To measure a garage door for replacement, start by measuring the height and width of the door to fit the opening. After that, measure the sides of the walls next to the door as well as the depth of the garage.
Begin measuring with the width of the finished garage door opening at the widest point. As you measure the opening, make note of any irregularities to the opening that must be cut down or finished with the addition of a jamb to make the width as consistent in size as possible. Next, measure the height of the opening, again using the highest point. Look for irregularities in floor level that can be fixed before installing the new door.
Measure the sides of the door opening to make sure that there's enough space available for installing the garage door tracks. Usually, tracks require up to 3.75 inches of space on each side of the door for clearance. Measure the headroom above the opening as well, to ensure that there is enough space after door installation to allow the door to roll up inside the garage.
As a final measurement, measure the depth of the garage space available for the door. The depth measurement should cover the entire garage depth or until the first obstruction that the door may meet when being pulled into the garage while it is opening. The space available will determine the type of garage door that can be installed.