About

Adult BMI calculations do not apply to children. While an adult with a BMI of 22 is considered "normal" regardless of age, a 6-year-old with a BMI of 22 would be considered obese. Children's body composition changes rapidly as they grow, and it differs between boys and girls.

This tool calculates the child's BMI and then compares it to the CDC's historical data for their exact age and gender. The result is expressed as a "BMI Percentile." For children, BMI is not a diagnosis but a screening tool to identify potential weight problems.

Formulas

First, standard BMI is calculated:

BMI = weight (kg)height (m)²

Then, the BMI Z-Score is derived using age/gender constants (L, M, S):

Z = (BMI / M)^L − 1L × S

Reference Data

Percentile Range	Weight Status Category
Less than 5th percentile	Underweight
5th to less than 85th percentile	Healthy Weight
85th to less than 95th percentile	Overweight
95th percentile or greater	Obesity

Frequently Asked Questions

Standard calculators are static. They don't account for the fact that a healthy 5-year-old boy has a much lower BMI than a healthy 15-year-old boy. Using an adult calculator will almost always give incorrect weight status classifications for children.

Typically, a child's BMI drops after age 1 and reaches a minimum around age 5-6. After this, it increases through adolescence. This increase is called 'adiposity rebound'. An early rebound (before age 5) is often associated with a higher risk of adult obesity.

No. BMI is a screening tool, not a diagnostic one. It does not measure body fat directly. A muscular child might have a high BMI percentile but normal body fat. A healthcare provider performs further assessments (skinfold thickness, diet history) to diagnose obesity.