10 The last measurement is to determine the depth of the chest cavity. Have the driver stand with his back against a wall or smooth door. Again, the back should be flat against the surface from the hips all the way to the shoulders. Place the measuring tool underneath the armpit until it’s flat against the door or wall and measure to the front of the chest. The driver must be relaxed and not flexing or puffing out his or her chest for this to be a valid measurement.