How this calculator works
The CDC 2000 reference supplies sex-specific L, M and S values. Exact ages between published half-month rows use linear interpolation of those parameters, as allowed by CDC. The calculation describes a position in that reference population. Repeated accurate measurements and clinical context matter more than one point. The original CDC LMS method becomes less reliable at extreme high BMI; it is not the CDC extended severe-obesity method. A known BMI mode avoids reconstructing measurements when a BMI has already been recorded. The result is an estimate for information only and is not medical advice.
Formula
z = ((X/M)^L - 1)/(L x S), or ln(X/M)/S when L = 0. Percentile = 100 x Phi(z), where Phi is the standard normal cumulative distribution. BMI = kg/metres². CDC categories are below 5th: underweight; 5th to below 85th: healthy weight; 85th to below 95th: overweight; 95th or above: obesity.
Worked example
For a boy aged 120.5 months, BMI 16.64613844 equals the CDC median. The result is z = 0 and percentile = 50%. A girl at 24 months with BMI 16.42339664 also has percentile = 50%.
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Frequently asked questions
Can I enter a BMI from a clinic?
Yes. Select Known BMI and enter the value in kg/m². Weight and height fields are hidden in that mode.
Do adult BMI categories apply?
No. Children need an age- and sex-specific reference. Adult numerical cutoffs cannot replace BMI-for-age screening categories.
Does this cover severe obesity?
It uses the original CDC 2000 LMS reference. Clinical assessment of very high BMI may require the CDC extended charts and other measurements.