An adult with a BMI of 22 is in the normal range, but a 7-year-old with a BMI of 22 is well above it. Children’s bodies change with age: BMI falls through the preschool years, rises again from about age 6, and differs between boys and girls once puberty starts. For this reason the CDC does not use fixed cut-offs for children and teenagers. Instead it compares each child’s BMI with the BMI of other children of the same age and sex, and reports a percentile. This calculator computes the BMI, finds its percentile in the CDC 2000 BMI-for-age charts, and gives the CDC weight category and, for BMI at or above the 95th percentile, the obesity class.
BMI is a screening tool. It does not diagnose, and a child’s doctor will look at the whole picture.
How to use the child and teen BMI calculator
- Select the child’s sex.
- Enter the age in years, with a decimal for months. The charts cover 2 to under 20 years.
- Enter height and weight. Use the child’s height without shoes, and weight in light clothing.
- Read the BMI, percentile and category. The 95th-percentile figure shows how far a BMI is above the obesity threshold.
Child BMI formula
z = ((BMI ÷ M)L − 1) ÷ (L × S); percentile = Φ(z) × 100
BMI at the 95th percentile = M × (1 + L × S × 1.645)1/L
Percent of the 95th percentile = BMI ÷ BMI95 × 100
| CDC category | BMI-for-age percentile |
|---|---|
| Underweight | below the 5th |
| Healthy weight | 5th to below the 85th |
| Overweight | 85th to below the 95th |
| Obesity | 95th or above |
The percentile categories are those of the CDC (Child and Teen BMI Categories). The obesity classes follow the American Academy of Pediatrics clinical practice guideline (Hampl SE et al., Pediatrics 2023;151:e2022060640): class 1 from the 95th percentile to below 120% of it; class 2 from 120% to below 140% of the 95th percentile or a BMI of 35 to under 40; class 3 at 140% or more, or a BMI of 40 or more. L, M and S come from Kuczmarski et al., Vital Health Stat 11(246), 2002.
Worked example
A boy aged 10 years 1.5 months, 140 cm tall, weighing 33 kg.
- BMI = 33 ÷ 1.40² = 16.8
- CDC BMI-for-age percentile = 53th (z = 0.07)
- Category: Healthy weight. The BMI at the 95th percentile for this age is 22.2; his BMI is 76% of that.
BMI-for-age: 5th, 50th and 95th percentiles
| Age (years) | Boys 5th | Boys 50th | Boys 95th | Girls 5th | Girls 50th | Girls 95th |
|---|---|---|---|---|---|---|
| 2 | 14.7 | 16.5 | 19.3 | 14.4 | 16.4 | 19.1 |
| 3 | 14.3 | 16.0 | 18.2 | 14.0 | 15.7 | 18.3 |
| 4 | 14.0 | 15.6 | 17.8 | 13.7 | 15.3 | 18.0 |
| 5 | 13.8 | 15.4 | 17.9 | 13.5 | 15.2 | 18.3 |
| 6 | 13.7 | 15.4 | 18.4 | 13.4 | 15.2 | 18.8 |
| 7 | 13.7 | 15.5 | 19.2 | 13.4 | 15.5 | 19.7 |
| 8 | 13.8 | 15.8 | 20.1 | 13.5 | 15.8 | 20.7 |
| 9 | 14.0 | 16.2 | 21.1 | 13.7 | 16.3 | 21.8 |
| 10 | 14.2 | 16.6 | 22.2 | 14.0 | 16.9 | 23.0 |
| 11 | 14.6 | 17.2 | 23.2 | 14.4 | 17.5 | 24.1 |
| 12 | 15.0 | 17.8 | 24.2 | 14.8 | 18.1 | 25.3 |
| 13 | 15.5 | 18.5 | 25.2 | 15.3 | 18.7 | 26.3 |
| 14 | 16.0 | 19.2 | 26.0 | 15.8 | 19.4 | 27.3 |
| 15 | 16.6 | 19.9 | 26.8 | 16.3 | 19.9 | 28.1 |
| 16 | 17.1 | 20.6 | 27.6 | 16.8 | 20.5 | 28.9 |
| 17 | 17.7 | 21.2 | 28.3 | 17.2 | 20.9 | 29.6 |
| 18 | 18.2 | 21.9 | 29.0 | 17.6 | 21.3 | 30.3 |
| 19 | 18.7 | 22.5 | 29.7 | 17.8 | 21.6 | 31.0 |
Limits of BMI in children
- It does not measure body fat. A muscular child can have a high BMI with little excess fat.
- Puberty timing matters. Early and late developers can look different on the chart for reasons that are normal.
- A single reading is a snapshot. The pattern over several years says more.
- It should never be used to shame. Weight talk with children needs care; focus on health habits for the whole family.
Pair this tool with the Child Height Percentile Calculator and Child Weight Percentile Calculator, and see the adult BMI Calculator from age 20. For waist measurements the Waist-to-Height Ratio Calculator gives a complementary screen.
This page is for information only and is not medical advice. A single measurement does not diagnose a problem; growth is judged over time. Talk to your child’s doctor about any concern.
Sources
- Kuczmarski RJ, Ogden CL, Guo SS, et al. 2000 CDC Growth Charts for the United States: Methods and development. Vital Health Stat 11(246), 2002.
- Centers for Disease Control and Prevention. Child and Teen BMI Categories.
- Hampl SE, et al. Clinical practice guideline for the evaluation and treatment of children and adolescents with obesity. Pediatrics 2023;151:e2022060640.
Frequently asked questions
Why can’t I use the adult BMI categories for a child?
Children’s body fat changes with age and differs between boys and girls, so a BMI that is healthy at 6 may be too low at 16. The CDC therefore compares each child’s BMI with others of the same age and sex and uses percentiles instead of fixed cut-offs.
What are the CDC weight categories for children?
Underweight is below the 5th percentile; healthy weight is the 5th to below the 85th; overweight is the 85th to below the 95th; obesity is the 95th percentile or above. The percentiles come from the 2000 CDC growth charts.
What are the obesity classes?
The American Academy of Pediatrics (Hampl et al., Pediatrics 2023;151:e2022060640) defines class 1 as 95th percentile to below 120% of it, class 2 as 120% to below 140% of the 95th percentile (or BMI 35 to under 40), and class 3 as 140% or more (or BMI 40 or more). The calculator shows your BMI as a percentage of the 95th percentile for this reason.
Is BMI a diagnosis in children?
No. It is a screening tool. A healthcare provider considers growth pattern, family history, diet, activity and other measures before deciding whether a child’s weight is a concern.
What should I do if the category is not “healthy weight”?
Talk to your child’s doctor. They can check for causes, look at the growth curve over time and discuss supportive changes for the whole family. Avoid putting a child on a restrictive diet without professional advice.