What Size Of Pool Heater Do I Need? (And How To Calculate It)

Kenneth Wilson

It has become essential for many pool owners today to have a pool heater so that the pool can be used most part of the year. To enjoy your pool comfortably at a desired temperature, you need to invest in a good size heater for your pool. The question is what size of the pool heater do I need to buy?

The answer depends on many factors  but the foremost to consider are: the volume size of your pool, what type of pool heater you prefer, the cost of installation and maintaining it, the temperature you desire of the water in your pool and the weather in the area that you live.

On average the cost of a pool heater and it operating costs varies from $1300 - $9600 and therefore it is wise to consider all factors before making a choice for a unit that is suitable for you. This requires a bit of research and saves you lots of time and money before making the investment in. We made it easy for you to understand the how to choose the right pool heater size.

The first thing you need is to calculate how much energy output your pool heater will need to heat the pool water to the required temperature.


The energy output of a pool heater is measured in BTU (British Thermal Units).  One BTU is defined as the energy needed to increase the temperature of one pound of water by one degree Fahrenheit.

To Calculate the BTU that the pool heater needs to produce, you first need to do some math.

You need to use the following formula:

          Volume (in Gallons) X 8.3(per pound) X Ideal temperature = Total BTU required.

Let’s simplify this formula for you to understand the calculation by examining each component.

Volume (In Gallons)

For a rectangular pool, you need to measure the average length multiply by the width and its depth and then by 7.5

Let take an example, your pool length is 25ft, width is 16ft and the depth is 5ft. Therefore, the volume of your pool in gallons is:

25 X 16 X 5 X 7.5 = 15000 Gallons 

For other irregular shape pool, just use the average length multiply by average width and average depth and then by 7.

Ideal Temperature

To get the ideal temperature you need factor in the difference of the lowest temperature in your region from the temperature you need in your pool. 

In our example, if we needed the pool to be 85 degrees Fahrenheit and the lowest temperature in the region is 55 degrees Fahrenheit, the ideal temperature for calculation would be 30 (that is 85F – 55F).

Heater Size Calculator 

Using our example above, we can now calculate the BTU needed for our pool:

   Volume (in Gallons) X 8.3(per pound) X Ideal temperature = Total BTU required
15000 X 8.3 X 30 = 3,735,000 BTU

This total figure of 3,735,000BTU is the energy produced by the pool heater in 24hrs. To get the hourly BTU produced you need to divide by 24 - which is equal to 155,625 BTU.

So, for a 15000-gallon pool you need a pool heater capable of producing 155,625 BTU.

Once we know the size of the pool heater required from these calculations, then you need to decide what type of heater best suit your needs. You also need to take in consideration the installation and operations costs of the pool heater.

Types of Pool Water Heaters

There are three types of pool water heaters:

Gas Pool Heaters

These are the most powerful pool heaters producing about 100,000 to 500,000 BTU. They use gas or propane to heat the pool water very quickly to the required temperature. They are very efficient in heating, and they are ideal for pools which are only used periodically. Although they are cheaper to install, the gas pool heaters have the most expensive running costs of about $200 - $400 per month.

Electric Heat Pump 

The electric heat pump is not an electrical heater rather it works as a reverse air conditioner. The electric heat pump uses electricity to push ambient air over the evaporator coil to the compressor, which then pumps it to the heat exchanger which in turn heats the water from the pool. They are the most expensive to install but their running costs are much lower, between $100 -$200 per month. (Related: Understanding Your Pool Systems: Sanitation, Filter And More!)

The Solar Heater

The solar heater uses electricity from the solar solar panels. This type of pool heater hardly has a running cost but has a disadvantage that it depends on the climate of the region you live in -whether there is enough sunshine for the solar panels to generate electricity. Also, additional cost of the solar blanket should consider to cover the pools at night to preserve the temperature.


After taking all consideration you then can decide what size and type of the pool heater that is best for you. Calculation of the BTU helps in knowing the minimum amount of energy that the pool heater needs to produce to warm your pool to the desired temperature.

Also knowing the different types of pool heaters gives you a choice of what could be suitable for you. The gas and propane pool heaters are most powerful and very efficient, followed by electric pump which are less powerful and then the solar pool heaters. However, the operating costs of each of these types of pool heaters should be taken in consideration when making your decisions.

Kenneth Wilson
September 15, 2021

Kenneth Wilson

Retired contractor. Currently residing in Southwest Florida. Now in semi-retirement, I write and manage this blog focused on helping home owners make savvy decisions when it comes to finding contractors and getting their projects done. I also operate remodeling design service for homeowners.

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