Rough opening measurement example:
Let’s say you have an opening in your wall that measures exactly 24” x 36”. If that’s the case, then you’re done. That’s your rough opening measurement and you don’t need to adjust your numbers.
When you want to place on order, and you want to know how big to make your rough opening? If you placed the order in a window that’s 23” x 35” and you want the measurement of the opening, you’ll simply add about 3/4″ (19mm) – 1″ (25mm) to the measurements to get 24” x 36”. Remember, your window measurement should always be smaller than your rough opening measurement to ensure it fits!