How to Calculate Heat Loss Through Windows

Updated April 17, 2017

To determine a building’s energy efficiency, you can calculate heat losses from the structure. In a home or building, heat losses during winter can occur in various ways that include through walls, doors, windows, ceilings and floors. Because air leakage or infiltration through windows can cause heat losses that may increase your heating bill during winter, these calculations can be important factors to consider for your home’s energy efficiency. To calculate heat loss through windows, you can use a simple formula.

Calculate heat loss through windows by using the following formula:

HL = WA x U x (Del) T

In the formula, HL = heat loss, WA = the window area in meters squared, U= overall coefficient of heat transmission given in Watts/meter^2 deg Celsius (W/m^2 deg C) and (Del) T = the difference between the inside and outside air temperatures in degrees Celsius.

Utilise the formula in Step 1 to determine the heat loss through two windows that are 1.2 meters wide by 1.5 meters high and have a U-value of 2.5 W/m^2 deg C. The outside temperature is -17.8 degrees C Celsius and the inside temperature is 18 degrees Celsius. Note that you can find the overall transmission coefficient value (U) for different materials from tables found readily online.

Multiply the width and height of one window to find the area as WA = 1.2m x 1.5m= 1.80m^2

Subtract the outside temperature from the inside temperature to get the temperature difference (Del) T as 18 degrees C – 0 degrees C= 18 degrees C.

Calculate the heat loss through one window by using the equation HL = WA x U x (Del) T. Substitute into the formula the values for WA = 1.80m^2 , U = 2.5 W/m ^2 deg C and (Del) T = 18 degrees C, you find that heat loss (HL) =1.80m^2 x 2.5 W/m^2 deg C x 18 degrees C = 81 Watts

Multiply the heat loss (HL) found in Step 5 by two to get the total heat loss from both windows. The total heat loss is 2 x 81 Watts = 162 Watts.


The U-value, also called the thermal transmittance coefficient, is a measure of the heat flow through different materials. The U-value of 2.5 W/m^2 deg C, used in this example, is for a double-glazed wood frame window. You can use this same formula to calculate heat losses through walls and other surfaces.

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