BMI for Age Calculator
Calculate BMI percentile for children and teenagers aged 2–19 using CDC growth chart standards. Unlike adult BMI, children's healthy weight varies with age and sex — this calculator gives the correct percentile interpretation for each child's specific age and biological sex, with plain-English guidance for parents.
Enter child's details
BMI-for-age uses the same height and weight formula as adult BMI, but interprets the result against age- and sex-specific CDC growth chart percentiles rather than fixed category thresholds.
Why not adult BMI?
Children's body fat naturally changes as they grow. The same BMI value can be healthy at one age and unhealthy at another. Adult BMI categories (18.5–24.9 etc.) do not apply to anyone under 18.
Percentile explained
A percentile of 65 means the child's BMI is higher than 65% of children of the same age and sex. The CDC defines healthy as the 5th–85th percentile.
What is BMI for age?
BMI-for-age (also called BMI percentile) is the correct way to assess body weight in children and teenagers aged 2–19. Unlike adult BMI — where fixed thresholds (18.5, 25, 30) apply regardless of age — children's healthy weight ranges change significantly as they grow and go through puberty.
The CDC growth charts compare a child's BMI against thousands of children of the same age and sex. The result is expressed as a percentile — the percentage of children of the same age and sex who have a lower BMI. A child at the 70th percentile has a higher BMI than 70% of their peers and is in the healthy range.
CDC weight status categories
These thresholds are the same for all children regardless of sex, but the actual BMI values that correspond to each percentile differ significantly by age and sex — which is why adult BMI categories cannot be applied to children.
How BMI percentile is calculated
The BMI value is calculated the same way as for adults — weight in kg divided by height in metres squared. The percentile lookup uses the CDC LMS (Lambda-Mu-Sigma) method, which models the distribution of BMI values across age and sex:
This calculator uses CDC LMS table values interpolated for the child's exact age in months, giving a more accurate result than simple age-in-years lookups.
Important limitations
- It is a screening tool, not a diagnosis. A child above the 85th percentile is not automatically unhealthy — a muscular or tall child may show high BMI without excess fat. A paediatrician assesses the full picture.
- Puberty changes everything. During growth spurts, BMI can temporarily increase even as a child becomes leaner. A single measurement mid-puberty can be misleading without context.
- Ethnicity differences. CDC charts were developed primarily on US population data. Some research suggests different thresholds may be more appropriate for certain ethnic backgrounds.
- Track trends, not single values. A BMI percentile measured consistently over months and years by a clinician is far more informative than a one-off calculation.
Frequently asked questions
Why can't I use adult BMI categories for my child?
Children's body composition changes with age and puberty. The same BMI value that indicates healthy weight in a 6-year-old may indicate underweight in a 14-year-old. The CDC growth chart percentile method adjusts for these changes — adult thresholds of 18.5, 25, and 30 simply do not apply.
What is a healthy BMI percentile for children?
The CDC defines the 5th to below the 85th percentile as the healthy weight range for children and teens. Being between the 5th and 85th percentile means the child's BMI is within the normal range for their age and sex.
My child is in the overweight category — what should I do?
The first step is to speak with your child's paediatrician. A single BMI-for-age result should never prompt restrictive dieting in children — this can harm both physical and psychological health. A doctor will assess whether weight management guidance is needed and what form it should take.
Can a very muscular or athletic child have a high percentile?
Yes — just like adult BMI, BMI-for-age cannot distinguish muscle from fat. An athletic child with high muscle mass may show a higher-than-expected percentile. In these cases, clinical judgement from a paediatrician is essential to avoid misclassification.
At what age does adult BMI apply?
Adult BMI categories (underweight below 18.5, normal 18.5–24.9, overweight 25–29.9, obese 30+) are generally applied from age 20 onwards. For ages 18–19, some clinicians use adult BMI while others continue to use CDC percentiles. This calculator covers ages 2–19.
Related health calculators
These tools are useful alongside BMI-for-age assessment.
Disclaimer
This BMI-for-age calculator is for educational and screening purposes only. It uses CDC LMS growth chart data with interpolated values. Results should not be used to diagnose weight problems or initiate dietary changes without medical guidance. Always discuss a child's weight and growth with a qualified paediatrician or family physician who can assess full growth history, pubertal development, and clinical context.