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What Size Pool Heater Do I Need?

The size of your pool heater depends on several factors, including your pool size, your location’s climate, and the temperature change you desire. Go for a heater with a BTU (British Thermal Unit) rating that matches your pool’s volume in gallons. For example, a 5000-gallon pool will require a 400,000 BTU-rated heater to guarantee a 10-degree F increase in temperature.

Pool heaters use electricity, gas, or solar energy to heat the water in your pool. They can extend your swimming season and make your pool more enjoyable. That’s why choosing the right size is critical. Larger pools need bigger pool heaters. However, the size of your swimming pool is not the only determining factor.

Consider the temperature increase you seek and the climate of your location. For example, you won’t need a big heater in hotter US regions. I’ve been fixing home appliances for ten years and have helped many clients choose and set up pool heaters for optimal results. 

In this article, I’ll share my best tips and guides for choosing the right pool heater size for your pool. I will explain the factors that affect the size of the pool heater you need, such as the surface area of your pool, the desired temperature, the climate, and the type of heater. I will also give you tips on choosing the best pool heater for your needs and budget.

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03/07/2024 10:00 am GMT

Does Pool Heater Size Matter?

I get asked this question by a lot of clients. The short answer is Yes, the pool heater size matters. Here’s why:

The pool heater’s size determines how fast your pool gets heated. This means the heater’s size determines what water it can heat within a given period. An oversized pool heater (BTU rating exceeds what’s required for your pool’s volume) uses more energy, whether gas or electricity. This is because a lot of energy is wasted heating water quickly.

A smaller pool heater is inefficient and takes much more time to heat your pool. Even as you heat your pool, some energy is lost. Therefore, a small heater cannot heat the pool while keeping up with the energy loss.

The ideal size heater guarantees that your pool gets quickly heated to the desired temperature. It also saves you a ton of money on energy bills. Ensure the heater you’re buying is the right size for your pool.

How Do I Know What Size Pool Heater I Need?

The ideal size pool heater depends mainly on the size of your and volume. However, you must also factor in the climate of your location and your desired temperature increase.

The Pool heater’s size is calculated as a BTU rating. The BTU rating is a unit that measures the amount of heat required to raise the temperature of 1 lb of water by 1 deg Fahrenheit. So your goal is to determine the right pool heater BTU rating for your pool. Here’s how to do it:

  • Determine your pool’s volume. You’ll need your pool’s surface area and average depth for this. 
  • Measure your pool’s length and width. Multiply these (L x W) to get the surface area.
  • Measure the average depth of your pool.
  • Multiply the average depth by the surface area to get the volume of your pool in ft³. For example, for a 32×16 pool with an average depth of 6ft, the volume will be calculated as 32ft x 16ft x 6ft = 3,072 ft³.
  • Calculate the number of BTUs needed to heat the pool by 1 deg F (This is considered as the pool volume in gallons)1 cubic foot approximates 7.48052 gallons. Thus, the number of BTUs = pool volume x 7.5 gallons.

For our example above, the number of BTUs = 3,072 x 7.5 = 23,040 gallons.

Thus, pool volume = 23,040 gallons.

Calculate the estimated BTU rating needed for your pool heater

BTU = Pool volume in gallons x Temperature Rise x 8.33.

Note: The weight of 1 gallon of water = 8.33 lbs.

Suppose you own a pool with 23,040 gallons of water and want to raise the temperature from 75 deg F to 85 deg F. In that case, you should require a pool heater with a BTU rating of 23,040 x 10 x 8.33 = 1,919,232.

This formula determines the total amount of BTUs needed to raise the temperature of your pool by the desired number of degrees. It does not consider heat loss or how fast you need your pool to heat up.

Here’s how to calculate a more accurate BTU rating that considers your pool’s average monthly heat loss and the speed of heating you desire.

Account for desired temperature increase and heat-up time.

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03/07/2024 12:39 am GMT

What temperature increase do you want to get from your heater? If your pool’s current temperature is 75 deg F, and you want to get it to 85 deg F, then 10 deg F is your target temperature increase. We’ll factor that into the final formula.

Also, how quickly you’d want your pool to get heated? The smaller the heat-up time, the larger the BTU size required.

Calculate the Heat Loss

Knowing how much heat will leave your pool while your heater is at work is important. Use an online calculator to compute the heat loss specific to your pool. You’ll provide the length, width, and depth of your pool, the city you live in, and the thermal resistance of your walls. 

