Decide how warm you want your pool water to be. Most people prefer theirs in the high 70s or low 80s, but your individual taste may vary.
Determine the average temperature during the coldest month you wish to use the pool. You can usually do a quick estimation based on your own experiences, but if you want more exact formal numbers, check the National Weather Service for average temperatures in your area.
Subtract the average air temperature you determined in Step 2 from your desired water temperature. The result is the number of degrees Fahrenheit your heater will need to warm the pool to make it comfortable.
Calculate the surface area of your pool. If you have a rectangular pool, this is easy: Simply multiply the pool's length by its width in feet. An online pool surface area calculator will allow you to estimate the size of other pool shapes.
Multiply the surface area from Step 4 by the required elevation in pool temperature from Step 3. Then multiply the results by 12. That figure is the number of British thermal units per hour your heater will need to produce in order to adequately heat your pool.