Ask questions and get answers from experienced industry professionals
It can vary widely based on what kind of window you have now (aluminum, wood, vinyl, etcl) as well as if you have brickmould casing on the exterior etc. It can be up to 5" different. Just let the professionals who are going to order your windows take exact measurements for you so they can take all of that into consideration. I agree with Michael that you should defnitely NOT order based on those measurements. You don't want to be repsonsible for purchasing wrong sized product if it doesn't fit. They can't be returned.
Are you a building professional?
Why not answer these questions like a pro?Sign up free