Plot a child's weight, height and head circumference against the WHO growth curves and see where they land — percentile and z-score, from birth to 19 years.
A growth percentile answers one question: where does this child sit among the children of the same age and sex in a reference population? Percentile 40 means that about 40 out of 100 reference children measured less, and about 60 measured more. It is a position on a distribution, not a grade: percentile 10 is not a failing mark, and percentile 90 is not a prize. The reference population itself was chosen because its children were healthy, breastfed, non-smoking-household children from six countries — the WHO Multicentre Growth Reference Study, published in 2006, describes how children grow when nothing gets in the way, and that is the standard this tool uses from birth to 5 years. From 5 to 19 years it switches to the WHO 2007 Growth Reference, which was built from a different dataset and does not meet the first one exactly at the boundary: the median height of a boy steps from 110.0 cm to 110.3 cm between months 60 and 61, and the step is in the WHO tables themselves.
The calculation behind the percentile is a z-score derived from the LMS method (Box-Cox transformation): each age and sex has a triple of parameters — L for the skewness, M for the median, S for the coefficient of variation — and the z-score is ((measurement / M) raised to the power L, minus 1) divided by (L × S). The percentile is then the cumulative normal distribution of that z-score. This page interpolates the LMS parameters between the two neighbouring months rather than rounding to the nearest whole month, because a child of 7 months and 20 days is not a child of 8 months. Where the z-score falls beyond ±3, the WHO recommends flattening the tail to avoid absurd values, and the page follows that rule for weight and BMI.
A single point on a growth curve says surprisingly little. What tells a paediatrician whether growth is going well is the trajectory — the line the child draws over months and years. A child who has always been around percentile 15 and stays there is growing steadily; a child who drops from 75 to 25 deserves a look even though both numbers are comfortably inside the normal range. That is why this tool reports the percentile and then returns the reading to the doctor. It does not classify, does not diagnose, and does not recommend anything. It also does not store anything: the dates, measurements and results stay in your browser and are never uploaded.
z = ((value / M)^L − 1) / (L × S), using WHO LMS parameters. The percentile is Φ(z), the cumulative normal distribution evaluated at z.
Not necessarily. Healthy children are spread across the whole range — someone has to be at percentile 5, and that child can be perfectly well. What matters is the trajectory: a child who has always been around that level and stays there is growing steadily. A sudden drop from a higher percentile is what deserves attention, and the paediatrician is the one who decides.
From the WHO. The 2006 Child Growth Standards cover birth to 5 years and describe how children grow under ideal conditions. The 2007 Growth Reference covers 5 to 19 years and was built from a different study. Both are the tables paediatricians use worldwide.
Because after 10 years weight by itself does not mean much — it only makes sense next to height. The WHO stops publishing weight-for-age at that point and recommends BMI-for-age instead. This page works out BMI automatically when both weight and height are filled in.
Because the WHO 2006 standard and the 2007 reference are two different studies that do not meet exactly at the boundary. The median height of a boy goes from 110.0 cm at 60 months to 110.3 cm at 61 months. The step is in the WHO tables, not in this page.
No. The dates, measurements and results are calculated in JavaScript on your own device. Nothing is uploaded, stored or logged anywhere.
Length is measured lying down, height is measured standing. The WHO uses length until 2 years and height after that, and the two differ by about 0.7 cm for the same child. The measurement technique matters as much as the number.
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