BMI Calculator for Kids

The formula is identical to adults: BMI = weight (kg) ÷ [height (m)]². However, the meaning of the result is completely different. For children and teens, BMI is interpreted against age- and sex-specific reference curves, not fixed thresholds like 18.5/25/30.

The WHO classifies children's BMI-for-age using standard deviations (SD): below −3 SD is severely underweight, −3 to −2 SD is underweight, −2 to −1 SD is mildly underweight, −1 to +1 SD is normal weight, +1 to +2 SD is overweight, +2 to +3 SD is obese, and above +3 SD is severely obese.

Children's bodies change rapidly as they grow. A BMI of 17 may be perfectly healthy for a 7-year-old but indicate underweight for a 15-year-old. The WHO reference tables update the expected range every single month from birth to age 19, which is why accurate age entry is critical.

The healthy weight range corresponds to BMI values between −1 SD and +1 SD for the child's specific age and sex. It is calculated as: min weight = (−1SD value) × height² and max weight = (+1SD value) × height².

BMI-for-age is a useful screening tool recommended by the WHO and CDC. It can identify children who may be at risk of weight-related health problems. However, it does not measure body fat directly and should always be interpreted by a healthcare professional who can consider the full clinical picture.