Educational tool — not medical advice. Verify with a qualified healthcare professional before making clinical decisions.
How BMI-for-Age Percentiles Work
Body Mass Index (BMI) is calculated by dividing weight in kilograms by the square of height in meters (kg/m²). For adults, fixed BMI cutoffs define weight categories — but for children and teens, BMI changes dramatically with age and differs between boys and girls, making raw BMI numbers meaningless without age context.
That's why pediatric BMI uses BMI-for-age percentiles: your child's BMI is compared to a reference population of children the same age and sex. The CDC growth charts provide the reference data for US children ages 2 to 20.
CDC BMI Weight Categories
- Underweight — below the 5th percentile
- Healthy weight — 5th to less than the 85th percentile
- Overweight — 85th to less than the 95th percentile
- Obese — 95th percentile or above
These categories are screening tools — not diagnoses. A child at the 90th percentile who is tall and muscular may have a perfectly healthy body composition. Your pediatrician considers BMI alongside other factors like growth trends, body composition, family history, and overall health.
Why BMI Has Limitations for Children
BMI doesn't distinguish between muscle and fat. Active, muscular children may have elevated BMI percentiles without excess body fat. Conversely, a child with low muscle mass may have a "normal" BMI despite unhealthy body composition. During puberty, BMI naturally increases as body composition changes — this is expected and doesn't necessarily indicate a problem.
Despite these limitations, BMI-for-age remains the recommended screening tool for childhood weight status because it's simple, non-invasive, and correlates well with body fat at the population level.
BMI-for-Age FAQ
How do I calculate my child's BMI percentile?
Select your child's sex, enter their age (either by birth date or directly), enter their weight and height, then click "Calculate BMI Percentile." The calculator computes BMI from weight and height, then plots it on the CDC BMI-for-age chart for their age and sex.
What age range does this BMI calculator cover?
This calculator uses CDC BMI-for-age charts which are valid for children and teens ages 2 through 20. For children under 2, BMI is not a reliable indicator — use our
Weight-for-Age or
Height-for-Age calculators instead.
Is a high BMI percentile always bad?
Not necessarily. BMI doesn't distinguish between muscle and fat. A tall, athletic child may have a high BMI percentile with healthy body composition. BMI-for-age is a screening tool — not a diagnosis. Your pediatrician can assess whether a high percentile warrants further evaluation based on your child's individual factors.
Why is child BMI different from adult BMI?
Adult BMI uses fixed cutoffs (e.g., BMI ≥ 30 = obese) regardless of age. For children, healthy BMI changes with age and sex as they grow. A BMI of 18 might be normal for a 10-year-old but underweight for a 16-year-old. That's why pediatric BMI uses age- and sex-specific percentiles rather than fixed numbers.
What should I do if my child's BMI is above the 95th percentile?
A BMI above the 95th percentile is classified as obese by screening criteria, but it's important not to over-react to a single reading. Talk to your pediatrician, who can evaluate growth trends, body composition, diet, activity level, and family history to determine if intervention is needed. Focus on healthy habits — not the number.
How accurate is this BMI calculator?
The calculator uses the same CDC LMS reference data and statistical methods used in clinical settings. The accuracy of the result depends on the accuracy of the weight and height measurements you provide. For the most reliable readings, measure height and weight at the same time of day, without shoes, and with light clothing.