What it is
This calculator converts a child’s raw anthropometry into WHO Child Growth Standard z-scores — how many standard deviations a measurement sits from the median of the WHO reference population. It reports several indices at once: weight-for-age, length/height-for-age, weight-for-length/height, and (optionally) head-circumference-for-age. Each tells a different story: acute versus chronic, nutritional versus structural.
The method (LMS)
WHO charts are not symmetric bell curves, so a raw “percent of median” misleads. The LMS method models the distribution at each age/length with three parameters — L (Box-Cox power for skew), M (median), S (coefficient of variation). The z-score is:
z = ((value / M)^L − 1) / (L × S)
The calculator interpolates L, M and S for the exact age or length, applies WHO’s extrapolation rule beyond |z| > 3 (which matters most for detecting SAM and macrocephaly), and auto-selects lying length for children under 2 years and standing height for ≥2 years. A measurement outside the valid WHO range is flagged, never scored as 0 — a deliberate guard against a wasted child silently reading “normal”.
The cut-offs
| Index | z < −2 | z < −3 | z > +2 / +3 |
|---|---|---|---|
| Weight-for-length/height | Moderate wasting | Severe acute malnutrition (SAM) | Overweight / obese |
| Length/height-for-age | Stunting | Severe stunting | — |
| Weight-for-age | Underweight | Severely underweight | — |
| Head circ-for-age | Microcephaly (< −2) | — | Macrocephaly (> +2) |
The headline is weight-for-length/height — the most acute nutritional indicator. The tool surfaces the worst finding so a stunted or wasted child is never reported as “normal” just because one other index is fine.
When to use it
At every well-child visit, on every malnutrition screen, and whenever a single plotted point looks off. Always interpret z-scores as a trajectory across visits, not one snapshot — crossing centiles downward is the signal.
Worked example
A 9-month-old boy, 6.4 kg, 68 cm. Weight-for-length comes back around z −2.3 → moderate wasting; length-for-age near the median; weight-for-age low. The acute (weight-for-length) and chronic (length-for-age) pictures diverge — here, acute wasting on a near-normal length, prompting a feeding and illness review rather than a chronic-stunting workup.
Pitfalls and caveats
- Weight-for-age cannot distinguish wasting from stunting — a tall thin child and a short stocky child can share it. Always pair it with the length/height indices.
- Weight-for-age becomes unreliable after ~10 years (puberty); for 5–18 y in India, plot on IAP 2015 charts — WHO references differ.
- Measurement technique dominates the result — wrong length/height mode, an un-zeroed scale, or a squirming infant swamps the math. Confirm before acting.
- Out-of-range values are flagged, not scored. Treat a flag as “re-measure and re-check”, not “normal”.
Run it: Growth z-scores (WHO)
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.