 400,000 BTU gas heaters so they can warm up faster.
Is a bigger heater wasteful?
No. Heating a pool to a given temperature takes the same total energy whatever the heater size — a bigger unit simply gets there sooner and then runs less. The cost difference is in the purchase, not the fuel bill.
Gas heater or heat pump?
Gas heats fast and works in cold weather, which suits occasional weekend use. A heat pump is far cheaper to run but heats slowly and fades as the air cools, which suits holding a temperature all season.
Does a cover really make that much difference?
Yes. Most of a pool's heat leaves by evaporation from the surface, and a cover stops it. It routinely halves heating costs, and no equipment upgrade comes close to that for the money.