how to size a pool heater

Sizing Your Pool Heater

When I first started working in the pool industry, one of the areas I always tried to avoid was helping customers size their pool heaters. I’m not sure what made me uncomfortable about sizing heaters but I avoided it like the plague. One thing I did know was that when it came down to heaters and heat pumps, the bigger the better. Although true, I learned that some pool owners actually prefer efficiency over size. Eventually, I realized that my “bigger is better” line wasn’t always the best advice for every pool set up and sooner, rather than later, I’d have to learn how to size a heater for a pool owner.

So, exactly how do you size a heater for your pool? Let’s do this together. 

Sizing Your Pool Heater

There are a lot of decisions pool owners make during the heater buying process. Besides the norm like choosing a manufacturer and if you’re using natural gas or propane, pool owners also must know how big or how small their heater needs to be. From experience, this is the area where most pool owners have tons of questions. And I don’t blame them.

Back to my original, “bigger is better” statement. It is true that larger heaters heat your pool faster than smaller ones. A 400k BTU heater will roughly heat your pool twice as fast as a 200k BTU heater, but it also burns gas twice as fast.  Heating up a pool versus a pool and spa combination requires less BTU power. Usually, if you have a pool and spa combination, we recommend purchasing the largest BTU size available because you’re not just heating up a single body of water. But, even if you want to perform the calculations for sizing your heater, you can do that as well.



Step 1: Calculate your Surface Area (Pool Length x Pool Width)

When you are determining which size pool heater to buy, the first step is calculating your pool’s surface area. A lot of pool owners assume that you use your pool’s gallon size to determine your heater size. In fact, there are some websites that DO use the number of gallons. I mean, the gallon size is important, just in a roundabout way. Let’s think about it this way. 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.

To calculate the surface area of your pool, multiply the length times the width.

For example, if your pool is 15 x 30, then your surface area is 450 square feet.


Step 2: Divide your Surface Area by 3 (Surface area/ 3)

Once you determine your pool’s surface area, divide it by three. This answer is the minimal BTU size recommended for that particular surface area.  Continuing the example above with the 15 x 30 pool, after dividing by 3, you get 150. Therefore, the minimum size heater that is recommended for a 15 x 30 pool is 150,000 BTUs.

Keep in mind, this is simply the recommended minimum size. Meaning, I wouldn’t go lower but I might need to go a little higher. 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 over night. Without a solar cover, there is no way to make up for the loss. That’s when having a higher BTU size comes in handy.

Step 3: Consider Your Variables

Now that you have the absolute smallest BTU size recommended for you pool, next, you have to determine if you need to go larger. And if so, how much larger? Before making a decision, consider all the variables that may affect the efficiency of your heater such as owning a solar cover, wind speed, and how often you swim.

For a 15 x 30 pool without a solar cover, I might recommend the 250 k or 300 k BTU model, depending on the customer’s preferences. That way it accounts for heat loss, evaporation, and unexpected windy conditions. It also accounts for pool owners who prefer to swim at night.

By adding a solar cover, you’re changing the game. Using a solar blanket in conjunction with your heater is the best tag team in terms of providing the best heat in the shortest amount of time. With a solar blanket, you might not need a 300 K BTU heater.  You might settle on the 200K or 250k heater. Sometimes, it’s cheaper to simply invest in a solar blanket than paying for a larger heater.


Raypak Residential Gas Heater Sizing

The really cool thing about technology is that it makes all of our lives easier. Raypak has a handy-dandy gas heater calculator that configures your city and state with your desired water temperature, average air temperature, and desired temperature rise. This online calculator allows pool owners to input really specific information about their pool and location and configure what size heater is best, not only for their pool but taking your physical location into consideration as well.



The calculator also has the ability to calculate the cost of natural gas and propane. This tool allows customers to compare energy costs and discover which unit is truly right for them. It goes much further than I could ever tell you, that’s for sure.

You can use the Raypak Residential Gas Heater Sizing Calculator here.

Check out our one stop- spot for the heater-related content we’ve already covered.

15 thoughts on “Sizing Your Pool Heater

  1. I had a Heat Pump for my 24′ round. The guys at the pool store said it was the right size. Nope!
    1. My pool mostly in the shade due to the tree cover.
    2. I’m on a cliff with a clear path to the beach (Revere Beach in MA).
    3. Always windy.

    Existing heater cost about 3k. The next size up cost 7k. Did the solar cover, to much stuff form the trees would fall including animals.

  2. The key part of the article is about gas consumption. A smaller heater will burn longer to get the desired temperature rise, a larger heater will get that the pool to temperature faster. Roughly the same amount of gas will be consumed either way, so don’t assume that the larger one will cost more to operate.
    Spa combos almost universally have a 400K btu heater.
    If considering adding a heater, do not forget to consider the price of running the gas line to it. That expense has been a deal killer on many heater installations.

  3. I am in southeast GA. I recently put a (roughly 14k to 15k gal) pool in. 15×30, 3.5′ at shallow, 7′ at deep end. Rough estimates on day light temperatures are 60 degrees. I am looking at a heat pump due to this, but… according to BTU calculator, Hayward does not offer a large enough heat pump… HELP!!

  4. After reading the information maybe I missed something but I didn’t see information on ELECTRIC HEATERS.
    Please advise,

    1. Electric pool heaters are not very common because of their high unit and operational cost. Even the smaller electric heater can pull 100-125 amps on single phase circuits. But most of the electric pool heaters require a three-phase power source, another rarity in residential installations.

      But the main reason for them not being included was their rarity. But Coates has a sizing calculator.

  5. Hello there.
    How do I calculate the BTU needed for a pool 640 SQF pool that has solar blanket and Solar heating panels, and its in Los angels, California, full of sunshine days.
    I need to boost the heat only to extend the swimming season and rise Temp to 85′ from April, May, June and November .
    TNX for your help!

  6. What factors should I consider if I have an indoor pool? the poolhouse is heated so I won’t get the quick variations in ambient temp to cool my pool? My pool is 15k gallons unsure of the dimensions…


  7. I have a typical 15 x 30 pool & spam combo. Had a 400k BTU LP heater so the gas install is in place. Live in central Florida, no screen enclosure. Pool temp is fine for 8+ months of the year. Never used the heater for pool, too expensive.

    I just want a replacement LP heater, electronic ignition, decent quality, for the occasional spa use only. What would you recommend?

  8. I recently purchased a 350000 BTU Hayward universal heater for my 18 x 36 inground pool. upon looking at my gas meter it only has 3/4 inch piping and the chart says that I should have at least one and a quarter inch all the way to the heater. Do I need to get a new gas meter or can I just go up to a one and a quarter inch all the way to the heater and then reduce it back to 3/4 in at the heater inlet?

  9. I just purchased 400,000 btu hayward heater. I have run
    90 feet of 1 inch natural gas pipe to heater and reduced it at last 4 feet to 3/4 inch. I may be undersized on pipe. How will that affect heater performance?

    1. You’d be starving the heater of gas, so obviously it is not good for performance. Think about what your car do if you cut the gas supply to a fraction of what it needed?

      Hayward’s LowNox heaters require a 1-1/4″ gas line for a 50-100 foot run.

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