How to Calculate What Pool Heater Size Is Needed


Pool heaters help keep the temperature comfortable whenever you want to take a dip, but it can be hard to determine what heater size is best. if your heater is too small, it won't maintain the proper temperature, but if it is too big, then you have probably spent more money than you require. You can calculate the proper heater size for your pool by making a few basic measurements and applying a simple mathematical formula.

  • Determine the ideal pool temperature for you. The exact number varies by taste, but most people consider 78 to 82 degrees Fahrenheit to be adequately comfortable.

  • Calculate the average temperature of the area surrounding the pool during the coldest month you intend to use it. If the pool is indoors, you can simply check the thermostat. If it's outdoors, consult the National Weather Service to determine average temperatures in your area.

  • Subtract the number in Step 2 from the number in Step 1. The result is the number of degrees your heater will need to heat the pool in order to make it comfortable.

  • Measure the surface area of your pool in square feet. You can do this yourself with a tape measure, or consult the calculator at if your pool is of an unusual shape. The more accurate you can be, the better, though if you only have generalized figures, you can still make a reasonable estimate of the size heater you will need.

  • Multiply the pool's surface area in feet by the figure derived in Step 3, and then multiply that figure by 11. The result is the approximate BTU/hr (British Thermal Units per hour) the heater requires in order to heat your pool properly.

Tips & Warnings

  • Experts recommend purchasing a slightly larger heater than the one you need for your pool. It will warm up the water more quickly and with less strain on its mechanics, which will help the heater last longer. Check the heater's efficiency rating when you buy it: it reflects how much actual energy it puts out. For example, if a heater produces 200,000 BTU and has a 90% efficiency rating, then for all practical purposes, it is only producing 180,000 BTU.
  • This formula is reliable, but very basic. You should also consider humidity, wind chill and night temperatures when making your estimate. If you live in a particularly dry area, if wind speeds near your house are high (more than 3 or 4 mph on average), or if temperatures drop a great deal at night (as they would if you lived in a desert environment), you should plan for a larger heater.

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