Complete Pool Heater Sizing Guide

The most important thing about purchasing a pool heater is determining the correct size for your above ground pool this will save you money as a pool heater with the correct amount of BTUs will limit the overall run time and reduce your electric bill throughout the season. More importantly, throughout the colder months when you would like to use your heater the most, you will shorten the overall heat time of your pool and in turn also limit the amounts of electricity required to run the unit.

From unmatched energy efficiency to the ability to heat your pool with, or without the sun — the pool heater provides countless benefits to your pool.

It is important to get the right size so I’ve put together this guide for you to calculate exactly what you need.

There are only a few simple things to consider beforehand. And in this article, you’re going to have everything you need to know to size a pool heat pump for your pool.

So, what to consider?

The size of your pool.

One the most important things to keep in mind when sizing a pool heater is the size of your pool. And this is because the amount of water a heat pump needs to heat (the size of your pool) directly correlates to how powerful it needs to be.

So by that logic, the bigger your pool (volume), the stronger/bigger your heat pump should be. When you size a pool heat pump, you need the surface area of your pool, which is easy to find and explained later in this post.

In order to make an informed decision about what size of pool heater you need for a given application, you need to know the surface area of the pool which is the main factor in determining the capacity of the right heater for your pool. This can be loosely calculated by multiplying the average width (ft./m) and the average length (ft./m) of the pool. i.e.

Pool Heater Capacity Formula

Average length x average width = surface area.

Formula for the Volume of a Pool

You might also like to know the volume of the pool (although not really necessary). The volume of a straight-walled pool can be loosely calculated using this formula (in feet):

Average length x average width x average depth x 7.5 = volume in gallons

Formula for Freeform Pools

If you’re working with a freeform pool (any pool without all straight walls), here’s a slightly more accurate formula:

Average length x average width x average depth x 7 =  volume in gallons

A lot of pool owners assume that you use your pool’s gallon size to determine your heater size. If you have a pool with a large surface area, the heat has more space to escape and subsequently, more water to heat. As a result, the heater needs to be large enough to compensate for surface area and the pool size.

Where you live (Climate).

Another equally important factor to consider like the size of your pool, is where you live. Specifically, the climate.

If you live in a climate that’s mostly warm year round, then most of the time, your pool heat pump won’t have to work all that hard. For someone with an average sized pool, using a smaller to standard sized pool heat pump would be ideal.

On the flip side, for pool owners living up north or in more diverse climates, it’s a little bit different. If your pool has to brave all four seasons, then it’s important to know how cold it can get during the swim season.

This is because of the way pool heat pumps work. Since they harvest the natural heat in the air, they tend to perform best in warmer climates.

But a heat pump can still do its job even in cold weather. It just has less starting heat to work with, so to compensate, it works a bit harder.

How often you use your pool.

Although this don’t really factor into the actual calculations needed to size a heat pump, but if you’re someone who’s in the pool almost every other day, then heating speed may be important to you.

Well, with a stronger pool heat pump comes stronger heating ability. So even if you’re already in a fairly warm climate, it can sometimes make sense to choose a stronger unit if you’re in the pool often and have the extra room in your budget.

Of course, pool heat pumps by nature are designed to provide resilient, and reliable heating in a full range of climates. So even with a standard sized heat pump, you can’t go wrong.

All that being said, let’s now talk about what we really want to know which is knowing the right heater for any given pool size.

Calculating the right size for your pool heater

The British Thermal Unit (BTU) is the standard unit for measuring the capacity and efficiency of most heating devices. To know the minimum BTU size your heater recommended for your pool,  you’ll divide your pool's surface area by 3.

For example, sizing a 10 × 45 pool, your pool's surface area will be 450 ft. sq. After dividing by 3, you get 150. Therefore, the minimum size heater that is recommended for a 10 × 45 pool is 150,000 BTUs.

This is simply the recommended minimum size. It’ll be best going for a higher one though. Especially when it comes to gas heaters, you always want to oversize the unit. The job of a pool heater is to replenish the heat loss at the surface of your pool.

Most heat loss happens overnight. Without a solar cover (which helps in reducing heat losses at the surface of pools), there is no way to make up for the loss. That’s when having a higher BTU size comes in handy.

However, there are other ways and methods to go about sizing your pool heater, the given method above is one of the simplest and fastest way to size your pool heater.