How to Calculate Heat Loss Through Windows

Between 12 and 30 percent of your yearly heating bill goes to make up for heat loss through windows, estimates the University of Wisconsin Cooperative Extension. Knowing the cost of the heat that lost through the glass of each window annually can help you take steps to reduce heating bills. A savvy homeowner can calculate this loss and, coupled with proper window treatments, make a substantial change in the cost of heating the home. Proper window treatments can reduce a heating bill by up to 25 percent, the U.S. Department of Energy notes.


Measure and record the width and length of the window in inches. Calculate the square footage of the window by multiplying the width by length and dividing by 144.


Retrieve the cost per unit of heating product from your heating bill. For example, electricity cost is per kilowatt hour, or kWh, oil is per gallon, and natural gas is per 100 cubic feet. Record this figure.


Determine the heating degree days, or DD, for your area. This is the number of degrees that the average daily temperature is below 65 degrees Fahrenheit. The provider of your heating fuel, a weather office or airport can typically provide this figure. One day of 20-degree weather, for example, provides 45 DD.


Calculate the cost of heat lost per square foot of the window by multiplying your fuel cost per unit by the number of degree days. Multiply the result by 38.82 for electricity, 1.57 for oil and 2.03 for natural gas and divide the result by 10,000. The end figure is the cost of heat lost per square foot of a double-paned window. For example: Cost of electricity: 98 cents, multiplied by 5,000 DD equals 4,900, multiplied by 38.82 equals 190,218, divided by 10,000 equals $19.02 per square foot of window annually. For triple-paned windows, multiply the result by 0.65. For a single-glass window, multiply the result by 2.27.