People looking to be healthy often check out their body-mass index, a simple number that researchers use to classify people as underweight, normal, overweight or obese.

BMI is calculated by dividing your weight, in pounds, by your height, in inches, then multiplying by 703.

A BMI of 18.5 to 24.9 is considered normal. Above 30 is considered obese.

While the measurement has gained popularity, more people are also pointing out that it may not be the most reliable measure of fitness or of your level of body fat.

In fact, when it was initially developed, BMI was meant to be used to find averages in large populations, not represent something about individuals.

Part of the problem is that it does not take into account what makes up your weight. A person with a lot of muscle -- a body builder, for instance -- would have a high BMI, even if he had a very low body-fat percentage.

Experts also say that even when BMI is right that you're carrying too much fat, it does not address where the fat lives on your body. Fat deposits in the abdominal area are the most dangerous, but a simple measure of weight and height fails to account for that difference.

Other measures may be more helpful in determining if you need to shape up.

Waist circumference, for example, can check for abdominal obesity, the type most associated with obesity-related diseases such as diabetes and heart disease.

You can also check out your waist-hip ratio, measured at the widest part, around your bottom. Experts say WHR is god at predicting heart trouble.

Another way to test is to have a caliper test or get dipped in water to determine your percentage of body fat.