Measure the length and width of each window. Use a tape measure and measure in inches. Only measure the window itself. Do not include any molding or trim.
Divide the length of the window by 12 to find the linear feet in terms of length. For instance, if your window is 48 inches long, divide 48 by 12 to get 4 linear feet.
Divide the width of the window by 12 to find the linear feet in terms of width. For example, if your window is 30 inches wide, divide 30 to get 21/2 linear feet.
Add linear feet if necessary in order to estimate the amount of materials you will need. For instance, if you plan to install trim around the entire perimeter of a window that is 48 inches long by 30 inches wide, you would add the linear foot equivalents of 4 and 21/2 to get a total of 61/2 linear feet.