The time will always come that your garage door will need replacing. You may find that it is already worn and broken, or you might just want to replace it with a new model. Nevertheless, determining the garage door size is one of your top priorities for you to be able to buy the perfect fit!
Using a tape measure, measure the inside length of the door. Start from one side of the door-frame, to the other side. Make sure you measure the widest area.
Once you are done repeat the first step, but instead of measuring the length this time measure the height.
Measure both sides of the garage door-frame. Start measuring from the left edge of the frame to the wall. Do the same on the other side.
Measure the space between the upper garage door-frame to the ceiling, this is known as the headroom clearance. You might need some assistance with this, or a ladder to reach the ceiling.