WHO Weight-for-Age Z-Score Calculator

The World Health Organization growth standards describe how children should grow under optimal nutrition and care, based on the Multicentre Growth Reference Study (2006). The weight-for-age Z-score compares a child’s weight to the expected median for their age and sex, expressed in standard deviation units.

Z = (observed weight − median) / standard deviation

A Z-score of 0 means the child is exactly at the median. Z = +1 means one standard deviation above the median (about the 84th percentile). Z = −2 means two standard deviations below (about the 2.3rd percentile, the conventional cutoff for underweight).

Standard interpretation thresholds (WHO):

• Z > +2: weight noticeably above expected (consider obesity assessment, especially with elevated BMI-for-age)
• −2 ≤ Z ≤ +2: within normal range
• −3 ≤ Z < −2: moderately underweight
• Z < −3: severely underweight (medical emergency in young children, often a marker for severe acute malnutrition)

This calculator uses tabulated median and standard-deviation values at key ages (birth, 1, 2, 3, 4, 5, 6, 9, 12, 18, 24, 36, 48, 60 months) and linearly interpolates between them. The full WHO standards use the LMS method, which fits a flexible distribution at every age. The simplified linear interpolation here is accurate to within about 0.1 standard deviations of the official LMS values for the same input.

Worked example: a 12-month-old boy weighing 9.6 kg. WHO median for 12-month boys is 9.65 kg with SD about 1.02 kg. Z = (9.6 − 9.65) / 1.02 = −0.05. Right at the median; weight is normal.

Limitations:

• For ages beyond 60 months, BMI-for-age replaces weight-for-age as the better indicator.
• Z-scores of weight-for-age alone do not distinguish wasting (low weight-for-height, recent malnutrition) from stunting (low height-for-age, chronic malnutrition). For full assessment, also compute height-for-age and weight-for-height Z-scores.
• Premature infants need correction for gestational age; the standards apply to term infants from birth.

Routine well-child visits track Z-scores over time. A single low value triggers investigation; a falling trend across visits is more concerning than a single low number with stable trajectory.