How this instrument works
Body mass index (BMI) alone doesn't say much about a teenager's health — it has to be read against other teens of the same sex and age, because puberty timing varies widely and can shift BMI noticeably within a year or two even for teens on a perfectly typical growth path. That's why pediatric and adolescent BMI is always reported as a percentile, and why this calculator is scoped to ages 13 through 19.
The math runs in two stages. First, BMI is computed the standard way — weight divided by height squared. Second, that BMI number is measured against the CDC's 2000 BMI-for-age reference (ages 2 through 20), which stores three LMS shape parameters — L, M, and S — at each sampled age; the entered BMI becomes a z-score against those parameters, and a logistic function (the same one behind this site's IQ percentile tool) turns that z-score into the reported percentile.
Two things worth flagging up front. This calculator's underlying table samples 10 ages across the full 2-to-20-year CDC range even though this page's inputs and defaults are scoped to roughly 13 through 19 — whatever age you enter still snaps to the closest of those 10 stored points rather than being interpolated. And CDC is explicit that BMI-for-age is a screening measure, not a diagnosis in itself; a percentile far from the middle of the curve is a cue to talk with a doctor about weight, growth, and puberty together, not a standalone conclusion.
- Choose Boy or Girl — the reference curves are sex-specific.
- Enter the teen's age in months (roughly 156 to 228, i.e. 13 to 19 years).
- Enter height in centimeters and weight in kilograms.
- Read the computed BMI, the BMI percentile, the z-score, and the reference median BMI matched for that age and sex.
- Discuss a percentile far from 50, or a fast change over time, with a doctor rather than acting on it alone.
Worked example — three teens checked against the CDC BMI curve
Consider a 16-year-old girl (192.5 months) at 163 cm and 58 kg. Dividing gives BMI = 58 / 1.63² ≈ 21.83, compared with a CDC reference median BMI of 20.453 for girls that age — a gap that works out to z ≈ 0.41 and lands around the 67th percentile, on the upper side of typical for the reference group.
Next, a 14-year-old boy (168.5 months) at 165 cm and 50 kg gives BMI = 50 / 1.65² ≈ 18.37, under the 19.158 reference median BMI for boys that age. That produces z ≈ −0.33 and roughly the 36th percentile — squarely typical, and a reminder that boys' BMI curves often trail girls' somewhat during mid-puberty.
Finally, an 18-year-old girl (216.5 months) at 160 cm and 54 kg computes to BMI = 54 / 1.60² ≈ 21.09, almost matching the 21.275 reference median for that age. The resulting z ≈ −0.06 lands close to the 48th percentile, right at the center of the reference distribution.
Questions
Why is there a separate BMI calculator just for teens?
Puberty timing varies by a year or more between individual teens, and BMI can shift noticeably during that transition even for someone growing entirely typically. This version of the calculator scopes its default age range to roughly 13-19 years, matching this site's general bmi-percentile-child and bmi-kids calculators, which use the identical CDC reference table but default to a wider or younger age range.
Does a high or low teen BMI percentile mean overweight or underweight?
Not on its own. CDC treats BMI-for-age strictly as a screening number, not a diagnosis — an unusual percentile simply flags a teen whose weight relative to height stands out for their age and sex, which is a reason to look closer, not a conclusion in itself. A doctor factors in pubertal stage, growth trend, family history, activity level, and a physical exam before saying anything about a teen's weight status.
Why does this only sample 10 ages instead of every month?
CDC's official BMI-for-age file is far more granular than 10 points, but this calculator's formula engine works from a fixed 10-age subset drawn straight from that file and rounds an entered age to whichever of the 10 is closest — it never blends or estimates between them. A 15-year-and-3-month-old and a 15-year-and-9-month-old could both snap to the same stored age and get identical reference numbers, so treat results as approximate for ages between the stored points.
What do the 85th and 95th percentile cutoffs actually mean?
By CDC convention, a teen's BMI-for-age at or above the 85th percentile is labeled overweight and at or above the 95th is labeled obese, with below the 5th labeled underweight; everything else counts as a typical range. This calculator just reports the raw number and leaves those labels — and what, if anything, to do about them — to a doctor, since the same percentile can carry different weight for different teens.
Is the reported percentile exact?
No — the final step swaps a logistic function in for the true normal cumulative distribution function, the identical shortcut this site's IQ percentile calculator uses elsewhere. That shortcut typically lands within about half a percentile point of an exact table lookup, close enough for a screening estimate but not a substitute for a clinician's own plot on the official chart.
Can I use this calculator for someone older than 19?
The underlying CDC BMI-for-age reference table stops at 20 years, matching this calculator's math, but adult BMI is generally interpreted with fixed thresholds rather than an age-and-sex percentile — this site's separate adult BMI calculator is the more appropriate tool once someone is past their teens.
References
- CDC — 2000 CDC Growth Charts Data Files (LMS parameters, BMIAGEREV 2-20yr BMI-for-age)
- CDC — About Child and Teen BMI: screening measure, not a diagnostic tool
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.