A pool heater is sized according to how long it should take to warm up your pool. While the sizing formula is the same whether you're shopping for a natural gas model or a propane-fueled one, the key variables that shape your final choice are:
- Pool surface area
- Desired swim temperature
- Ambient temperature — what time of year will the pool be in use?
- Desired rate of heat rise over time
- Whether a solar blanket will be used
How Big Is My Pool?
The very first step when sizing your pool heater is figuring out the surface area of the pool in square feet. For a rectangular pool, you simply multiply the length by the width. As an example, we'll work with an 18-foot by 36-foot pool throughout this guide. An 18 ft × 36 ft pool has a surface area of 648 sq. ft.
Minimum BTUs
Pool heaters are rated in British Thermal Units (BTUs), the standard for measuring the energy needed to raise one pound of water by one degree Fahrenheit. Dividing your pool's surface area by three gives you the minimum BTU output your heater should deliver. In our example, that works out to a minimum of 216,000 BTUs.
216 × 1,000 = 216,000
Keep in mind, this is only the minimum BTU rating — it isn't necessarily the best choice. Now that we've set a baseline, let's look at the other factors that influence the final sizing decision.
Time to Heat
The larger the pool, the longer it'll take to warm up, but patience isn't always an option. Upgrading to a bigger heater cuts down on the time needed to bring your water up to your ideal swim temperature.
To calculate how long it will take to heat your pool:
Subtract the average air temperature from your desired water temperature to find the required temperature difference.
Take the total gallons of water in your pool and multiply by 8.34. This yields the pounds of water contained in the pool.
Divide the pounds of water by your heater's minimum BTU rating to determine the hours required to raise the water temperature by 1°F.
Now multiply that per-degree figure by the temperature difference from step 1. This tells you how many hours the heater needs to reach your target.
For instance, suppose our 18 × 36-foot pool has an average depth of 5 feet. The formula to calculate total gallons is Length × Width, multiplied by the average depth, and then multiplied one more time by 7.5.
The math breaks down as follows:
- 85 − 70 = 15 degree difference
- 18 × 36 = 648 sq. ft.
- 648 × 5 = 3,240 sq. ft.
- 3,240 × 7.5 = 24,300 gallons
- 24,300 × 8.34 = 202,662 lbs
- 202,662 ÷ 216,000 = 0.93825 hours to raise water 1°F
- 0.93825 × 15 = 14.074 hours to heat the pool
So, for our 18 × 36-foot pool, it would take roughly 14 hours to reach the target swim temperature if we ran the pool pump 8–10 hours a day. So how do we decide what size is actually ideal for our needs?
Raypak Calculator
I get it — doing these calculations by hand isn't for everyone, but there's still a way for every pool owner to learn what they need. Raypak offers a convenient online calculator where you input your info, push a button, and bam: custom recommendations on what size heater you should be buying!
Doing the Math
If you'd rather tackle it yourself, here's a two-step formula that will tell you how many BTUs you require.
Take your desired hours-to-heat value, then divide it by your temperature difference.
Divide the pounds of water in your pool by the figure from step 1. That's how many BTUs your heater should have.
In our example, we'll say the heater needs 9 hours to reach our target temperature. The numbers shake out as:
- 9 ÷ 15 = 0.6
- 202,662 ÷ 0.6 = 337,770 BTUs
That means the right heater for our pool would be right around 399,000 BTUs.
Why Get a Solar Cover?
A solar cover works wonders for locking heat inside your pool by preventing warmth from escaping overnight when the air turns cool and dark. If you're deciding between, say, a 250,000 BTU model and a 300,000 BTU model, you could go with the smaller unit paired with a solar cover to save some money.
Ready to Heat Your Pool Efficiently?
The Blue Torrent 65,000 BTU Heat Pump is engineered for pools up to 13,000 gallons — dependable, energy-efficient, and built to keep your swim season going strong.
Shop Blue Torrent CFT 65