For children and teens aged 2 to 19, BMI is compared with other children of the same age and sex and given as a percentile on the CDC growth charts: below the 5th percentile is underweight, the 5th to below the 85th is a healthy weight, the 85th to below the 95th is overweight, and the 95th or above is obesity. Adult cut-offs such as 25 and 30 are never used for children.
Enter the child's sex, age in years and months, height and weight. The calculator gives the BMI, the percentile, the category and the healthy weight range for the child's height at that age — all from CDC's reference data, including the 2022 extended charts for children above the 95th percentile.
Worked example
Child is a: GirlAge: 10 yearsand: 0 monthsHeight: 4 ft 7 inWeight: 75 lb
BMI-for-age percentile: 59th percentile Healthy weight
That is a BMI of 17.4, at the 59th percentile for a 10-year-old girl — a healthy weight. At her height the healthy range is 61 to 85 lb.
Open this example in the calculator →How to use the BMI Calculator for Kids & Teens
- Choose girl or boy — boys and girls grow differently, so their charts differ.
- Enter the age in years and months. Use the age on the day the child was measured.
- Enter a measured height and weight, without shoes and in light clothes.
- Read the percentile, the category and the healthy weight range at the child’s height.
What a BMI percentile means
A percentile compares one child with a reference group of children of the same age and sex. A girl at the 70th percentile has a higher BMI than 70 out of 100 girls her age in the CDC reference. It is not a score out of 100 and it is not the percentage of body fat.
Percentiles are used because a healthy BMI changes a lot through childhood. It falls from age 2 to about 5 or 6, then rises through the school years and puberty. A BMI of 17 is in the healthy range for a 10-year-old girl, but would be high for a 4-year-old and low for a 17-year-old.
Child and teen BMI categories
| Percentile | Category |
|---|---|
| Below the 5th | Underweight |
| 5th to below the 85th | Healthy weight |
| 85th to below the 95th | Overweight |
| 95th or above | Obesity |
| 120% of the 95th or more | Severe obesity (class 2; class 3 from 140%) |
The severe-obesity classes come from the American Academy of Pediatrics' 2023 clinical guideline. They are given as a percentage of the 95th percentile rather than as a higher percentile, because at that level percentiles bunch together: almost every child with severe obesity would otherwise be "the 99th".
How this calculator works it out
It uses the CDC 2000 BMI-for-age reference — the L, M and S values for every month from 24 to 240 months, taken from CDC's own data — and interpolates between months as CDC's software does. Below the 95th percentile the percentile comes from the LMS formula. At and above it, it uses CDC's 2022 extended method, which models the upper tail directly and gives meaningful percentiles all the way up; CDC now publishes the charts that way. Age in whole months is placed at the middle of the month, as CDC recommends. It is the same method as CDC's own software, and it reproduces CDC's published check value: a boy of 50.5 months with a BMI of 22.6 is at the 99.77th percentile.
Measuring a child accurately
- Height: shoes off, heels, back and head against a wall, looking straight ahead. Lay a book flat on the head and mark the wall.
- Weight: on a hard floor, in light clothes, at about the same time of day.
- A one-inch error in height moves a 7-year-old's percentile by more than ten points. If the result surprises you, measure again.
If the result is outside the healthy range
One measurement is a snapshot; doctors look at how a child moves along the chart over time. For a child above the 85th percentile, the usual aim is not weight loss but slowing weight gain while height catches up, with changes the whole family makes together. The AAP recommends that children with obesity be offered treatment early rather than "watchful waiting". Below the 5th percentile can be normal for a small, slim child who is growing steadily — or a sign worth checking. Either way, a pediatrician is the right next step.
CDC charts or WHO charts?
In the United States, the CDC charts are used from age 2 to 19; the WHO growth standards are used under 2. Many other countries use the WHO reference for ages 5 to 19, which reports z-scores: above +1 is overweight and above +2 is obesity. The two systems give similar but not identical answers. This calculator follows CDC.
Frequently asked questions
What is a healthy BMI for a child?
Any BMI between the 5th and the 85th percentile for the child’s age and sex. There is no single healthy number, because a healthy BMI changes as children grow: roughly 13.5 to 17 for a 6-year-old, 15 to 22 for a 12-year-old, and 18 to 26 for an 18-year-old.
How is BMI calculated for kids?
The BMI itself is calculated the same way as for adults — weight in kilograms divided by height in meters squared. The difference is the interpretation: a child’s BMI is compared with children of the same age and sex on the CDC growth charts and expressed as a percentile.
What does the 85th percentile mean?
That the child’s BMI is higher than that of 85% of children the same age and sex in the CDC reference. The 85th percentile is where overweight starts; the 95th is where obesity starts.
Is BMI accurate for teenagers?
It is the standard screening tool from age 2 to 19, but like adult BMI it cannot tell muscle from fat, and puberty timing affects it — an early developer may sit higher on the chart for a while. A pediatrician interprets it alongside growth over time.
At what age do you use adult BMI categories?
From age 20. Until then, BMI is read as a percentile for age and sex, even for an 18- or 19-year-old.
Why does the calculator need the age in months?
Because the healthy range moves month by month. For a 10-year-old, six months can shift the percentile by several points. Enter the months for the most accurate result.
My child is above the 95th percentile but not shown as 99th. Why?
Above the 95th percentile this calculator uses CDC’s 2022 extended method, which spreads children out across the upper tail instead of bunching them at the 99th. The % of the 95th percentile shown with the result is how pediatricians grade severity.
Sources
- Child and Teen BMI Categories — Centers for Disease Control and Prevention
- CDC Extended BMI-for-age Growth Charts (2022) — Centers for Disease Control and Prevention
- CDCAnthro: reference data and code for the CDC growth charts — CDC Division of Nutrition, Physical Activity, and Obesity
- Hampl SE et al. Clinical Practice Guideline for the Evaluation and Treatment of Children and Adolescents With Obesity. Pediatrics 2023;151(2):e2022060640 — American Academy of Pediatrics
- Obesity and overweight (fact sheet) — World Health Organization
- Rubino F et al. Definition and diagnostic criteria of clinical obesity. Lancet Diabetes Endocrinol 2025;13:221–262 — The Lancet Diabetes & Endocrinology
For general information only — not a diagnosis or a substitute for advice from a doctor. How we calculate.