BMI Calculator for Kids

Body mass index compares weight with height. For a child, the BMI number alone is not interpreted with the fixed adult cutoffs. Children are still growing, and typical body composition changes substantially with age and puberty. Pediatric assessment therefore compares a child's BMI with values for other children of the same age and sex. This calculator first divides weight in kilograms by height in meters squared, then estimates where that value falls in an age- and sex-relative distribution. A percentile describes relative position, not a test score. A result at the 60th percentile means the calculated BMI is higher than the BMI of about 60 percent of comparable children and lower than the BMI of about 40 percent. Common clinical screening bands classify values below the 5th percentile as underweight, the 5th through 84th percentiles as healthy weight, the 85th through 94th percentiles as overweight, and the 95th percentile or higher as the obesity range. These labels are screening terms rather than diagnoses. Accurate measurements matter. Weigh the child in light clothing, without shoes, on a level scale. Measure standing height against a flat wall with heels together and eyes facing forward. Even a small height error can noticeably change BMI because height is squared in the equation. Growth is best understood as a pattern, so one measurement should be considered alongside prior measurements rather than in isolation. BMI cannot directly measure body fat, muscle, bone density, nutrition, or fitness. Athletic children, children entering puberty early or late, and children with some medical conditions may need more individualized interpretation. Official CDC or WHO growth charts use detailed reference tables and exact age in months. This calculator provides a convenient estimate for general education. Discuss unexpected results, rapid percentile changes, eating concerns, or questions about growth with a pediatrician, who can review the complete growth curve and the child's overall health.