Pool Heater Size Calculator
Enter your pool size and the temperature rise you want to get the heater BTU your pool needs. Sizing the heater right means warm water faster, lower running costs, and a heater that holds your target all season.
Round pool? Use the diameter for both length and width, or multiply diameter times diameter times 0.785 to get your surface area and enter that as length with width 1.
The difference between the water temperature you want and the average air temperature when you swim. Want 85°F water with 65°F average air? That is a 20° rise.
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recommended BTU
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surface area (sq ft)
Best value tip: a solar or insulating pool cover traps heat overnight and cuts how long any heater runs. It will not change the size you buy, but it lowers your bill every month. A heat pump costs far less to run than gas if you can heat slowly in warm air, and solar is cheapest of all when you have sunshine.
Heating gear for a sq ft pool
Auto-matched to your result. Sizes are starting points; confirm against your gas line or electrical capacity.
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How to Size a Pool Heater
Sizing a pool heater comes down to two things: how much water surface you are heating and how many degrees you want to raise the temperature. The standard surface-area method for a gas heater multiplies the pool surface area in square feet by the temperature rise in degrees Fahrenheit, then by 12. Surface area is simply length times width for a rectangular pool. A 32 by 16 foot pool is 512 square feet, and if you want to raise the water 20 degrees, the math is 512 times 20 times 12, which is about 123,000 BTUs. You then round up to the next common gas heater size, which in this case is a 150,000 BTU unit.
Gas pool heaters come in standard sizes, usually 100,000, 150,000, 200,000, 250,000, 300,000, and 400,000 BTU. The calculator above rounds your result up to the nearest of these, because a heater that is slightly oversized just reaches your target faster and then idles, while an undersized one may never quite catch up on a cool or windy day. When you land between two sizes, round up rather than down.
Round and Oval Pools
The formula only needs surface area, so any shape works once you know its square footage. For a round pool, multiply the diameter by itself and then by 0.785 to approximate the area of the circle. A 24 foot round pool is roughly 24 times 24 times 0.785, or about 452 square feet. For an oval pool, multiply the longest length by the widest width by 0.785. Take that square footage and treat it as your area, and the rest of the BTU math is identical to a rectangle.
Understanding Temperature Rise
Temperature rise is the gap between the water temperature you want and the average air temperature during your swim season. If you like 85 degree water and your average air temperature is 65 degrees, your rise is 20 degrees. Use a realistic seasonal average rather than the single coldest morning of the year, because designing around an extreme makes you buy far more heater than you need most of the time. The larger the rise you demand, the bigger the heater, which is exactly why trapping heat with a cover matters so much.
Gas, Heat Pump, or Solar
Each heating type has a clear personality. Gas heaters heat fast and work in any weather, including cold snaps and early spring, which makes them ideal when you want warm water on demand or you only fire the pool up occasionally. The tradeoff is that gas costs the most to run. Heat pumps are far cheaper to operate and shine when you keep a pool warm all season, but they heat slowly and need reasonably warm air, generally above 50 degrees, to pull heat from. Solar systems are the least expensive to run once they are installed, yet they depend on sunshine and a large panel area, so they work best as a season extender rather than a guaranteed on-demand source.
The Cover Is the Best Upgrade
Whatever heater you choose, a pool cover is the highest-value addition you can make. Most heat escapes from the water surface through evaporation, so a solar or insulating cover dramatically cuts heat loss and holds warmth overnight. A cover does not change the heater size you should buy, but it slashes how long that heater has to run to hold your target temperature. That shorter runtime shows up directly on your gas or electric bill, month after month, which is why so many owners pair even a large heater with a simple solar cover.
Heat-Up Time and Volume
Heater size sets how fast you reach your target. To estimate heat-up time, take your pool gallons times 8.34, times the degrees of rise you want, divided by the heater BTU output, which gives a rough number of hours. A big heater on a small pool can warm up in a few hours, while a modest heat pump on a large pool may take a full day or more. Knowing your gallons makes both heating estimates and chemical dosing easier, so it is worth calculating your volume first.
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Frequently Asked Questions
How many BTUs do I need to heat my pool?
A common rule for gas pool heaters is to multiply the pool surface area in square feet by the temperature rise you want, then by 12. Surface area is length times width. So a 32 by 16 foot pool that is 512 square feet, raising the water 20 degrees, needs about 512 times 20 times 12, or roughly 123,000 BTUs, which rounds up to a 150,000 BTU heater. Sizing up is almost always better than sizing down, since a slightly larger heater simply finishes faster and cycles less.
What is temperature rise and how do I figure mine out?
Temperature rise is the difference between the water temperature you want and the average air temperature when you swim. If you want 85 degree water and your average air is 65 degrees, your temperature rise is 20 degrees. Use the typical air temperature for your swim season, not the coldest possible day. The bigger the rise you ask for, the more heater you need, which is exactly why a pool cover that traps heat overnight pays for itself so quickly.
How do I size a heater for a round or oval pool?
The formula uses surface area, so you just need the surface area of your shape. For a round pool, multiply the diameter by itself and then by 0.785 to approximate the circle area. A 24 foot round pool is about 24 times 24 times 0.785, or roughly 452 square feet. For an oval, multiply the long length by the width by 0.785. Plug that square footage in as your length times width and the BTU math is the same.
Should I get a gas heater, a heat pump, or solar?
Gas heaters heat fast and work in any weather, so they suit pools you heat on demand or use in cold months, but they cost the most to run. Heat pumps cost far less to operate and are great for keeping a pool warm all season, but they heat slowly and need warm air around 50 degrees or higher to work well. Solar is the cheapest to run once installed but depends on sunshine and a large panel area. Many owners pair a heater with a solar cover to cut runtime.
Is a bigger pool heater always better?
Bigger is generally safer than smaller because an oversized heater simply reaches your target temperature faster and then idles, while an undersized one may never catch up on a cool, windy day. The main downside of going large is a higher purchase price and gas line or electrical requirements. Match the heater to your gas supply and pad size, and when you are between two common sizes, round up rather than down.
Does a pool cover really reduce how much heater I need?
Yes, and it is the single best value upgrade for a heated pool. Most heat loss happens at the surface through evaporation, so a solar or insulating cover can cut heat loss dramatically and trap warmth overnight. A cover will not change the heater size you should buy, but it slashes how long that heater has to run to hold your target temperature, which directly lowers your gas or electric bill every single month.
How long will my heater take to warm the pool?
Heat-up time depends on volume, heater size, and the temperature rise you want. A rough estimate is to take your pool gallons times 8.34, times the degrees of rise, divided by the heater BTU output, to get hours. A large heater on a small pool warms up in a few hours, while a modest heat pump on a big pool can take a day or more. Knowing your gallons helps, so calculate your volume first and keep a cover on while heating.