Body Mass Index by Age

The body mass index formula determines your weight status. Weight statuses are generally defined as underweight, normal weight, overweight, obese and morbidly obese. Each weight status corresponds with a BMI range. The numeric value assigned to your body mass index places you in one of these ranges. Age and gender can affect whether your body mass index is healthy.

Calculation

Taking your height in inches and your weight in pounds, you can determine your body mass index.
There are a couple of equations, but the numeric value is the same regardless of which one you choose.
BMI = (weight x 703) / (height x height)
BMI = [weight / (height x height)] x 703
If you weigh 160 lbs. and are 5 feet 8 inches--68 inches--tall, calculate: (160 x 703) / (68 x 68) to arrive at a body mass index of 24.3. For the second equation, [160 / (68 x 68)] x 703 = 24.3.

Children

With a child, adolescent, teen or young adult between the ages of 2 and 20, the BMI value is placed on a chart comparing individuals of the same age and gender. Where the child falls within this chart designates weight status. If a child falls below the fifth percentile, she's usually considered underweight. Healthy weight is any value that places you between the fifth and 84th percentile. The 85th to just under the 95th percentiles are considered overweight, while the 95th percentile or greater is considered obese.

Adults

Adults between the ages of 20 and 65 typically use the numeric value of BMI alone to determine weight status, according to the Centers for Disease Control and Prevention. A numeric value under 18.5 generally means you're underweight. Anywhere between 18.5 and 24.9 is considered normal weight, 25.0 to 29.9 is overweight and 30 to 39.9 is obese. A designation of morbidly obese is anything over 40.

Young Women

A caveat in BMI for adults involves women between the ages of 20 and 33. The Mayo Clinic explains that the problem with this age group is in the overweight and obese categories. A study by the Department of Obstetrics and Gynecology in conjunction with the Center for Interdisciplinary Research in Women's Health found that the current cutoff failed to identify nearly half of the women within this age range suffering from obesity. BMI values in the upper range of overweight may actually mean that a woman is obese. Further studies are necessary to determine if all ranges need to be adjusted for this gender and age.

Elder Adults

Most adults older than 65 use the same BMI ranges and weight statuses to determine body composition as adults between the ages of 20 and 65. The only difference is with what's considered normal weight and overweight. An ideal body mass index for someone of this age is between 25 and 27, advises the National Institutes of Health. Though higher BMI reduces the risk of osteoporosis, a degenerative disease affecting bone density.