Baby Weight Percentile Calculator
Enter your baby's age, sex, and weight to see an estimated weight-for-age percentile on the official WHO Child Growth Standards curve (0–24 months). The math uses the published WHO LMS parameters, so the result matches WHO's own z-score method. This is a quick educational estimate — not a medical device. For clinical decisions, always use official growth charts and your pediatrician.
Baby Details
How the WHO Percentile Is Calculated (LMS Method)
The WHO Child Growth Standards describe each age-and-sex point with three numbers — L (Box-Cox power), M (median), and S (coefficient of variation). Your baby's weight is converted to a z-score, then to a percentile on a standard normal curve:
z = ((weight ÷ M)L − 1) ÷ (L × S) | percentile = Φ(z) × 100
Because weight distributions are slightly skewed, the L exponent reshapes them before the z-score is taken — that is why WHO uses LMS rather than a simple mean-and-SD table. This calculator embeds the published WHO LMS values for every month from 0 to 24 and interpolates between them, so the result matches WHO's own method.
WHO Median Weight by Age (Reference)
These are the WHO median (50th percentile) weights. Use them to sanity-check where the 50th line sits at each age.
WHO weight-for-age median (kg)
| Age months | Boy median kg | Girl median kg |
|---|
Values are the WHO P50 (median) weights. A baby tracking steadily along their percentile band is growing healthily even if the number isn't 'average'.
Reading the Result: What the Percentile Tells You
A 60th percentile means about 60% of same-age, same-sex babies weigh less. What matters most is the trajectory: steady growth along a percentile band is healthy even if the number isn't average. Sudden drops across two or more bands warrant a pediatrician visit. Below the 3rd or above the 97th percentile can flag under- or over-weight concerns but always needs clinical context.
- 3rd–15th percentile: below average — common and fine if the trend is steady; watch for drops.
- 15th–85th percentile: the healthy mainstream range where most babies sit.
- 85th–97th percentile: above average — usually fine; discuss if coupled with rapid gain.
- Below 3rd / above 97th: outside the typical range; worth a clinician's review.
Frequently Asked Questions
What does a growth percentile mean?
It's your baby's relative position: 60th = heavier than ~60% of peers the same age and sex.
Is this calculator a medical tool?
No — an educational estimate from the official WHO LMS data. Use official WHO/CDC charts and your pediatrician for decisions.
How accurate is the WHO LMS method?
The z-score and percentile math match WHO exactly; only the between-month interpolation is approximated, and that is negligible.
When should I be concerned?
Persistent drops across two+ bands, or below 3rd / above 97th percentile — discuss with your clinician.
Why separate charts for boys and girls?
Average weight and growth tempo differ by sex from infancy, so sex-specific curves compare like with like.
Weight-for-age vs weight-for-length?
Weight-for-age ignores proportions; weight-for-length adds context on whether weight suits length. This tool reports weight-for-age only.
References & Sources
- World Health Organization. "WHO Child Growth Standards: Methods and development." Geneva: WHO, 2006. Source of the weight-for-age LMS parameters used here (boys and girls, 0–24 months).
- WHO Multicentre Growth Reference Study Group. "WHO Child Growth Standards based on length/height, weight and age." Acta Paediatrica, 2006;95(S450):76–85.
- Centers for Disease Control and Prevention (CDC). "Clinical Growth Charts." Recommends WHO charts for children from birth to age 2 in the United States.
This tool is for educational use only. It is not a medical device and does not replace professional medical advice. Growth assessment should always combine weight, length/height, head circumference, and clinical judgment.