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 PoolBright.com 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.