Start at the bottom inside edge of the door opening. Measure from the finished wall on one side of the opening, to the finished wall on the opposite side of the door opening. Use a tape measure.
Repeat the side-to-side measurement in the center and in the top of the door opening. Use a pencil and paper to write down the shortest of these three measurements as the total width for the two bypass door sections.
Measure the height of the door opening. Start at the top of the door threshold, if there is a threshold in place, or start at the floor, using the tape measure. Measure up to the casing at the top of the door opening.