How to Find the Right Size Pool Heater for You

Kelly Weimert
Written by Kelly Weimert
Updated August 12, 2021
A couple discussing while sitting at the edge of their pool
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Learn what size pool heater you need for comfortable three-season swimming

Few activities are more fun than plunging into a refreshing pool during the height of summer. But if you want to extend that fun to the spring and fall seasons, then you probably want a pool heater to avoid jumping into frigid temperatures. Here, we cover everything you need to know about how to find the perfect-sized pool heater for your home.

How Are Pool Heaters Measured?

When figuring out what size pool heater you need, it helps to understand how pool heaters are measured. All pool heaters are measured using British Thermal Units (BTUs), a unit that measures energy. One BTU represents the amount of energy needed to increase the temperature of 1 pound of water by 1 degree Fahrenheit. 

Generally, if a pool heater has a high number of BTUs, then it can heat a larger pool than a heater with a lower number of BTUs.

Calculate Your Pool's Surface Area

Your first step in figuring out the right heater for your pool is to calculate your pool's surface area. The larger your pool's surface area, the easier it is for heat to escape, which means a large pool requires a large heater. 

All you need to do to calculate the surface area is multiply the length of the pool by the width. So, if you have a pool that's 15 feet wide and 20 feet long, then your pool's surface area is 300 square feet.

Determine Your Pool's Average Depth

Once you calculate your pool's surface area, you need to calculate its average depth. To find your pool's average depth, all you need to do is determine the depth of the shallowest part and the deepest part, then add those two numbers together and divide them by two. 

For example, if your pool is 3 feet deep at its shallowest and 12 feet deep at its deepest, then your pool's average volume is 7.5 cubic feet, since 3 + 12 = 15 and 15/2 = 7.5.

Calculate Your Pool's Approximate Volume in Gallons

To find the number of BTUs you need in a pool heater, you then need to calculate your pool's volume in gallons. You can find this number by first multiplying your pool's surface area by its average depth. This number will result in your pool's average cubic feet. In order to convert cubic feet to gallons, you then need to multiply the number of cubic feet by 7.48. 

For example, if your pool's surface area is 300 and its average depth is 7.5, then its volume in cubic feet is 2,250 because 300 x 7.5 = 2,250. To convert this number to gallons, simply multiply it by 7.48. In this case, 7.48 x 2,250 = 16,830, so the pool holds approximately 16,830 gallons.

Calculate the BTUs You Need in a Pool Heater

Now that you know your pool's approximate volume in gallons, you can figure out about how many BTUs you'll need in a pool heater. But first, you should consider your location's average temperature during a given season. 

As you might imagine, your pool heater will need to work harder (i.e. need more BTUs) to heat your pool during cooler weather than warmer weather, so always purchase a heater for the coldest months you'll be using the pool. 

The following calculations are based on approximately how many BTUs you'd need to heat your pool to 80 degrees Fahrenheit in 24 hours during different outdoor temperatures.

Minimum BTUs Needed for 70-degree Fahrenheit Weather

To find the number of BTUs needed to heat a pool to 80 degrees Fahrenheit on a typical 70-degree day, you should multiply your pool's number of gallons by four. 

So, a 15,000-gallon pool will need a pool heater size that can accommodate at least 60,000 BTUs to heat it during a 70-degree day because 15,000 x 4 = 60,000.

Minimum BTUs Needed for 60-degree Fahrenheit Weather

You can find the BTUs needed to heat your pool on a 60-degree day by multiplying your pool's gallons by five. 

For example, a 15,000-gallon pool will need a pool heater with at least 75,000 BTUs.

Minimum BTUs Needed for 50-degree Fahrenheit Weather

If you want to keep your pool warm during a 50-degree day, then you should multiply its gallons by six to find the minimum BTUs you'll need in a pool heater. 

For instance, a 15,000-gallon pool will need a heater with 90,000 BTUs. 

Important Pool Heater Considerations

Father swimming with his kids in a pool
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The calculations above are a great start, but there are several additional factors you'll want to consider in order to find the perfect size heater for your needs. You can also talk to a local pool heater repair professional for more help deciding on the right heater for you.

What's Your Climate Like?

If you live somewhere that's particularly cold or windy, your pool heater will have to work harder to heat your pool than it would in more temperate climates. In this case, you'll want to get a heater with more than the minimum number of recommended BTUs. 

So, if you live in a very windy area and the minimum recommended BTUs for your pool's size during an average 70-degree day is 60,000, then you probably want a heater with at least 75,000 BTUs to account for the extra wind.

What Are Your Personal Preferences?

Some people like their pool to feel like bathwater, while others prefer it to be much colder. Knowing how warm you want your pool to be during different seasons will help you determine just how much power you need in a heater. 

If you love that bathwater feeling, then opt for a heater with a high number of BTUs, which will heat your pool to your desired temperature faster than one with lower BTUs.

What Type of Pool Heater Do You Want?

There are several types of pool heaters to choose from, including gas, electric, and solar heaters. Typically, gas heaters can heat pools faster than electric and solar heaters, but each type has its pros and cons, so you should consider factors like your budget, lifestyle, and personal preferences before committing to a heater. 

You can also look for pool heater installation in your area to get help finding the right type for your household.

How Fast Do You Need Your Pool Heater to Work?

Pool heaters with a high number of BTUs can heat your pool faster than heaters with lower BTUs. If you only use your pool occasionally, then a heater with lower BTUs that you can turn on a couple of days before you plan to use it is probably fine. 

However, if you frequently use your pool, particularly during cooler seasons like spring and fall, then you might want a heater that can work a little faster, in which case you should lean toward the higher BTUs recommended for your pool's size and locale.

Do You Have an in-Ground or Above-Ground Pool?

Above-ground pools lose heat faster than in-ground pools, so you'll want to lean toward the higher end of the recommended BTU spectrum if you have an above-ground pool.