How the Teen BMI Calculator Works
Body Mass Index is calculated the same way at every age: BMI = weight (kg) ÷ height (m)², or in imperial units, BMI = 703 × weight (lb) ÷ height (in)². What changes for teens is the interpretation. Because adolescents are still growing and puberty affects body composition differently by age and sex, the CDC does not use the fixed adult ranges (underweight, normal, overweight, obese at 18.5/25/30). Instead, a teen's BMI is compared against the distribution of BMI values for other kids of the same age and sex, producing a percentile. This calculator computes the exact BMI from your inputs, then estimates that percentile by interpolating standard CDC-pattern BMI-for-age reference curves.
Why percentiles instead of a fixed BMI number
A 13-year-old and an 18-year-old with an identical BMI of 22 are in very different situations: that number would be high for the younger, pre-growth-spurt teen but unremarkable for the older, more fully-grown one. Percentiles correct for this by asking "how does this BMI compare to other teens of the same age and sex?" rather than judging the raw number in isolation. The CDC defines four weight-status categories from the percentile: underweight (below the 5th percentile), healthy weight (5th to under the 85th percentile), overweight (85th to under the 95th percentile), and obesity (95th percentile or above).
How the percentile estimate is calculated
The calculator converts your height and weight to metric units, computes BMI = kg ÷ m², then looks up the approximate 5th, 50th, 85th, and 95th percentile BMI values for the entered age and sex from standard growth-chart reference points (interpolating between ages). Your BMI is placed within those bands to estimate a percentile and category, and the same reference points are used in reverse to show a healthy weight range (5th-85th percentile) at the entered height. Because this uses interpolated reference points rather than the full clinical monthly growth-chart tables, treat the percentile as a close educational estimate — for an exact, clinically-documented percentile, use the CDC's official BMI Percentile Calculator for Child and Teen or ask a pediatrician.
What this number does not capture
BMI-for-age does not distinguish muscle from fat, so a muscular, athletic teen can show a higher percentile without excess body fat. Puberty timing also varies widely — two 14-year-olds can be at very different growth stages — so a single measurement is less informative than a trend plotted over several checkups. This tool is a screening estimate, not a diagnosis; unusual or borderline results are best discussed with a pediatrician, who can factor in growth history, diet, activity level, and family history.