The face of a garage door tells only a portion of the story; behind the door lies an entire system of components that allow the door to work. The doors extend beyond the finished trim you see from the front, a fact that must be taken into consideration when framing and preparing the area for installation. The rough opening is equal to the size of the door, but you need to figure back, head and side room before choosing and installing the new garage door.
Measure the old door if you are replacing an existing door with one of the same size, or the new door of choice if you are building a new garage. Look at standard sizes and price your options, then note the measurements you will be using.
Add 18 inches to the height of the door. This is how much you need for backroom, or the space from the top of the door back towards the garage interior.
Add 6 to 14 inches to the height of the door for headroom, or the space directly above the closed door. Refer to the headroom requirements for the specific door you are using to see if your existing space can accommodate the new door without modification.
Add 7 1/2 inches to the width of the door for side room (3 3/4 inches on each side).