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WHO z-score calculator online

Calculate weight-for-age (WAZ), length/height-for-age (HAZ), weight-for-length/height (WHZ) and BMI-for-age (BAZ) z-scores for children from birth to 10 years, using the official WHO Child Growth Standards and WHO Growth Reference. Results show stunting, wasting, underweight and overweight at a glance.

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For research and learning. Clinical decisions should follow your local protocol and a full clinical assessment.

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How to read WHO z-scores

A z-score (standard deviation score) compares a child’s measurement with the median of healthy children of the same age and sex in the WHO reference population. A z-score of 0 means the child is exactly at the median, −1 is one standard deviation below it, and +2 is two standard deviations above it. Because z-scores are on the same scale at every age, they can be averaged, compared between groups and analysed with ordinary statistical tests, which is why public-health surveys and clinical studies report them instead of percentiles.

IndicatorBelow −3 SDBelow −2 SDAbove +2 SDAbove +3 SD
Length/height-for-age (HAZ)Severe stuntingStuntingTallVery tall
Weight-for-length/height (WHZ)Severe wastingWastingOverweightObesity
Weight-for-age (WAZ)Severe underweightUnderweightCheck weight-for-height
BMI-for-age (BAZ)Severe thinnessThinnessOverweightObesity

Under 5 years, a WHZ or BAZ between +1 and +2 is reported as a possible risk of overweight.

Which WHO growth standard is used?

For children from birth to 5 years, the calculator uses the WHO Child Growth Standards (2006), built from the Multicentre Growth Reference Study of breastfed children in six countries. For children aged 5 to 10 years, it uses the WHO Growth Reference (2007). Z-scores are computed from the official LMS tables by exact age in days, and values beyond ±3 SD are adjusted with the WHO restricted method, the same method as WHO Anthro and AnthroPlus.

Tips for accurate measurements

  • Measure length lying down before 2 years and height standing from 2 years.
  • Use dates of birth and measurement when you have them: age in days is more precise than whole months.
  • Weigh with minimal clothing and record weight to 0.1 kg and length to 0.1 cm.
  • With bilateral pitting oedema, weight-based z-scores are not interpretable, so only HAZ is shown.

Z-scores for a whole cohort, automatically

Running a nutrition survey or a paediatric study? In Library Medicine® you add a calculated field to your data-collection form and every child’s WAZ, HAZ, WHZ and BAZ is calculated as data is entered, online or offline. Compare groups with built-in statistics and export the dataset to SPSS, Stata or R.

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Frequently asked questions

What is a WHO z-score?

A z-score tells you how many standard deviations a child’s measurement is above or below the median of the WHO reference population of the same age and sex. A z-score of 0 is the median; −2 and +2 are the usual cut-offs for undernutrition and overweight.

Which WHO standards does this calculator use?

The WHO Child Growth Standards (2006) for children from birth to 5 years and the WHO Growth Reference (2007) for children aged 5 to 10 years, using the official LMS tables and the WHO restricted method beyond ±3 SD.

What do the cut-offs −2 and −3 mean?

Below −2 SD is moderate and below −3 SD is severe: height-for-age below −2 is stunting, weight-for-height below −2 is wasting and weight-for-age below −2 is underweight. Weight-for-height above +2 is overweight and above +3 is obesity.

Why is weight-for-height empty for older children?

WHO defines weight-for-length/height only up to 5 years (lengths 45–110 cm, heights 65–120 cm). From 5 years, use BMI-for-age instead. Weight-for-age is defined up to 10 years.

Should I measure length or height?

Measure recumbent length (lying down) before 2 years and standing height from 2 years. The calculator uses the length tables before 731 days of age and the height tables afterwards.

Can I calculate z-scores for a whole study automatically?

Yes. In Library Medicine® you add a calculated field to your data-collection form and the z-scores are computed for every child as data is entered, then included in your exports and statistics.

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