Understanding Percentiles

Your child is in the 10th percentile for BMI.

Your child is in the 95th percentile for height.

Is the 95th percentile good? Is the 10th percentile bad?

How do you understand percentiles?

Start by imagining the entire population of the United States divided into 100 groups. Each group is a percentile.

You can divide people by anything. You could have all the people in the world divided by the number of turtles they own. The first percentile would have the fewest — none. The 100th percentile would include people who own the most turtles.

In health care, the term “percentile” is most often used for height and weight. This tells how one individual compares with other individuals in the community — usually, in the United States.

Narrow it down.

We could sort all the people in the U.S. by height. The shortest Americans are in the first group and the tallest ones are in the 100th group. Obviously, this would put all the babies in the first group, so it won’t work.

That’s why percentiles are narrowed down by other factors. “Your child is in the 10th percentile for BMI” really has to mean “… for 8 year old girls.”  Or some other smaller set of all the people in America.

So lets start with something narrower: birth weight for girls in the United States. Now we can divide all the newborn girls by their birth weight, putting the littlest girls in the first group and the biggest ones in the 100th group.

The babies in the first group would weigh four pounds or so. The ones in the 100th group would be a remarkable 10 lbs.

In the middle, we’d see those average 7 pound babies.

We’re not saying that any of these birth weights are bad; we’re just sorting everyone out. In fact, the six to eight pound girls in the middle might be healthier than the ones at the edges — the 10th or 100th percentile.

Why use percentiles?

For children, percentile measurements make the most sense. You might have seen a rule of thumb saying that a 4 year old should weigh about 40 pounds. But if you visit your local preschool’s 4 year old class, you can see kids of many different heights and frame sizes. They shouldn’t all weigh 40 pounds.

  • A small-framed girl at the 19th percentile for height could be at the 5th percentile for weight and be at a healthy weight.
  • But a girl at the 90th percentile for height and the 5th percentile for weight could be dangerously underweight.

Children have different heights and different natural patterns of growth. Looking at percentiles helps you focus on healthy growth without being distracted by normal differences.

You can calculate your height and weight percentiles, but adults usually are measured by Body Mass Index and given a desirable weight range, rather than going by percentiles. Kids vary a lot in size and they grow all the time, so setting a specific weight for health can be difficult. Many American adults, however, are heavier than is ideal for their health. Being at the 50th percentile for weight as an adult might not be a healthy weight for you.

Percentiles can be used to measure anything, but they are most commonly used to measure kids’ sizes. If you’re not sure what your child’s percentile numbers mean, be sure to ask your doctor.