Remember that windows that are custom built can’t be returned, so measure carefully. To measure the width, raise the lower sash and measure the jamb-to-jamb width in three places: near the top of the window, at the middle and close to the bottom. To measure the height, measure from the head jamb to the sloped sill to just past where it meets the inside window stool. Don’t assume all windows are the same size. Draw a sketch of your house and measure each window separately.