A bone density or bone mass chart can be found on the official website of the National Institute of Arthritis and Musculoskeletal and Skin Diseases, as of 2015. The chart defines normal bone density and low bone mass along with density ranges of osteoporosis.
Bone density is measured by comparing density to the standard deviation to establish a T-score, notes the National Institute of Arthritis and Musculoskeletal and Skin Diseases. A score of zero is optimal, since it means that bone mass density is normal. The standard deviation is the difference between a person’s bone mass density and the bone mass density of a normal, healthy young adult. A T-score of 1 to -1 is normal, while lower than -1 indicates low bone mass and -2.5 or lower is an indicator of osteoporosis.