We were told to get help. We measured and DIYed both our doors and windows. We liked doing them. If you just measure the door with the trim on, you are not going to get an accurate reading. We had that happen and it turned out it was a standard 36. We had HD and Lowes each come out to measure and their measurements were crazy off.

As PP indicated, they are probably specifying the Rough Opening (R.O.), which would be 37.5 give or take on a 36 inch door.

That all said, do yourself a favor - get some help. Doors usually are not returnable and based on your questions its pretty obvious that you should not be installing it. Go to a good local lumberyard - they can help you out.

Check the rough opening

Thanks! I am so confused. The door listing says 37.5 inch door, but then specifications say it is a "common width" of 36. Huh?!?

Your are better to buy slightly smaller than bigger. You can shave down the jamb a bit if it is wood but its very difficult to do. You can also remove the jamb if your have some kind of framing to attach the door to and it comes with its own jamb. It really depends on the set up. I would remove the trim and see how the current door is put in to measure. You will get a better measurement as the trim may be covering a large gap.

When buying an exterior door, how much leeway is there on width and heights in terms of the door itself. If I buy too big, can the jamb be adjusted for size?