Mike Sloggatt: The first step when ordering a replacement window is to get really good measurements to make sure that the window we order is going to fit just right into the opening.
First, we want to measure from where the stop contacts the jamb, from one side to the other, at the top and in the center of the window. Then we’ll open the window to get the same measurement across the bottom. Compare those three measurements and you will likely go with the smallest one, in case there’s a bow in the existing window frame.
Next, we want to measure the height from just above the stop at the top of the window to just inside the bottom edge of the stool. We’ll take that measurement on both sides of the window, and again use the smallest measurement when ordering our window.
For some manufacturers, sill angle is is an important factor, so we’ll measure that as well. With a level sitting on the stool, measure down to the sill right at the back of the stool and then again four inches away from the stool. Then you can use a sill-angle chart to determine the rise height that you will need for your replacement window.
Another thing I like to check is whether or not the window is square. A quick comparison of the corner-to-corner diagonal measurements will work to figure that out. You can also get an idea of how square a window is by seeing if the top and bottom sash line up nicely where they meet. If it looks like the window frame has shifted significantly, you’re going to need to do some extra measuring to make sure you order a window that is going to fit properly.