What the z-scores mean
WHO classifies growth by how far a z-score is from 0. The labels below are WHO's population definitions; for an individual child, a value outside ±2 is a reason for a closer look, not a diagnosis. In the US, the CDC uses the same ±2 SD cutoffs (the 2.3rd and 97.7th percentiles) on WHO charts for children under 2.
| Z-score | Weight-for-length/height | Length/height-for-age | Weight-for-age |
|---|---|---|---|
| Above +3 | Obesity | Tall for age | Use weight-for-length/height instead |
| Above +2 | Overweight | Tall for age | Use weight-for-length/height instead |
| −2 to +2 | Normal range | Normal range | Normal range |
| Below −2 | Wasting | Stunting (short for age) | Underweight |
| Below −3 | Severe wasting | Very short for age | Very low weight for age |
Sources: WHO NLIS (−2 and +2 definitions); WHO fact sheet (obesity above +3); WHO eLENA (severe wasting below −3). “Tall”, “very short” and “very low” are descriptive, not WHO categories.
How the calculator works
Each z-score comes from the official WHO LMS tables (the same files WHO ships with its Anthro software), interpolated to your child's exact age in days: z = [(X/M)L− 1] / (L × S). For weight-based indicators beyond ±3, WHO's adjusted method is applied, and results so extreme that WHO's software would flag them (for example a weight-for-age above +5) are marked as likely measuring or typing errors.
Under 24 months the calculator uses weight-for-length; from 24 months, weight-for-height. BMI uses the same length or height after the 0.7 cm adjustment. For what percentiles and z-scores mean in everyday terms, see what a z-score on a growth chart means, what baby percentiles mean and WHO vs. CDC growth charts.
Frequently asked questions
What is a z-score on a growth chart?▾
A z-score (or SD score) says how many standard deviations a measurement is from the median for a child's age and sex. 0 is the median (50th percentile), −2 is about the 2.3rd percentile and +2 about the 97.7th. Unlike percentiles, z-scores stay evenly spaced at the extremes, which is why clinicians and researchers use them.
What z-score is considered normal?▾
WHO treats z-scores between −2 and +2 as the normal range. Below −2 for length/height-for-age is called stunting, below −2 for weight-for-age is underweight, and below −2 for weight-for-length/height is wasting. Above +2 for weight-for-length/height is overweight and above +3 is obesity in children under 5.
Why does the calculator ask whether length was measured lying down or standing?▾
WHO standards use lying-down length under 24 months and standing height from 24 months. Children are about 0.7 cm shorter standing, so WHO adds 0.7 cm to a standing height under 2 and subtracts 0.7 cm from a lying-down length at 2 and older. The calculator does the same.
How are z-scores beyond ±3 calculated?▾
For weight-based indicators, WHO's Anthro software measures values beyond ±3 SD in units of the distance between the 2 and 3 SD curves, instead of using the LMS formula directly. That keeps very high or very low weights from producing exaggerated z-scores. This calculator follows the same method.
Does my pediatrician use z-scores?▾
US pediatricians usually talk in percentiles and use WHO charts under 2 and CDC charts from age 2. Z-scores are common in specialist clinics, hospitals and nutrition programs, and for children far from the median, where percentiles bunch up near 0 or 100.
Sources
- WHO: Malnutrition in children (definitions of stunting, wasting, underweight and overweight)
- WHO fact sheet: Obesity and overweight (cutoffs for children under 5)
- WHO eLENA: Identifying severe acute malnutrition in infants under 6 months (weight-for-length below −3)
- WHO Child Growth Standards: methods and development (2006)
- WHO Anthro software, source code (z-score method, 0.7 cm length/height adjustment, implausible-value flags)
- CDC MMWR (2010): Use of WHO and CDC growth charts for children aged 0–59 months in the United States