What it is
In children, body mass index is age- and sex-dependent because body composition changes throughout growth. A raw BMI value is therefore meaningless without context — it must be expressed as a BMI-for-age z-score (standard deviations from the reference median) or percentile against a growth standard. This calculator uses the WHO references.
The method
BMI is calculated conventionally:
$$ BMI = \frac{weight\ (kg)}{height\ (m)^2} $$
That value is then compared to the age- and sex-specific WHO distribution to produce a z-score (and percentile). WHO classification cutoffs differ by age band:
Under 5 years (WHO Child Growth Standards):
- Wasting: < −2 SD; severe < −3 SD
- Risk of overweight: > +1 SD
- Overweight: > +2 SD
- Obese: > +3 SD
5–19 years (WHO 2007 reference):
- Thinness: < −2 SD; severe < −3 SD
- Overweight: > +1 SD (≈85th percentile)
- Obese: > +2 SD (≈97th percentile)
Note the cutoffs shift at age 5 — the same z-score does not carry the same label across that boundary.
When to use it
Use at well-child visits and any nutritional assessment to screen for under- and over-nutrition and to track trajectory over time. Trend across serial measurements is more informative than a single point.
Worked example
A 7-year-old boy, 30 kg, 122 cm:
$$ BMI = \frac{30}{1.22^2} = 20.2\ kg/m^2 $$
Against the WHO 5–19 reference for a 7-year-old boy (median ≈15.5), this BMI falls above +2 SD — classified obese, warranting evaluation despite a “normal-looking” absolute number.
Pitfalls and caveats
- Never apply adult BMI thresholds (25/30) to children — they will misclassify across most age bands.
- WHO vs CDC vs IAP references differ. CDC (US) uses percentile-based overweight (≥85th) and obese (≥95th); IAP has India-specific charts. State which reference you used; do not mix them within a patient’s record.
- BMI does not distinguish fat from lean mass — athletic or oedematous children may be misclassified.
- Measurement error in young children (length vs height, clothing, scale calibration) materially shifts z-scores; technique matters.
- Extreme z-scores (e.g., < −5 or implausibly high) usually signal a data-entry or unit error — re-check inputs.
- Correct for prematurity using corrected age in infancy.
Run it now: BMI-for-Age Calculator
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.