Introduction
This BMI Percentile Calculator shows how a child's body mass index (BMI) compares to other kids of the same age and sex. It works for children and teens ages 2 to 20, and it uses the CDC growth chart data that doctors use.
Enter the child's date of birth, the date you took the measurement, their sex, height, and weight. You can use US units (feet, inches, pounds) or metric units (centimeters, kilograms). If you switch unit systems, the numbers you already typed are converted for you, so you never have to retype them.
You get the child's BMI, exact age, BMI percentile, Z-score, and CDC weight status: underweight, healthy weight, overweight, or obese. You also see a growth chart with the child's point plotted on it, plus a step-by-step math breakdown so you can see how the answer was found.
Kids grow fast, so a child's BMI number alone does not tell you much. The percentile is what matters, because it compares your child to other children the same age and sex. Use this tool to track growth over time, and talk to your child's doctor about what the results mean.
How to use our BMI Percentile Calculator
Enter your child's birth date, measurement date, sex, height, and weight. The calculator shows the BMI, exact age, BMI-for-age percentile, Z-score, CDC weight status, and a growth chart.
Date of Birth: Type the day your child was born (mm/dd/yyyy). This sets the exact age used for the percentile.
Date of Measurement: Type the day the height and weight were taken. It starts as today's date, so change it only if you measured earlier.
Computed Age: This box fills in on its own. Read it to make sure your dates are right. The tool works for ages 2 to 20 years.
Gender: Pick Male or Female. Boys and girls use different CDC BMI-for-age curves.
Unit System: Choose US (inches and pounds) or Metric (cm and kg). If you switch, your numbers change over for you.
Height: Enter feet and inches, or centimeters. Measure with shoes off, heels together, and eyes looking straight ahead.
Weight: Enter pounds or kilograms. Weigh your child in light clothes with no shoes.
Calculate: Press Calculate to see the BMI percentile, Z-score, weight status, chart, and step-by-step math. Press Reset to start over with sample values.
What Is BMI Percentile for Children and Teens?
BMI stands for Body Mass Index. It is a number that compares a person's weight to their height. For adults, one BMI number fits everyone. For kids, it does not work that way. Children grow fast, and boys and girls grow in different ways. So a child's BMI is compared to other kids of the same age and the same sex. That comparison is called the BMI percentile.
How BMI Is Found
BMI uses a simple formula: weight in kilograms divided by height in meters squared (kg/m²). If you enter pounds and inches, they are changed to metric first. The result is one number, like 17.5.
What the Percentile Means
A percentile shows where a child stands in a group of 100 kids of the same age and sex. If a child is at the 60th percentile, about 60 out of 100 kids that age have a lower BMI. The percentile comes from the CDC growth charts, which are based on data from thousands of American children ages 2 to 20.
CDC Weight Status Categories
- Underweight: below the 5th percentile
- Healthy Weight: 5th to below the 85th percentile
- Overweight: 85th to below the 95th percentile
- Obese: 95th percentile or higher
What the Z-Score Means
The Z-score tells how far a child's BMI sits from the average, counted in standard deviations. A Z-score of 0 is right at the middle (50th percentile). A score of +1 is above average, and −1 is below average. Doctors like Z-scores because they still work well when a number is very high or very low, where percentiles all look close to 99 or close to 1.
Why Age and Sex Matter So Much
BMI drops during the toddler years, hits a low point around age 5 or 6, and then climbs through the teen years. This is normal growth. That is why an exact age in months is used, not just the year. A few months can shift the percentile.
Things to Keep in Mind
BMI is a screening tool, not a diagnosis. It does not measure body fat directly. A child with a lot of muscle may show a high BMI but have little fat. Growth spurts, body build, and bone size all affect the number. One reading also matters less than the pattern over time, so doctors like to plot BMI on a growth chart at each checkup and watch the curve.
Always talk with a pediatrician or family doctor before changing a child's diet or activity. They can look at growth history, health, and family background before giving advice.