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What Size Pool Heater Do I Need? An Easy Guide to Sizing

Last Updated: March 13, 2020

pool heater

Would you like to extend the swimming season in your swimming pool? Maybe open it a month or two earlier and close it a month or two later? Wouldn’t it be great to have it available for more than just a few months in the summer?

Well, you can. But no one wants to swim in ice water – unless you’re a member of the Coney Island Polar Bear Club and you like torturing yourself in freezing cold water – so, to extend the swimming season, you need to heat the water. But what size what heater do you need?

That’s what we’re here for. We’ve already done all the hard work for you, so just sit back, receive our words of wisdom, and you’ll be able to make an informed decision on the heater that’s best for you.

BTU’s

The size of all swimming pool heaters is measured by their BTU’s (British Thermal Units), which is defined as how much work, or heat, is needed to heat one pound of water by one degree Fahrenheit. Obviously, the higher the BTU rating on a heater, the faster it will heat the water and the faster you’ll get to go swimming.

In order to figure out how many BTU’s you need for your swimming pool, you’ll need to calculate how many gallons of water are in your pool. That’s the foundation, the ground floor so to speak. After that, you should also figure out how much surface area your pool has – which will determine how much heat it loses through evaporation.

Your physical location plays into it as well. A swimming pool in southern Arizona won’t need as much heating as one in upper Maine. And finally, you should consider a pool cover of some kind to help retain the water’s temperature when the pool isn’t being used.

Water, Water Everywhere

“Water, water everywhere, nor any drop to drink,” says the Rime of the Ancient Mariner. But how much water is what you need to know. There are two ways to figure out how much water is in your swimming pool, the hard way and the easy way. Let’s look at the hard way first.

inside swimming pool

Image credit: Autopilot, Wikimedia

The Hard Way

If you’re interested in doing the math you can whip out your calculator and solve the following equation to find out how many gallons of water are in your swimming pool. The equation is;

Gallons of water = (D x (π r2))/231), where D = depth of the water in inches and r = radius of the pool in inches.

The inner part of the calculation – π r2 – will tell you the surface area of the pool. By the way, though, this calculation assumes a circular swimming pool. If your pool is shaped differently you’ll have to calculate the volume differently based on its shape.

The Easy Way

Most people don’t want to do all the math, so here is a swimming pool volume calculator you can use instead. You just plug the numbers into the fields and hit the “calculate” button, and bing, bang, boom—out pops your answer.

Pool Size and Heater Size

We’re almost done. Now that you know how many gallons of water are in your pool, you’re ready for the next step. Below is a helpful little table that will tell you what size heater you need based on how many gallons of water are in your pool.

Gallons of WaterSize of Heater
1000-10,000100-200 BTU
10,000-20,000200-300 BTU
20,000-40,000300-400 BTU
40,000-80,000400 BTU

If your pool holds more than 80,000 gallons of water, you’ll need to consult with a local swimming pool contractor. You might need more than one heater and/or more than one pool pump for a pool that large.

Conclusion

Running a pool heater can become costly unless you’re using a heat pump (which has its own expenses). Help your budget by investing in a pool cover. There are a lot of resources available online to help you choose the right one for your pool.


Header image credit: Bill Jacobus, Flickr