Doctors and scientists have been estimating body fat with Body Mass Index (BMI) for nearly 200 years. Looks like it may be time for an update.

BMI is a ratio of height and weight. It's a relatively simple calculation which is why it's often used to evaluate obesity-related health risks. (Math-phobics can check out our BMI calculator.) However, it's not always reliable, in particular for measuring athletic body types. For example, under the current BMI guidelines a rock solid body builder who is 5-foot-5 and 150 pounds has a BMI of 25. This is considered overweight, the same as a chip-eating couch dweller of similar height and weight.

A new measure known as Body Adiposity Index (BAI) makes body fat estimates based on hip-width and height measurements rather than bodyweight. One potential advantage over BMI is that it gives a clearer snapshot of how much unhealthy flab a person carries which eliminates much of the guesswork of whether or not a person is truly overweight. It also seems able to differentiate how much of a person's weight is fat and how much is muscle and fat free mass - although like BMI, it still doesn't reveal anything about where an individual's fat is deposited.

BAI was recently developed by a team of researchers at the University of Southern California in Los Angeles. When they analyzed 1700 people for an extensive series of physical characteristics they found that hip circumference and height correlated strongly with body fat percentage as measured by a highly reliable but expensive scanning method known as DEXA, or dual-energy X-ray absorptiometry.

By the scientist's own admission, BAI still needs some fine tuning. But so far results have been promising. Recent studies with various ethnic groups have also produced good results.

If its accuracy holds up, BAI just might replace BMI as the body fat test of choice. Any doctor, nurse, dietitian or personal trainer should be able to provide you with a number quickly and easily. You can even figure out your own BAI with one of the calculators already posted on the web.

Just a reminder that body fat percentage is only one number medical experts use to evaluate a person's overall health and level of "fatness". There aren't any universally accepted guidelines for the ideal percentage of body fat. The American Council on Exercise recommends that women strive for between 16-26 percent body fat and men between 12-22 percent. Athletic men and women often have lower body fat percentages than that. Those above 38 percent body fat are generally considered to be obese.

Liz Neporent is the best selling writer of 15 health books including The Winner's Brain and Fitness for Dummies, 4th edition. Follow her on twitter @lizzyfit and ask her anything you want to know about getting in shape and losing weight.