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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.
They are definitely measured differently. The insert window you would measure inside the existint window frame, which in most cases is built out of 3/4" lumber all the way around. For a full replacement window, you can remove the interior casing and include that frame within your measurement. In regards to our answer, better late than never. If this process becomes a pain in the "sash" simply consult a professional. Make it a great day!
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