Measure the width of the opening Measure the distance between the two sides of the opening. You want to make sure that you have enough room to get your truck or car inside without hitting any obstacles. If you don’t have a tape measure, use a piece of string and tie it off at both ends. Measure the height of the opening This should be measured from the floor to the top of the door. Make sure that you take measurements from the center of the door. Calculate the square footage Multiply the width times the height to determine how much space you need. Determine if you need a larger or smaller door If you have a lot of stuff stored in your garage, you may want to consider getting a bigger door. However, if you only store a few things, you might not need a big door.