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. This historical reference is provided for comparison with CDC 2000 charts, rather than as a replacement for the WHO standards recommended under age 2. Measure length lying down. These charts are not a premature-infant reference and do not apply a gestational-age correction. 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. Reference curves invert the LMS equation: X = M(1 + LSz)^(1/L), or M exp(Sz) for L = 0.
Worked example
A newborn boy weighing 3.530203168 kg, measuring 49.98888408 cm long and with head circumference 35.81366835 cm is at the median of all three CDC male birth references: each percentile is 50%.
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Frequently asked questions
Which chart should guide a baby's care?
CDC recommends WHO standards for infants under age 2. This calculator explicitly uses the older CDC 2000 reference so it can be compared with those published charts.
Does the graph show my baby's growth over time?
It shows reference curves and one entered measurement. It does not store previous measurements or predict future growth.
Why are there three different percentiles?
Weight, length and head circumference describe different measurements and have separate distributions. They do not need to have the same percentile.