Decide the average water temperature you want for your pool.
Find out the average air temperature for the coldest month of the pool season in your local area. Your local extension office will be able to help you locate that information.
Calculate the temperature rise that the pool heater will need to achieve by subtracting the average air temperature from step 2 from your desired water temperature from step 1. For example, if the average air temperature in your coldest swimming month is 50 degrees and your desired water temperature is 75 degrees, the temperature rise is 25 degrees.
Measure the width and length of the pool. Multiply the two numbers together to calculate the surface area of the pool. For example, if your pool is 40 feet long and 18 feet wide, the surface area is 720 square feet.
Determine the BTU per hour required to maintain the desired temperature by multiplying the pool area by the temperature rise by a factor of 12. Continuing the example, multiply 25 times 720 times 12 to get 216,000 BTU.