Keep in mind that these measurements are for initial pricing purposes only and need not be exact. Measuring your current window openings is the first step to buy your windows. With a tape measure, you will take three measurements each of the width and of the height, as described below, and use the smallest of the results for width and height as your replacement window dimensions.

Width

The width is given first when reading replacement window dimensions. Measure the width between the inner jambs as shown on the diagram, taking three measurements: across the top, middle and bottom. Be sure to measure the existing window’s frame at its widest points, rather than the narrower distance between inside trim pieces.
Enter replacement window width here (narrowest of the three, in inches)

Height

For the height, you will measure from the point where the inside sill meets the outside sill stool, up to where the header meets the inside stop at the top of the frame. Take three measurements: near the left jamb, in the middle and near the right jamb. See diagram.