How baby percentiles work
The WHO Child Growth Standards describe how children should grow under optimal conditions, based on the WHO Multicentre Growth Reference Study of breastfed infants across six countries. For babies under 24 months the standard uses recumbent length — measured lying down — rather than standing height.
- Age in months — enter the baby's completed age in months (decimals like 14.5 are fine).
- Length — measured lying flat, head to heels, in cm or ft/in.
- Result — percentile, z-score, and the WHO median length for that exact age and sex.
Why 24 months is a boundary
At 24 months the WHO standard switches from recumbent length to standing height — and the published tables contain a real ~0.7 cm step at that boundary, because the measurement method changes. Our engine preserves that step exactly as published instead of smoothing it away, and never interpolates across it.
Once your child turns 2, switch to the boys percentile calculator or girls percentile calculator (CDC reference, ages 2–20), or predict their adult height with the child height predictor.