BMI calculator for children and teens
Enter age, height and weight, get the BMI percentile and category.
How is BMI worked out for children and teens?
BMI itself is the same formula as for adults — weight in kilograms ÷ height in meters squared, or pounds ÷ inches² × 703. But for ages 2 through 19 the CDC compares it with children of the same age and sex on its BMI-for-age growth charts, and the result is a percentile.
What are the BMI categories for children?
By the CDC: underweight is below the 5th percentile, healthy weight from the 5th to below the 85th, overweight from the 85th to below the 95th, and obesity at the 95th percentile or above. Severe obesity is a BMI of 120% of the 95th percentile or more, or a BMI of 35 or more.
Example
The CDC’s own example: a 12-year-old girl who is 5 feet tall and weighs 136 pounds has a BMI of 26.6. That is the 96th percentile for her age — higher than 96% of 12-year-old girls in the reference data — so she is in the obesity category.
What are the CDC extended BMI-for-age charts?
The 2000 charts have no percentile lines above the 95th and cannot plot a BMI over 37, so in December 2022 the CDC extended them for children with very high BMIs. Up to the 95th percentile nothing changed; above it, the percentile is 90 + 10 × Φ((BMI − 95th-percentile BMI) ÷ σ), with σ set by age and sex. The calculator uses that method above the 95th percentile and also shows the BMI as a percentage of the 95th percentile.
Why not use the adult BMI categories?
In the CDC’s words, because children and teens are growing, their BMI must be read against other children of the same sex and age. The adult cutoffs (25 and 30) are for ages 20 and up — for adults, use the BMI calculator.
What age range does the calculator cover?
Children and teens from age 2 up to 20. The calculator needs the age in whole months, from 24 to 239, because the CDC tables are listed by month. For ages 20 and older use the adult BMI calculator.
Is a high percentile a diagnosis?
No. The CDC says BMI-for-age is a screening tool. A high or low percentile is a reason to talk with a pediatrician, who also considers growth history, family build, puberty and other health information.
How the calculation works
The calculator first finds the BMI as for an adult. It then looks up the CDC reference values for a child of the same sex and age in months. These values are three numbers, L, M and S, that describe how BMI is distributed among children of that age and sex. The child's BMI is converted to a z-score with them, and the z-score to a percentile. Percentile categories are underweight below the 5th, healthy from the 5th to below the 85th, overweight from the 85th to below the 95th and obesity at the 95th or above. Above the 95th percentile, the extended CDC method is also shown.
Worked example
Take a boy who is 10 years old (120 months), 140 cm tall and weighs 36 kg. His BMI is 36 ÷ 1.40² = 18.4. For his age and sex that corresponds to the 76th percentile, in the healthy weight range; the 5th, 85th and 95th percentile BMIs for boys at this age are about 14.2, 19.4 and 22.2. If the same boy weighed 50 kg, his BMI would be 25.5, which is 115% of the 95th percentile BMI and the 97.6th percentile on the extended chart: the obesity category.
Common mistakes
- Using the adult categories. A BMI of 20 means different things for a 6-year-old, a 12-year-old and an adult. Always use the age-specific percentile.
- Entering age in years. The calculator uses whole months; 10 years is 120 months, and 10 years and 6 months is 126.
- Judging from one measurement. A pediatrician looks at the trend over time. A child can move between percentiles during growth spurts.
Sources and related calculators
Percentiles and categories: CDC growth charts (2000) and the CDC 2022 extended BMI-for-age charts. Related: BMI calculator, baby growth percentile calculator and child height predictor.
For general information only — not medical advice. Talk with your doctor or another health professional about your own health. How we check our calculators.