Find out the temperature of the air outside. You can find this information on the Internet, on the local news or by reading an outdoor thermometer if you happen to have one handy. This will be measured in degrees Fahrenheit.
Use a specialized pool thermometer to record the temperature of the surface of your pool water. Do not record any deeper than the surface, as heat loss occurs at the surface of the water and not below. This will be measured in degrees Fahrenheit.
Subtract the temperature of the surface of your pool water from the temperature of the air around your pool. This will give you your temperature difference in degrees Fahrenheit.
Find your pool's surface area. Multiply the length and the width (measured in feet) for a square or rectangle pool. This will give you the area in square feet. For a circular pool, measure across your pool to find the diameter. Divide this number by 2 to find the radius. Multiply the radius by itself and then by pi (3.14) to find the area of your pool, measured in square feet.
Multiply the surface area of your pool by the temperature difference you recorded in "Step 3" by the number 5. This will give you an estimate of the heat loss for your swimming pool, measured in BTUs per hour. For example a 20-by-30-foot pool heated at 75 F with an air temperature of 65 F will lose 30,000 BTUs per hour: hsurface = (5 Btu/hr ft2 oF) ((75 F) - (65 F)) (30 ft) (20 ft)