Child BMI Calculator
Calculate BMI and CDC percentile for children and teens ages 2-20. Unlike adult BMI, children's BMI is interpreted using age- and sex-specific percentile charts.
Quick Answer
For children ages 2-20, BMI is calculated the same way as adults (weight/height²) but is interpreted using CDC percentile charts that account for age and sex. Below the 5th percentile is underweight, 5th-84th is healthy weight, 85th-94th is overweight, and the 95th percentile or above is obese. A child's percentile shows how their BMI compares to other children of the same age and sex.
Your Child's Results
Percentile Scale
What This Means
A BMI of 16.3 at age 10 places your child at the 42th percentile for boys of the same age. This means their BMI is equal to or greater than 42% of children their age and sex in the CDC reference population.
CDC Weight Status Categories
About This Tool
Body Mass Index for children and adolescents ages 2 through 20 is calculated using the same formula as adults, weight in kilograms divided by height in meters squared, but is interpreted very differently. Because body composition changes dramatically throughout childhood and differs between boys and girls at every age, a raw BMI number has no fixed meaning for children. Instead, a child's BMI is compared to a reference population of children of the same age and sex using percentile rankings developed by the Centers for Disease Control and Prevention (CDC). These growth charts, last updated in 2000, are based on national survey data collected from the 1960s through the 1990s and represent the historical distribution of BMI among American children.
How Percentiles Work
A percentile indicates where a child's BMI falls relative to other children of the same age and sex. For example, a 10-year-old boy at the 75th percentile has a BMI equal to or greater than 75% of 10-year-old boys in the CDC reference population. This is fundamentally different from adult BMI categories, which use fixed thresholds regardless of age or sex. The percentile approach accounts for the natural changes in body fat that occur during growth. Infants and toddlers typically have higher body fat percentages that decrease during the preschool years. Body fat then increases again during the pre-pubertal period, and the timing and magnitude of these changes differ between boys and girls. By using age- and sex-specific percentiles, the CDC system captures these normal developmental patterns.
Understanding the Weight Categories
The CDC defines four weight status categories for children and teens. Underweight is defined as a BMI below the 5th percentile for age and sex, which means fewer than 5% of children in the reference population have a lower BMI. Healthy weight spans from the 5th to the 84th percentile, encompassing the majority of the reference population. Overweight is defined as the 85th to 94th percentile, indicating a BMI higher than most peers but below the obesity threshold. Obesity is defined at or above the 95th percentile. In recent years, the CDC has also introduced the category of severe obesity, defined as a BMI at or above 120% of the 95th percentile, recognizing that children well above the 95th percentile face significantly elevated health risks compared to those near the cutoff.
Limitations of BMI Percentile in Children
Like adult BMI, child BMI percentile has important limitations. It does not directly measure body fat and cannot distinguish between weight from muscle, bone, or fat. Athletic children with above-average muscle mass may have elevated BMI percentiles despite having healthy body composition. Conversely, a child at a normal BMI percentile could still have an unhealthy body fat percentage if they are physically inactive and have low muscle mass. The timing of puberty significantly affects BMI, as children who enter puberty earlier tend to have higher BMIs than late-maturing peers, even when both are developing normally. Additionally, the CDC growth charts were developed primarily from data on American children and may not perfectly represent growth patterns in children of all ethnic backgrounds. The WHO growth standards, used in many other countries, may be more appropriate for international populations, particularly for children under 5.
When to Talk to a Pediatrician
A single BMI measurement is less informative than tracking BMI percentile over time. Consistent trends, such as a child whose percentile is steadily increasing or decreasing, are more clinically significant than any single data point. If your child's BMI percentile falls outside the healthy weight range, or if you notice a significant change in their growth trajectory, discuss it with their pediatrician. The doctor can evaluate growth in the context of family history, pubertal development, diet, physical activity, and other health markers. They may order additional assessments such as blood pressure, fasting glucose, lipid panels, or thyroid function tests if warranted. It is important to approach weight discussions with children sensitively, focusing on health behaviors rather than numbers, to support a positive relationship with food and body image.