How to measure replacement windows
You have a rough opening size (if you have removed the old window first) and the actual &quot;in the wall&quot; size of the replacement window, which must be smaller than the rough opening size to allow for shimming, out of square, etc. The default is to have it quite a bit smaller to allow for lots of shimming, but if you have checked the actual (.vs. planned in new construction but not yet built) opening you have, and checked that it is square and the sill is level, you can use less generous amounts of slop. i.e. I use &quot;Masonry&quot; rough opening sizes (smallest relative to the actual size) for the window openings in my structural insulated panel walls - they are not made of concrete, but they don't have the slop that studwalls are prone to.
When looking at windows you need to check what size they are claiming in the large print .vs. what the actual size (smaller, usually) is because of this, and the tendency to go for the bigger number for market purposes. There's also a matter of if you are doing the work in a way where you can use a &quot;new construction&quot; window that involves getting the window flashing under the siding, or if you need a &quot;replacement window&quot; that may be fitting into the old window's gutted frame, if the old window's gutted frame is solid, so it can use the old window's flashing and no siding disturbance is needed. Those are always smaller (glass area) than the window they replace, since you now have two frames around the glass.