1. Measure the Width
From the inside of your home, use a tape measure, and measure the distance from the inner edge of one side jamb to the other. The side jamb is the vertical piece of trim on each side of the window. If your window is finished with drywall instead of jambs, simply measure the distance between the drywall on each side.
2. Measure the Height
Measure the distance between the sill of the window and the head jamb. The sill is the lowest horizontal piece of trim. If your window is finished with drywall at the top instead of a head jamb, simply measure up to the drywall. Round up to the nearest inch and record your measurement.