When ordering a new exterior door, you must measure the jamb size. If the size is incorrect, the door may not fit into the opening or not open completely. It is important to purchase the right size door jamb along with the door to fit your particular application. Older homes often require a special order door to accommodate the wall thickness. Ordering the correct door jamb along with your exterior door will save you time, money, and create a more professional look.
Open the door and measure the width from the hinges to the opposite side. Add 1-1/2 inches to allow for the thickness of the jamb. Write this number down, which is the overall width of the jamb.
Measure the distance from the sub-floor straight up to the inside of the jamb. Add 3/4 of an inch for the thickness of the jamb. The sub-floor is the wood beneath your carpet, wood flooring, tile or other flooring. The door threshold will be seated in caulking on the sub-floor during installation.
With the door open and your back to the hinges, measure the distance between the interior door casing and the exterior trim. This is the jamb width and the total thickness of your wall. Add 1/16 inch to this measurement to allow for irregularities in the wall.