How to use it
- Choose the Sex, then enter the Date of birth and the Measurement date, or leave the date empty for today.
- Enter the Weight in kilograms and the Length or height in centimetres.
- Choose how the child was measured in How was it measured?, lying down or standing.
- Add the Head circumference if you have it, then read each row of the WHO z-scores and percentiles table.
How the z-score and the percentile are calculated
For each age and sex, WHO publishes three numbers, L, M and S. The z-score of a measurement X is ((X ÷ M)^L − 1) ÷ (L × S), or ln(X ÷ M) ÷ S when L is 0. A z-score of 0 is the WHO median (M); a z-score of −2 or +2 is two standard deviations from it.
The percentile is 100 times the normal distribution function of the z-score: the share of children of the same age and sex who measure less. WHO gives percentiles only between −3 and +3 SD, so beyond those lines the tool shows a bound instead.
From birth to 1,856 days (under 61 months) the WHO Child Growth Standards of 2006 are used, read by each day of age. From 61 to 228 months the WHO Growth Reference of 2007 is used, read by month, with values between two months worked out in between. This is not medical advice: the result is an estimate from the published tables, and a doctor should look at any measurement that worries you.
z = ((X ÷ M)^L − 1) ÷ (L × S)
percentile = 100 × Φ(z), for −3 ≤ z ≤ 3Which measure is read at which age
Length (lying down) is read up to 730 days, which is 24 months, and height (standing) from 731 days. A standing height under 731 days has 0.7 cm added to make it a length; a lying length from 731 days has 0.7 cm taken off to make it a height. These are the WHO Anthro rules.
- Weight-for-age: standards from birth to 5 years, reference from 5 to 10 years. The reference has no weight-for-age after 10 years.
- Length or height-for-age and BMI-for-age: standards to 5 years, reference from 5 to 19 years.
- Weight-for-length: standards from birth to 2 years. Weight-for-height: standards from 2 to 5 years.
- Head circumference-for-age: standards to 5 years only.
What this calculator does not include
- Premature birth: the WHO tables describe children born at term, and the tool does not correct the age for prematurity. Ask your doctor which age to use for a premature child.
- Illness, feeding, family height and other causes of slow or fast growth, which a single measurement cannot show.
- Ages above 19 years, where the WHO tables stop. For BMI-for-age with the classes WHO states from 5 years, use the child BMI percentile calculator.
Worked example: girl of 18 months, 82.0 cm lying down, 10.2 kg
From 1 May 2025 to 1 November 2026 is 549 days: 1 year 6 months 0 days. The age is under 731 days, so the length is read lying down, and the WHO standards are used.
The WHO girls' tables at 549 days give the z-scores and percentiles below, which the tool shows for this child.
| Measure | z-score | Percentile |
|---|---|---|
| Weight-for-age | -0.03 | 48.8 |
| Length-for-age | 0.43 | 66.7 |
| Weight-for-length | -0.34 | 36.6 |
| BMI-for-age | -0.42 | 33.8 |
| Head circumference-for-age | -0.18 | 42.8 |
Frequently asked questions
What does a percentile of 50 mean?
It means half of the WHO children of the same age and sex measure less and half measure more. The 50th percentile is the median, the middle of the reference, not a target: children well above or below it can be healthy.
Is a z-score below -2 a problem?
WHO uses -2 SD as the line for a closer look at a child, not as a diagnosis. A doctor should check the measurement and how it has changed over several visits, because one reading says little on its own.
Why does the tool ask whether the length was measured lying down?
WHO reads length, lying down, under 2 years and height, standing, from 2 years. If the child was measured the other way, WHO adds or takes off 0.7 cm, and the tool does the same so the result matches the WHO table.
Which WHO tables does the tool use?
Birth to 1,856 days (under 61 months) uses the WHO Child Growth Standards of 2006. From 61 months to 228 months (19 years) it uses the WHO Growth Reference of 2007, which covers 5 to 19 years.
Why is a row missing from the table?
A row is left out when WHO has no table for that measure at this age, or the value is outside the table: for example, weight-for-length needs a length between 45 and 110 cm, and head circumference is only in the standards.
Are the figures I enter sent to a server?
No. The calculation runs in your browser and nothing you enter is sent to us.
Sources
- WHO Child Growth Standards: weight-for-age, length/height-for-age, weight-for-length/height, BMI-for-age and head circumference-for-age (expanded tables)
- WHO Growth reference data for 5 to 19 years: LMS tables and the computation note of the z-scores
- WHO Anthro software manual: age, length and height, and the z-score colour bands
Figures and text last checked on Tuesday, November 10, 2026.