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From what to what do you measure when knowing what size pre-hung exterior door will fit?

For what I think should be the standard 36"x80", the contractor through the orange store came back with entry unit size to order = 37.5" x 81.5".

So with the guy at the store saying i gotta go by what they say will fit there's no door size that is specifically 37-1/2 x 81-1/2.

the exterior of the house is sided, it's a one level ranch. looking from the outside the door is a left hand in swing. On the exterior to the left of door the wall juts out 90° so the rough opening can't really be opened up on that side or shouldn't. But on the right side there's plenty of room to work with, as well as about a foot above the door to roof soffit which is flashing not siding.

  • Is there a door already installed in the space? If so all you need to measure is the size of the door itself and by the same size door. If there is no door installed then you just measure the rough opening which is stud to stud. If you know the size of the rough opening the guy at the orange Store should be able to tell you what size door to buy to fit that rough opening. Seems pretty straightforward unless there’s something we don’t know. – Alaska Man Sep 22 at 22:56
  • for width do you measure inside to inside of jam? – ron Sep 23 at 0:11
  • I have the interior trim removed. Or does a new pre hung door width reference the overall width including the pre-hung frame? With interior trim removed do I instead measure the overall door frame width? How does height work considering the sill at the bottom? It apparently matters to 1/2" accuracy since that what's been written. I want to know how they got that exact measurement, and why a new door within 1" width or height cannot be made to work... given the rough opening is larger by ? inches and shims are used – ron Sep 23 at 0:15
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    If you have a door already installed you just measure the door itself. JUST THE DOOR, not the jamb. Open the door and and measure the door itself. Doors come in standard sizes I.E. 36", or 34" or 32" and so on. If your door is a 36 inch door then you by a 36 inch pre-hung door and it should fit the opening you have because that is the size of door your are replacing. It sounds like you may need someone with some knowledge to help you install it so you can ask them to help you buy it. youtube for how to hang a pre-hung door so understand how it fits in the rough opening with shims. – Alaska Man Sep 24 at 2:30
  • I would mostly agree but have found a builder that also owned a modular house factory and there door sizes were slightly off or smaller than standard so it may be an unusual size. But standard sizes are usually good. – Ed Beal Sep 24 at 15:18

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