How this calculator works
BMI uses the existing weight-over-height-squared equation, then the CDC 2000 LMS transform compares it with age- and sex-specific reference data. The real CDC table is bundled with this page; its half-month LMS points are linearly interpolated for exact ages between rows. The reference covers ages 24 through 240 months, primarily children and teens aged 2 to 19, with the 20th-birthday endpoint. It is not for children under 2, who need other growth charts. These are growth screening categories, not diagnoses. The original LMS percentiles become less reliable at extreme high BMI and are not the CDC extended severe-obesity method. The result is an estimate for information only and is not medical advice. The result is an estimate for information only and is not medical advice.
Formula
BMI = kg / metres². For L not equal to zero, z = ((BMI/M)^L - 1) / (L x S); for L = 0, z = ln(BMI/M) / S. Percentile = 100 x standard normal CDF(z). Below the 5th percentile: underweight; 5th to below 85th: healthy weight; 85th to below 95th: overweight; 95th and above: obesity.
Worked example
For a boy aged 120.5 months, CDC gives L=-2.765648008, M=16.64613844 and S=0.120112464. A BMI of 16.64613844 gives z=0 and the 50th percentile, in the healthy weight category. Synthetic LMS values in the formula sheet are tested only as maths fixtures, never used as growth reference data.
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Frequently asked questions
Why is the adult category different?
Children grow and body composition changes with age. CDC child categories compare BMI with children of the same age and chart sex rather than adult cutoffs.
Where do the LMS values come from?
They are the CDC 2000 BMI-for-age CSV, stored locally with source URL and checksum. Age interpolation is documented by CDC; there is no external data request when you calculate.
Can this diagnose a growth problem?
No. Growth over time, measurements and clinical context matter. A health professional can interpret the result and use extended charts when appropriate.