Baby Percentile Calculator
Pediatricians plot every well-baby visit on a growth curve, and the percentile they quote — “she is in the 60th for weight” — compares your baby against a reference population. This calculator uses the WHO Child Growth Standards (the international benchmark for breastfed infants from birth to 24 months) to convert raw measurements into percentiles and z-scores for weight-for-age, length-for-age, and head-circumference-for-age.
How to read your baby's percentile
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1
Enter date of birth and measurement date
The tool calculates exact age in days to match the WHO age-in-months LMS tables.
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2
Enter sex, weight, length and head circumference
Use kilograms and centimetres for best precision; imperial units convert automatically.
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3
The tool applies the LMS method
Each measurement is compared against the WHO reference distribution for age and sex to produce a z-score and percentile.
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4
Read the three percentiles
A child in the 50th is exactly average; 10th-90th is the normal range. Consistency over time matters more than any single number.
What the numbers mean
Percentiles say “X% of same-age same-sex babies in the reference population weigh less than this”. Percentile 50 is median. Percentile 97 means heavier than 97 out of 100 peers.
Z-scores measure in standard deviations: 0 is the mean, +1 is one SD above, -2 is two SD below. The WHO defines flags at ±2 SD (roughly 2nd and 98th percentile) and ±3 SD for severe wasting/overweight.
WHO percentile bands
| Percentile | Z-score | Clinical note |
|---|---|---|
| < 3rd | < -2 | Flag: possible underweight/stunting |
| 3rd-15th | -2 to -1 | Lower end of normal |
| 15th-85th | -1 to +1 | Normal range |
| 85th-97th | +1 to +2 | Upper end of normal |
| > 97th | > +2 | Flag: possible overweight/macrosomia |
WHO vs CDC charts
The WHO standards (2006) describe how breastfed babies should grow under optimal conditions — recommended for all children under 24 months by the CDC.
The CDC charts (2000) describe how a mostly formula-fed US sample did grow in the 1970s-90s — used for children 2-20 in the US.
This calculator uses WHO for ages 0-24 months. For older children, switch to a child growth calculator that uses CDC references.
Tracking over time
Single data points matter less than trajectory. A baby who has stayed near the 25th percentile since birth is perfectly healthy. A baby who dropped from the 75th to the 15th in two months warrants a check-up, even if 15th is technically within normal range.
Frequently Asked Questions
Not on its own. A steady 20th percentile is normal; genetics matter. A sudden drop across bands (crossing two major percentile lines) is what pediatricians watch for.
Breastfed infants grow rapidly in the first months then plateau; formula-fed infants gain more steadily. The old CDC charts (formula-skewed) can make healthy breastfed babies look underweight after 4-6 months. The WHO charts correct for this.
Good enough for trend tracking but not for diagnosis. Clinic scales are calibrated and babies measured naked; home scales include a diaper and clothes. Trust your pediatrician’s numbers over kitchen or bathroom scales.
No. All calculations run in your browser against locally-stored WHO LMS tables. No measurements leave your device.
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