How to Calculate Heat Loss Per Foot

Measure the height and width of each wall and ceiling of the room to obtain the total surface area. For example, if the interior walls are 8 feet by 10 feet, each wall would be 80 square feet in area. The ceiling is 10 feet by 10 feet, which would make the area 100 square feet. Add four walls and the ceiling together to get total surface area of 420 (80 + 80 + 80 + 80 + 100 = 420).

Measure the inside and outside temperatures, in Fahrenheit, and take the difference between the two temperatures and multiply by the total surface area found in the previous step. For example, if the temperature outside is 33 degrees Fahrenheit, the inside temperature is 69 degrees Fahrenheit and the surface area of the room is 420 square feet, the difference between the temperatures would be 36. Then, multiply 36 by 420 to get 15,120 BTUs.

Divide the BTU value found in the previous step by the R-value of the insulated wall material. Most insulated walls in common homes are made from R-19. Consult your builder's manual to find the exact R-value. For example, if the BTU is 15,120 and the walls are insulated with R-19, the amount of BTUs lost per hour is approximately 796 BTU/hour (15,120/19 = 796).

Find the volume of your room by multiplying the room's length by the width and height -- length * width * height. This will give you the cubic footage of your room. Then, take your BTU per hour rate found in the previous step and divide the total cubic feet of the room by the BTU per hour rate. For example, if the room is 1,000 cubic feet and BTU is 796 BTU per hour, the heat loss per foot would be 1.26 BTU per cubic foot in one hour (1000/796 = 1.26).