Your body mass index, or BMI, gives a rough approximation of your body fat level. Doctors use it as a screening tool to determine if you're at possible risk of complications related to being overweight or obese. BMI is calculated according to your height and weight. Your age does not factor into the equation unless you are a child or teen. Age may be a factor used by your doctor to interpret your results, however.
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Adult BMI Calculations Use Weight and Height, Not Age
The equation for BMI is based on metric measurements. It's equal to your weight in kilograms divided by the square of your height in meters. To use American measurements, plug pounds and inches into the equation and then use a conversion factor of 703.
The equation then reads: BMI = (weight in pounds / height in inches x height in inches) x 703.
With this equation, a man who weighs 180 pounds and stands 6 feet tall has a BMI of 24.4, regardless of his age. BMI = (180 / 72 x 72) x 703 = 24.4.
A healthy BMI falls between 18.5 and 24.9, so the man in the example is at a normal weight. If your BMI comes in under 18.5, you're considered underweight. People whose BMI is 25 to 29.9 may be overweight, and those with a BMI of 30 or more are considered obese.
Muscle Mass and BMI Limitations
BMI roughly correlates with body fat levels determined through more precise measures, including body fat scales, skinfold calipers or X-ray DEXA scans. But BMI presents some limitations, which is why it's considered a screening tool and isn't used to make a diagnosis.
Your BMI doesn't take into account the type of weight you carry. People who are extremely athletic and muscular may have a BMI number in the overweight range when they just have an abundance of muscle -- not fat. Muscle tissue is far healthier than fat, and having a lot of muscle doesn't put you at risk of disease.
The younger and fitter you are, the more likely a high BMI is due to high muscularity. Men in particular have more muscle mass than women and may have their body fat misinterpreted by the BMI equation. A trained medical professional should be able to tell from a physical examination and lifestyle questions that your athletic lifestyle is skewing the results.
Normal Weight Obesity
Even if your BMI registers in the normal 18.5 to 24.9 range, you may still carry too much fat. In fact, having a normal BMI but too much abdominal fat is a condition called normal weight obesity. This excess of belly fat increases your risk of heart disease and type 2 diabetes, as well as other health complications. A high body fat percentage -- over 20 percent for men and over 30 percent for women -- may indicate this excess.
Age comes into play because body composition changes as people advance in years. Older people, especially those who don't exercise, are more likely to have normal weight obesity. With the natural decrease in bone density and the loss of muscle mass, less weight comes from muscle and bones, and more comes from fat tissue. Smokers are also at risk of having a normal BMI but still having too much body fat.
Your BMI number may be in the healthy range, but it's still important to have regular checkups of your blood pressure, blood sugar and cholesterol level.(ref4) If your doctor suspects you have too much body fat, additional tests such as skinfold caliper tests or air displacement may be run. These additional tests are available in health centers and hospitals, and are performed only by trained professionals.
Children, Teens and BMI
Age does matter when interpreting the BMI for people younger than 20 years. The same equation is used to ascertain BMI, but the results are compared to those of other youth of the same age and gender. Children and teens experience great changes in body fat at various stages of development. For example, girls put on more body fat during puberty due to sex-specific hormones preparing the body for pregnancy and breastfeeding. Age- and gender-specific charts take these differences into account.
A child or teen whose BMI is less than the 5th percentile is considered underweight. Normal weight BMIs for children and teens fall in the 5th to 85th percentile. A BMI that's in the 85th to 95th percentile qualifies as overweight, and a BMI over the 95th percentile is considered obese. Your doctor can help you figure out where your child falls on the BMI-for-age charts. Several health organizations also offer online BMI calculators that take a child's age into account, including the Centers for Disease Control and Prevention. You can plug in your child's weight, height, gender and age to get a personalized assessment.