To find the width, measure from the inside of the jamb on one side of the window to the inside of the jamb on the other side of the window. Measure at the bottom, middle and top of the window. Use the narrowest measurement for the width of your replacement window.

To find the height, measure from the top of the sill to the bottom of the window's head jamb. Measure at the left, middle and right of the window. Use the shortest measurement for the height of your replacement window.

Using the smallest measurements ensures that the replacement window will fit inside the existing window frame.