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About this tool
BMI for ages 10-20 read against age- and sex-specific IOTF cut-offs instead of the adult 25 and 30 lines.
The Teen BMI Calculator computes body mass index as weight in kilograms divided by height in metres squared, then reads it against age- and sex-specific cut-offs rather than the adult lines at 25 and 30. It uses the International Obesity Task Force curves published by Cole and colleagues in the BMJ in 2000, which are constructed to pass through exactly 25 and 30 at age 18 — so a 12-year-old boy crosses into overweight at about 21.2, not 25. It is aimed at parents and teenagers who want the reading interpreted for the right age instead of an adult chart.
Open Teen BMI Calculator on AltFTool — it loads instantly in your browser.
Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
Reads BMI against the cut-off for that exact age and sex, interpolated between half-year points.
Displays what the adult 25/30 chart would have said, so misclassification is visible rather than hidden.
Converts each cut-off into the actual kilogram figure at that height.
There is no single number — the overweight line sits at about 21.9 for a 13-year-old boy and 22.6 for a girl, rising to 25 for both by age 18. A BMI below that line rules out overweight, but growth patterns vary so much through puberty that a clinician's growth chart remains the better tool for any borderline result.
Because body fatness changes sharply through childhood and puberty, so the adult lines at 25 and 30 misclassify adolescents. A BMI of 24 is already in the obesity range for a 10-year-old but entirely normal at 18, which is why age- and sex-specific curves are used up to the eighteenth birthday.
They are international child and adolescent overweight and obesity thresholds published in the BMJ in 2000 by Cole, Bellizzi, Flegal and Dietz. Each curve was drawn through data from six countries so that it reaches exactly 25 (overweight) and 30 (obesity) at age 18, making childhood readings comparable with the adult definitions.
No, and that is deliberate. A low adolescent BMI can reflect a normal late growth spurt, a constitutional build or a genuine problem, and telling them apart needs height, weight and growth velocity plotted over time by a clinician. If a teenager's weight is falling or they are eating very little, see a doctor rather than relying on any single index.
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