Child BMI Calculator

BMI for children and teens (ages 2–19) uses the same mathematical formula as adult BMI but is interpreted completely differently. Instead of fixed cut-off values, children’s BMI must be compared against age- and sex-specific growth charts using percentile rankings.

BMI formula (same for all ages): BMI = Weight (kg) ÷ Height (m)² BMI (imperial) = 703 × Weight (lb) ÷ Height (in)²

Percentile-based interpretation (CDC/WHO standards):

Why age and sex matter: Children have rapidly changing body composition as they grow. A BMI of 18 is healthy for a 12-year-old girl but underweight for a 17-year-old boy. Fat percentage naturally increases during puberty in girls and decreases in boys — fixed cut-offs cannot capture this variation.

BMI-for-age percentile uses z-scores: z = (BMI − median BMI for age/sex) / standard deviation The z-score is then converted to a percentile using the standard normal distribution.

The spread is the part people skip, and it is the part that matters. It is not one number. A 4-year-old’s BMI clusters tightly, with a standard deviation around 1.4, so half a point of BMI moves the percentile a long way. By 16 the spread has more than doubled, so the same half point barely registers. Any tool that uses one fixed spread for every age will misjudge the youngest children badly, and it will misjudge them in the direction of saying everything is fine.

Worked example: An 8-year-old boy weighs 28 kg and is 130 cm tall: BMI = 28 / (1.30)² = 28 / 1.69 = 16.6

The median for that age is about 15.8, so 16.6 sits a little above the middle, in the 60th to 70th percentile range. Comfortably healthy weight, and nowhere near the 85th percentile line that starts the overweight band.

The same BMI in a 14-year-old boy is a different story. The median there is around 19.2, so 16.6 sits well below the middle, somewhere around the 20th percentile. Still inside the healthy band, but a long way from where the same number sat six years earlier. That is the whole reason a fixed BMI cut-off is useless for children.

On the accuracy of this page. It fits a symmetric bell curve to a distribution that is genuinely skewed: the upper tail is stretched out and the lower one is compressed. That means the middle of the range is reasonably good and the tails drift, usually reading the low end as lower than it is. Clinicians use the CDC LMS parameters, which model the skew properly. Treat anything within a few points of the 5th, 85th or 95th percentile as “go and check a real chart”.

Clinical note: BMI is a screening tool, not a diagnostic one. Healthcare providers use additional assessments (skinfold thickness, dietary history, activity level) before any clinical classification. Never make dietary changes for children based on BMI alone without medical guidance.