If you are installing a new patio door or replacing an old one, you need to know the patio door measurements. Typically, home improvement stores sell patio doors based on rough opening sizes---the distance between studs and the header and subfloor.
Determine the rough opening of the patio doors by measuring horizontally from stud to stud. If you already have a finished patio door and do not have access to the studs, measure from inside the door frame, and add 1 1/2 to 2 inches for an estimated size of the rough opening.
Measure the height of the opening by measuring from the bottom of the top frame (header) to the top of the subfloor.
Make four measurements---two horizontal measurements on the top and bottom of the opening, and two vertical measurements on each side. This will tell you whether your patio doors are square. Take the smaller of the horizontal and vertical numbers. Write down the measurements on a piece of paper.
Consult a contractor or home improvement specialist if you are unsure about taking measurements of your patio door opening.