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.
It is also a good idea to measure the overall exterior dimensions of your door, especially if you have trim other than brick molding applied to your existing door frame. Exterior doors can be purchased without the trim applied if your trim must fit between brick or vinyl siding moldings. Measure the total width by hooking your tape on the exterior trim and measuring to the opposite side. Then, hook your tape on the top piece of trim and pull down to the bottom of the existing threshold. This is your overall exterior dimension.
Be careful to note that older doors may have sloping sills. New doors have flat sills. If this is the case with your door, the inside measurement is used to order the door.