Reference: The information on how to measure Abdominal circumference comes from AFI 36-2905 section 3.6 Body compensation assessment procedures.

To measure abdominal circumference, locate the upper hip bone and the top of the right iliac crest. Place a measuring tape in a horizontal plane around the abdomen at the level of the iliac crest. Before reading the tape measure, ensure that the tape is snug, but does not compress the skin, and is parallel to the floor. The measurement is made at the end of a normal expiration.