Measuring for your windows
Taking accurate measurement of your window openings is the vital first step in successfully buying replacement windows. You will take three measurements each of the width (at the top, middle and bottom) and of the height (at the left, center and right) using the smallest results as your replacement window dimensions.
The width is given first when reading replacement window dimensions. Measure the width between the inner jambs as shown in the image. Be sure to measure the existing window's frame at its widest points, rather than the narrower distance between inside trim pieces.
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 (head) at the top of the frame.