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I would like to know how I would have to measure it if you could tell me Please.
Door sizing goes by the door dimensions. For example: 36" door means 36x80, 32" door means 32x80, etc.
"Rough opening" is the opening in the house framing that will allow the door + the jamb to fit in.
To determine your door+jamb size, remove the casing (molding) around the door from the inside and measure from stud to stud and bottom to header.
If you don't know how to do it, get a finish framer/carpenter to help you.
don't forget to check the height too. some old doors are 78" high. if your door is not 80" or 78" high you might have to have a custom door made
When rough framing, the studs and header are made 2 inches larger than the proposed door. So the rough opening size of a 36" x 80" door is 38 x 82, unless you have a special replacement door.
Since the dawn of factory-made millwork, door dimensions have been called out in feet and inches. Bear this in mind so your notes aren't read at the millwork order desk as 3'6"x8'0" and not the desired 3'0"x6'8".
With doors, Pro's have always called the shots in feet and inches. The big-box stores use inches only because their people are mentally maxxed out at that point, so some suppliers are following suit :( For a whole-unit replacement (door and jamb) I like to measure the old opening where it meets brick or siding, also checking with a framing square on top and bottom, then measuring the unit I want to buy myself to be sure it will fit or can be altered to fit easily.
One of my main rules of remodeling is to never replace an exterior jamb unless you have to as there have been variances in the trim standards over the years so what you get today may not go- even though the door size is identical. I just saw where someone who didn't understand this replaced 2 perfectly fitting units with new ones that were 1/4 too small on 3 sides and to top things off, they didn't follow the original holes which were slightly out-of-pluimb. Their customer is buying it but I wouldn't. Had they replaced only the door it would have looked and worked beautifully just like the old ones did. Glad my name ain't on those!