Body mass index is a calculation that uses your height and weight to estimate how much body fat you have. Too much body fat is a problem because it can lead to illnesses and other health problems.

BMI, although not a perfect method for judging someone's weight, is often a good way to check on how a kid is growing.

The best way to determine your BMI is to have your doctor do it for you. That way, you'll know the number is accurate and your doctor can discuss the result with you and your mom or dad.

 

Four Categories

Once you know your BMI, you'll learn that you are in one of four categories:

1. Underweight: less than the 5th percentile

2. Healthy weight: greater than or equal to 5th but less than 85th percentiles (in other words: 5th to 84th percentile)

3. Overweight: greater than or equal to 85th but less than 95th percentiles (85th to 94th percentile)

4. Obese: greater than or equal to the 95th percentile

A kid whose BMI is at the 50th percentile is close to average compared with the kids of the same age and gender who were measured to make the chart. A kid at the 85th to 94th percentiles is considered overweight. And a kid who measures at or above the 95th percentile is considered obese, a term doctors use that means very overweight.

 

How BMI Can Change

It's important to remember that BMI is interpreted differently for adults. There are separate charts for men and women, but they don't use percentiles at all — just number ranges that are considered underweight, normal weight, overweight, or obese (which means very overweight). That's because adults have stopped growing and their age is no longer a major consideration when it comes to weight.

Kids, on the other hand, are growing. And it's common for kids to gain weight during certain times in childhood, such as puberty. The charts take growth into account.

The following example shows that in action: Here's a look at a boy as he grows yet stays in the 50th percentile for BMI, which means he's average. Notice that his BMI goes up and down, but he continues to stay at the same percentile.

AgeBMIPercentile

2 years16.550th

4 years15.850th

8 years16.050th

12 years17.550th

 

Where BMI Can Fall Short

BMI is not the whole story when it comes to someone's weight. A more muscular kid might have a higher weight and BMI but not have too much body fat. A smaller kid could have an ideal BMI, but might have less muscle and too much body fat.

Also, it's very common for kids to gain weight quickly — and see the BMI go up — during puberty. Because of these and other considerations, it's a good idea to talk to your doctor if you have questions about whether you are at your ideal weight.

If your doctor tells you your BMI is high, don't let it get you down. Instead, talk to your doctor about what you should do to lower your BMI. Unlike adults, kids don't usually need to diet. But by eating healthier and getting more exercise, a kid can improve his or her BMI.

Controlling a weight problem while you're still a kid can help you avoid becoming an overweight adult and developing health problems like diabetes and heart disease. With a little effort, your BMI will be JWIOTB — just where it ought to be!