Use a thermal resistance calculator to determine the thermal resistance of your pool’s walls. You will need to know your pool’s thermal conductivity, which depends on its thickness. (The Average thermal conductivity for 200mm thick concrete pool walls is 1.93 W/m-k).

For our example above, I calculated the average monthly heat loss for a pool thermal resistance of 0.10363 and a temperature of 70 deg F to be 172,064.71 BTUs/h for May.

Calculate your ideal BTU size:

Here’s the final formula for heat-up time and loss: 

BTU size = ((Pool Volume in gallons x Temperature Rise x 8.33) ÷  Heat-up Time) + Heat Loss.

Finally, for our example, we’re assuming a heat-up time of 2 hours.

BTU size = ( (23,040 gallons x 10F x 8.33) ÷ 2h) + 172,064.71 BTUs/h = 1,131,680 BTUs/h.

These calculations are specific enough and consider the unique circumstances surrounding your swimming pool. However, seeking a professional to do more accurate calculations for better outcomes with your pool heater is always recommended.

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03/06/2024 08:20 pm GMT

Pool Heater Size Chart

The chart below compares BTU ratings with pool sizes. This is a generalized analysis; it does not consider specific factors such as pool location or pool heat loss. The chart also accounts for a desired temperature increase of 10 degrees Fahrenheit and a minimal heating time of 24 hours.

Pool Size (gallons)BTU Required (10 degrees F increase)Minimal Heating Time (hours)


  • The pool is in a mild climate.
  • The pool has a cover to minimize heat loss.
  • The pool is heated for 8 hours per day.

Pool Heater Sizing Calculator

Use this pool heater size calculator from Best Pool Supplies to quickly compute the estimated BTU rating for a heater you need to buy. The calculator accounts for specific factors such as the pool’s location, your desired temperature increase, average monthly heat loss, and especially the pool size. 

You’ll provide all the info needed, and the calculator will generate an estimated BTU for the heater you should buy.

If you’re new to this, I insist on getting a professional to help with computations.

Things To Consider When Sizing A Pool Heater

Things To Consider When Sizing A Pool Heater

As seen earlier, the pool volume (measured in gallons) is not the only parameter to consider when sizing a pool heater. Other things to consider include the following: 

Your pool’s specific heat loss

Calculate the heat loss for your pool. The higher its value, the higher the rating of the pool heater you should purchase. Your target BTU rating should reflect your pool’s heat loss (measured in BTUs/hour). The thickness of the concrete, the length, width, depth, and location of your pool are all factors that go into heat loss.

Your area’s climate

Pools in colder climates will require heaters with a higher BTU rating than pools in warmer climates.

Your Pool Cover or Blanket 

You could go for a lower-rated pool heater if your pool cover insulates the pool enough to reduce heat loss significantly.

Pool placement

Heaters better serve indoor pools with smaller BTU ratings than outdoor pools. Less heat is lost when you’re indoors.


Your pool heater’s size or BTU rating determines how fast the pool heats up. An ideal-size heater minimizes energy loss and overall energy costs. To choose the right size pool heater, first, evaluate the size of your pool, then the climate of your area, and how fast you’ll need your pool to heat up. In this guide, I’ve simplified how to assess the ideal BTU rating for a pool heater to buy.


Is it better to oversize a pool heater?

It is not always better to oversize a pool heater. Ideally, you want a heater large enough to heat your pool efficiently, to your desired temperature, and in your set timeframe. An oversized pool heater will lead to heat loss and higher energy costs.

How do I choose a pool heater?

To choose a pool heater for your needs, consider the following factors:

  • Pool size: The larger the size of your pool, the larger the size (BTU-rating) of the pool heater you should get.
  • The climate of your area: Colder climates require heaters with much higher BTU ratings because it gets harder to heat a swimming pool to desired temperatures.
  • The type of pool heater: Electric and gas pool heaters are on the market. Decide which of these options works best for you.

Check out more of our helpful swimming pool guides, where we share expert tips to help you enjoy the best possible experience with your pools.


  • Raoul Hayes

    Raoul Lobo is a seasoned expert in the realm of home appliances and environmental comfort. As a prominent author at, Raoul's passion and expertise shine through in his insightful articles and guides. With years of hands-on experience, he has become a trusted source for readers seeking advice on pools, washers, dryers, and a wide range of other appliances.

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