For children & teens
BMI Calculator for Teens and Children
You can calculate a child or teen's BMI number below — but for anyone under 20, the number is interpreted against growth-chart percentiles, not the adult categories. Here's what that means.
Why the adult categories don't apply
Children and teenagers are still growing, and body fat changes with age and differs between boys and girls. A fixed cut-off like 25 would mislabel many healthy kids. Instead, doctors use BMI-for-age percentile charts (such as the CDC growth charts) that compare a child's BMI with others of the same age and sex.
How BMI-for-age percentiles work
For ages 2–19, a BMI below the 5th percentile is considered underweight, the 5th to under the 85th percentile is a healthy weight, the 85th to under the 95th is overweight, and the 95th percentile or above is obesity. So a BMI of 21 might be perfectly healthy for a 16-year-old but high for an 8-year-old.
What parents should do with the number
One measurement means little on its own — what matters is the trend over time, plotted on a growth chart at check-ups. If your child's percentile is climbing or sits above the 85th, bring it up with your pediatrician rather than restricting food at home. Focus on family habits: shared meals, active play, limited sugary drinks, and good sleep.
A note on teenagers and body image
The teen years are when body image is most fragile. Frame any conversation around health, energy, and strength — never weight or appearance — and avoid putting a teenager on a diet without medical guidance. If you notice signs of disordered eating, talk to a doctor promptly.
Key facts about BMI
For anyone under 20, the adult bands below do not apply. The CDC's BMI-for-age growth charts compare a child or teen against others of the same age and sex — the age table shows how the assessment differs.
Adult BMI categories
How BMI is read by age group
| Group | How it's assessed | Healthy / lowest-risk |
|---|---|---|
| Children & teens (2–19) | BMI-for-age percentile, by sex | 5th to under 85th percentile |
| Adults (20–64) | Fixed adult bands | 18.5 – 24.9 |
| Older adults (65+) | Adult bands, interpreted with care | Lowest risk seen around 23 – 29.9 |
| Asian adults | Lower action points suggested | 18.5 – 22.9 (risk rises from 23) |
Children's results use CDC BMI-for-age growth charts. A meta-analysis of 32 studies in adults over 65 found the lowest mortality at BMI 23–29.9, and a WHO expert consultation published in The Lancet recommended lower action points for Asian populations.
Limitations of BMI
- It doesn't measure body fat. In a study of over 13,000 adults, BMI ≥30 missed about half of people with excess body fat.
- Muscle counts as weight. Athletes can score "overweight" while lean.
- Fat location matters. Belly fat carries more risk; waist-to-height ratio predicts heart and metabolic risk better than BMI. Aim for a waist under half your height.
- Age, sex and ethnicity change the picture, as the table above shows.
- Not for pregnancy or people with major fluid shifts or amputations.
Even so, large studies such as the Global BMI Mortality Collaboration (10.6 million adults) confirm BMI is a useful population-level screening tool. Use it as a starting point and discuss results with a health professional.
Frequently asked questions
What is a healthy BMI for a teenager?
There's no single number. For ages 2–19, healthy is defined as the 5th to under the 85th BMI-for-age percentile, which varies with age and sex. Your pediatrician plots it on a growth chart.
Why does this page show the adult categories in the result?
The calculator computes the BMI number itself, which is the same formula at any age. But the category label only applies to adults 20+ — for under-20s, ask a doctor to interpret the number against percentile charts.
Should I put my teen on a diet if their BMI is high?
Not without medical guidance. Restrictive dieting during growth can backfire. Pediatricians usually recommend family-wide habit changes instead — more activity, fewer sugary drinks, regular meals.
Is my child's information stored?
No. The calculation runs entirely in your browser — nothing is sent or saved anywhere.