For kids and teens ages 2–20 — enter age, sex, weight and height to estimate BMI-for-age percentile, based on CDC growth charts.
The formula itself is the same as for adults — weight in kilograms divided by height in meters squared — but the result is then compared to a BMI-for-age-and-sex percentile chart instead of fixed adult cutoffs, since a healthy BMI naturally shifts as kids grow.
A BMI of 18 might be perfectly typical for a 12-year-old but underweight for a 17-year-old, since body composition changes a lot through childhood and puberty. The percentile places a child's BMI relative to other kids the same age and sex, which is what makes it meaningful.
Under the 5th percentile is generally categorized as underweight, the 5th to under the 85th as a healthy weight, the 85th to under the 95th as overweight, and the 95th percentile and above as having obesity — the same category boundaries the CDC uses.
Children under 2 are tracked on WHO growth standards, while ages 2 through 20 switch to CDC growth charts — a different reference dataset built specifically for that older age range. That's why this is a separate tool from our Baby & Toddler BMI Calculator rather than an extension of it.
No — it's a general estimate based on approximated growth chart data, not a clinical assessment. A single BMI percentile reading, especially near a category boundary, is far less informative than a pediatrician tracking your child's growth pattern over multiple visits.