How this instrument works
Body mass index (BMI) — weight divided by height squared — means something different at every age in childhood, because the amount of body fat that's typical changes as children grow. That's why pediatric BMI is always reported as a percentile against other children of the same sex and age rather than as a raw number: a BMI of 17 might be perfectly typical for a 6-year-old and unusually low for a 14-year-old.
This calculator first computes BMI from the entered height and weight, then uses the CDC 2000 BMI-for-age growth chart (ages 2 through 20), built with the LMS method (Cole 1990): at each reference age, three parameters — L (skewness), M (median), S (spread) — describe the BMI distribution for that sex and age. The result is converted to a z-score against those parameters, then to a percentile using a logistic approximation to the normal curve, the same approximation already used by this site's IQ percentile calculator.
Two limits worth stating plainly. First, this calculator's reference table holds 10 representative ages spanning 2 to 20 years rather than the CDC file's full month-by-month resolution — it matches the entered age to the nearest of those 10 points rather than interpolating, so a result for an age between two table points is computed as if it were exactly at the closer one. Second, this is a screening tool, not a diagnostic one: CDC itself describes BMI-for-age as a screening measure, not something that diagnoses disease on its own — a result far from the 50th percentile is a prompt to discuss weight, growth, and health together with a pediatrician, not a diagnosis by itself.
One more nuance: for BMI far above the 97th percentile, CDC also publishes separate 2022 Extended BMI-for-Age growth charts designed to track very high BMI more precisely than the original 2000 curves this calculator uses — if a result here sits at or near the top of the scale, a pediatrician's own plot on the extended chart will be more precise than this tool's approximation.
- Choose Boy or Girl — the reference curves are sex-specific.
- Enter the child's age in months (24 to 240, i.e. 2 to 20 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 pediatrician rather than acting on it alone.
Worked example — a 10-year-old boy near the reference median
A 10-year-old boy (120.5 months) at 138 cm and 32 kg: BMI = 32 / 1.38² ≈ 16.80. The CDC 2000 reference median BMI for a boy that age is 16.646, so his BMI sits just above the median — z ≈ 0.08, which lands at about the 53rd percentile, essentially typical for the reference population.
Compare a 6-year-old girl (72.5 months) at 115 cm and 25 kg: BMI = 25 / 1.15² ≈ 18.90, well above the reference median BMI of 15.217 for that age. That gap gives z ≈ 1.66, landing around the 94th percentile — high enough for the reference population at that age that it's a fair topic for her pediatrician to discuss alongside her growth trend and family history, not something to interpret from a single calculation.
Now an 18-year-old boy (216.5 months) at 178 cm and 65 kg: BMI = 65 / 1.78² ≈ 20.52, just below the reference median BMI of 21.896 for that age. z ≈ −0.52, landing around the 29th percentile — comfortably within a typical range for the reference population.
Questions
Is a high or low BMI percentile a diagnosis of overweight or underweight?
No. CDC explicitly describes BMI-for-age as a screening measure, not a diagnostic one — a high or low percentile flags a child whose weight relative to height is unusual for their age and sex, which is a reason for closer clinical evaluation, not a standalone conclusion. A pediatrician weighs the percentile alongside growth trend, family history, diet, activity, and physical exam before drawing any conclusion about a child's weight status.
Why does the calculator only use 10 reference ages instead of every month?
The official CDC 2000 BMI-for-age file publishes LMS parameters at fine age resolution, but this calculator hard-codes a representative 10-point subset of those official values (roughly every two years from age 2 to 20) and matches the entered age to the nearest one — it does not interpolate between ages or invent any in-between value. Results for an age between two table points reflect the closer reference age, so treat them as approximate.
What do the percentile bands (85th, 95th) actually mean?
In CDC's convention, a BMI-for-age at or above the 85th percentile is classified as overweight and at or above the 95th percentile as obese for that age and sex, while below the 5th percentile is classified as underweight; the range in between is typical. This calculator reports the raw percentile and leaves that classification and its clinical implications to a pediatrician, since the same percentile can mean different things for different children.
Why doesn't this calculator handle BMI far above the 97th percentile precisely?
This calculator uses the original 2000 CDC BMI-for-age curves, whose statistical model becomes less precise at the extreme upper tail. CDC now publishes separate 2022 Extended BMI-for-Age growth charts specifically to track very high BMI more accurately; if a result here sits near the top of the range, a pediatrician's plot on the extended chart — or the CDC's own child and teen BMI calculator — will be more precise than this tool's approximation.
How accurate is the percentile number itself?
The percentile is computed with a logistic function as a stand-in for the exact normal cumulative distribution function, which is the same well-documented approximation this site's IQ percentile calculator uses. It differs from the exact table lookup by roughly half a percentile point in most cases for typical BMI values — close enough for a screening read, but not a substitute for a clinician's exact chart plot, especially at the extremes noted above.
What's the difference between this calculator and bmi-kids or bmi-teens on this site?
All three use the identical CDC 2000 BMI-for-age reference table and formula — they differ only in the age range they're scoped and defaulted to. This general calculator covers the full 2-to-20-year CDC range; bmi-kids is scoped to roughly 2-12 years and bmi-teens to roughly 13-19 years, for a more focused input experience within those bands.
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
- CDC Growth Chart Training — BMI-for-age growth charts, incl. 2022 Extended BMI charts
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.