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Pool Heater Size Calculator

Use this pool heater size calculator before you shop for a heater. Enter your pool volume, surface area, and how many degrees you want to gain, and the tool returns the BTU output you need along with a realistic heat up time. Undersized heaters run all day and never catch up.

Pool heater size calculator illustration of a pool heater warming pool water with a thermometer

Run the numbers

Formula used

BTU per Hour = (Gallons x 8.34 x Temp Rise) / Hours + (Surface Area x 12)

Your result

Heater output needed
75,644 BTU/hr
Gas heater input rating
90,052 BTU/hr
Suggested heater size
100,000 BTU
Total energy for this heat up
1,668,000 BTU

Results update as you type. Round up when you buy materials.

How the pool heater size calculator works

It takes 8.34 BTU to raise one gallon of water by one degree Fahrenheit. Multiply that by your gallons and your target rise and you have the raw energy needed.

Real pools also lose heat to the air and to evaporation the whole time the heater runs, so the tool adds a surface loss allowance based on square footage.

The formula, step by step

BTU per Hour = (Gallons x 8.34 x Temp Rise) / Hours + (Surface Area x 12)

  • Multiply gallons by 8.34 to get pounds of water.
  • Multiply by the degrees of rise you want. That gives total BTU.
  • Divide by the hours you are willing to wait for the heat up.
  • Add roughly 12 BTU per square foot of surface each hour to cover heat loss.

Worked example

A 20,000 gallon pool with 512 square feet of surface, raising the water 10 degrees in 24 hours.

20,000 x 8.34 x 10 = 1,668,000 BTU total. Divided by 24 hours = 69,500 BTU per hour.

Add 512 x 12 = 6,144 BTU per hour of loss. A 200,000 BTU heater covers this with room to recover after a cold night.

Watch it done

This short video walks through the same job in real life, so you can match what you see on screen to what you do at the pool.

Video guide: What Size Heater Do You Need for Your Pool? by Buildahottub. Video hosted on YouTube.

Tips before you start

Gas heaters are rated by input BTU and run around 84 percent efficient, so a 400,000 BTU model delivers about 336,000 BTU to the water. Heat pumps are rated by output and work best above 50 degrees ambient.

A solar cover is the cheapest upgrade you can make. It cuts evaporation loss by most of the total, which often halves the runtime your heater needs to hold a temperature overnight.

Pool Heater Size Calculator FAQs

What size pool heater do I need?

Most residential inground pools land between 200,000 and 400,000 BTU. Run your gallons and target rise through the calculator to narrow it down.

How long does it take to heat a pool?

A properly sized heater raises water 1 to 2 degrees per hour on a small pool and around half a degree per hour on a large one.

Is a heat pump better than a gas heater?

A heat pump costs less to run but heats slowly and needs warm air. A gas heater costs more per hour and heats fast in any weather.

Does a bigger heater waste money?

No. A larger heater reaches the set temperature sooner and then shuts off, so fuel use per degree stays about the same.