I have listed below the formula that we use to size pool heaters for our customers. This formula is used for an outdoor swimming pool. 1. Determine your desired water temperature. (75 degrees, 90 degrees, ect.) 2. Determine the average temp for the coldest month that you will be using the pool. Try www.weather.com. They are the best for breaking down the daily average temps by month. 3. Now subtract the average temp from the coldest month from the desired temperature of the water. (Subtract answer #2 from answer #1). This will give you the "temperature rise" needed. 4. You will need to calculate the swimming pool's surface area. (Length x Width) 5. Here is the tricky part: You will use the following formula to calculate the output of the heater. Pool surface Area x Temperature Rise x 12 (multiply answer to #4 by answer to #3.) This formula takes into consideration how much the temperature will rise per hour , what the average wind speed is at the pools surface. I would recommend calculating the formula yourself and then contacting a trained pool professional for accuracy. This formula is basic, but many other factors may be involved in your swimming pool that should be considered. I have had great service from Hayward and Jandy (Teledyne Laars) products. You can also call either manufacturer and they can recommend a pool professional in your area. Good Luck to you!!!