To provide general instructions for accomplishing ultrasonic thickness measurements. Measurements can be made from one side of a material when access to the opposite side is restricted. This Recommended Practice is intended for, but not limited to use at maintenance and overhaul facilities to inspect aerospace structures and hardware for material loss or remaining thickness after rework or fabrication processes. Due to the number of variables which can affect the reliability of ultrasonic thickness measurements, it is only recommended for use when mechanical methods of measurement are not practical.