Open the door you want to measure.
Measure the width of the door jam. Hook the tip of the tape measure on the left side of the door jam. The jam starts just behind the door trim. Pull the tape to the opposite side of the jam. The jam stops where the door trim on the opposite side starts. The door jam should be 4 9/16 or 6 9/16 inches wide.
Measure the height of the door jam. Place the tip of the tape measure against the floor, and pull the tape measure straight up to the top of the door frame. Read the measurement on the tape measure. This is the height of the door jam.
Measure the thickness of the door. Standard doors will be 1 3/8 or 1 3/4 inches thick. This is the door thickness that the door jam is designed for. Your door jam will measure out to something like 3 foot 0 1/4 inches by 6 foot 8 1/8 inches for a 1 3/8-inch-thick door. This is the information that a door supplier will need to know if you are ordering a new door slab.
Determine the door swing. Place your back against the door hinges. If the doorknob is to your right side, the door has a right-hand swing. If the doorknob is to your left side, the door has a left-hand swing. The door supplier will need this information.