The familiar height-and-weight-based body mass index (BMI) is easy to calculate and widely used to estimate how much body fat a person has.

It's also wrong much of the time.

BMI measurements misclassify about 60% of women and 13% of men when used to determine if someone falls into the "obese" category, one tipoff doctors use to determine if someone is at a higher risk of developing diabetes or heart disease.

Researchers at the Cedars-Sinai Diabetes and Obesity Research Institute took on the challenge of designing a measurement that would be more accurate than BMI at predicting health risks, but just as simple to calculate.

"The main problem with BMI is that it doesn't distinguish between fat mass and non-fat mass," says Dr. Orison Woolcott, leader of a recent study to test out a new way to measure body fat.

"So a bodybuilder who is heavy on the scale would be considered obese or overweight, and of course that's not correct. A person who appears lean could have a risky amount of fat mass, but their BMI doesn't detect that."

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These discrepancies matter when you weigh the BMI's most important function: identifying who is at highest risk of developing serious medical conditions.

Dr. Woolcott and his co-author, Dr. Richard Bergman call the new measure the relative fat mass index, or RFM. It plugs your height and your waist circumference into a formula and the resulting number is roughly equal to your body fat percentage.

Their recent study found this simple measure is better at predicting body fat percentage than BMI.

More studies need to be completed before they can make a definite call on what the cut-off values should be for a healthy body fat percentage versus one that's putting someone at risk, Dr. Woolcott says.

In the meantime, it's a useful way to track your own personal data, he suggests. For example, if you're working on losing weight, this is a simple and efficient way to track how much body fat you're shedding.

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The beauty of RFM is that it can be determined with nothing more sophisticated than a tape measure and some simple math.

First, grab a tape measure and record your height and your waist circumference. To get the best results for your waist, put the tape measure at the top of your hip bone and wrap it around your body. (You can measure in any unit you want: inches, centimeters, etc., since the relative fat mass is a ratio—just be sure to use the same nit for both measurements.)

Then plug the numbers into the formula:

• Men: 64 – (20 x height/waist circumference) = RFM
• Women: 76 – (20 x height/waist circumference) = RFM