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. Head circumference is measured around the widest part of the head. This is the historical CDC reference; CDC recommends WHO growth standards under age 2 for current clinical monitoring. Do not use this term-infant reference for premature infants without clinical guidance. 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.
Worked example
At birth, the male reference median head circumference is 35.81366835 cm. Entering that circumference gives z = 0 and percentile = 50%.
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Frequently asked questions
What does the 50th percentile mean?
The measurement equals the reference median for the selected age and chart sex. Half the reference distribution lies below it.
Can a percentile diagnose a growth problem?
No. Measurement technique, health history and the pattern over time need assessment by a health professional.
Which units and ages can I use?
Age accepts months, years or days. The reference range is 0 to 36 months. Lengths convert to centimetres and weight to kilograms before the LMS calculation.