What percentile means, how the WHO standard works, the LMS method, and how a z-score turns into a percentile.
A baby growth percentile compares one measurement — weight, length, or head circumference — with a reference group of babies of the same age and sex. The result is expressed as a percentage: the 50th percentile means the measurement sits exactly in the middle, with about half of babies measuring less and half measuring more. In the United States, the CDC and the American Academy of Pediatrics recommend using the WHO Child Growth Standards for infants from birth to 24 months, since the WHO reference was built from healthy, predominantly breastfed infants and better reflects optimal growth in that age range. This tool implements that WHO reference directly.
What does a percentile actually tell you? Is the 50th percentile "best"?
Quick answerA percentile is a rank, not a grade. The 50th percentile is not a target — the 15th and the 85th can be equally healthy. What matters far more than any single number is whether a baby's own curve stays consistent over successive well-child visits. A sudden jump or drop across percentile bands is generally more worth discussing with a pediatrician than a stable value at either end of the typical range.
How is the percentile calculated? (the WHO LMS method)
Quick answerFor every month of age and each sex, WHO publishes three parameters: L (skewness), M (median), and S (coefficient of variation) — together called the LMS method. Your baby's measurement is converted into a z-score, then the z-score is converted into a percentile.
The formula is: if L is not zero, z = ((measurement ÷ M)L − 1) ÷ (L × S); if L is zero, z = ln(measurement ÷ M) ÷ S. The z-score is then converted to a percentile using the cumulative standard normal distribution (this tool uses the Abramowitz–Stegun approximation). For example, z = 0 → 50th percentile, z = −1.88 → 3rd percentile, and z = +1.88 → 97th percentile. For ages that fall between whole months, the L, M, and S values are linearly interpolated between the two neighboring months, so an age like "6 months 12 days" is evaluated correctly.
What percentile range is normal? When is a low or high percentile a concern?
Quick answerThe 3rd to 97th percentile is the range generally considered typical. Values below the 3rd or above the 97th are flagged as "outside the typical range" and are worth a conversation with a pediatrician — though even then, many babies at the extremes are perfectly healthy.
- Low percentile (under 3rd): can reflect feeding, absorption, or an underlying issue, but is often simply constitutional (genetically small) if the curve is stable.
- High percentile (over 97th): a rapid rise, especially in weight, is worth monitoring. A sudden jump in head circumference can prompt a neurological evaluation.
- Crossing percentile lines: a jump or drop across two major bands (for example, from the 50th to the 15th) between visits is generally a stronger signal than a single extreme value.
What do weight, length, and head circumference percentiles each tell you?
Weight-for-age reflects short-term nutrition and can shift relatively quickly with illness or feeding changes. Length-for-age (measured lying down, or recumbent, through 24 months — the WHO/AAP standard for this age range) reflects longer-term growth and is slower to change. Head-circumference-for-age is an indirect marker of brain growth and is watched closely in the first two years; it is measured with a soft tape around the largest part of the head, just above the eyebrows and ears.
What is weight-for-length, and why check it alongside weight-for-age?
Quick answerWeight-for-length compares a baby's weight with same-sex babies of the same length, not the same age. It's a proportionality check: a shorter baby is expected to weigh less, so a low weight-for-age percentile can simply reflect a shorter baby who is perfectly proportionate for their length — which weight-for-length would show as typical. Enter both weight and length above and this tool calculates it automatically alongside the age-based percentiles.
Why does the US use WHO standards for babies but CDC charts for older children?
Quick answerThe CDC and AAP recommend the WHO growth standards for birth through 24 months, and the CDC growth charts for ages 2 through 20. WHO's infant data describes how healthy, breastfed infants in optimal conditions actually grow (a "standard"), while the CDC charts describe how a large sample of US children were observed to grow (a "reference"). This calculator implements only the WHO portion (birth–24 months); it intentionally does not attempt CDC percentiles for children 2 and older, since that requires a different dataset and would be inaccurate if approximated here.
What is the difference between a percentile and a z-score?
Quick answerA z-score states how many standard deviations a measurement is from the median (0 = median, negative = below, positive = above). A percentile is the same information restated as a rank out of 100. Near the extremes (very low or very high), z-scores are more informative than percentiles, because percentiles compress toward 0 or 100 and can't distinguish, say, the 0.1st percentile from the 0.01st the way a z-score of −3 versus −4 can.