Baby Percentile Calculator
HealthEstimate your baby's weight-for-age percentile from 0-24 months using WHO growth reference medians. See how your baby's weight compares to same-age peers.
Weight-for-Age Percentile
32.8
WHO Median Weight for Age
7.9
z-score
-0.44
What is a Baby Percentile?
The Baby Percentile Calculator estimates a baby's weight-for-age percentile using simplified World Health Organization (WHO) growth reference data for infants from birth to 24 months. Enter your baby's age, sex, and current weight, and the calculator compares that weight against the reference median for same-age, same-sex babies to produce a percentile ranking.
This is one of a family of infant growth tools on this site, alongside the Head Circumference Percentile Calculator and Birth Weight Percentile Calculator, each tracking a different growth measurement.
How to use this Baby Percentile calculator
- Enter your baby's age in months (0-24).
- Select your baby's sex โ Boy or Girl.
- Enter your baby's current weight in kilograms.
- Read the Weight-for-Age Percentile result instantly.
- Compare the result to the WHO Median Weight shown to see the reference gap directly.
- Re-run the calculator at each checkup to see how the percentile trends over time.
Formula & Methodology
The calculator interpolates a reference median weight and standard deviation (SD) for the entered age and sex from WHO growth reference points, then computes: z-score = (Baby's weight โ Reference median) รท Reference SD Percentile = ฮฆ(z-score) ร 100, where ฮฆ is the cumulative standard normal distribution. Worked example โ a 6-month-old boy weighing 8.5 kg: Reference median at 6 months (boys) โ 7.9 kg, SD โ 0.9 kg z-score = (8.5 โ 7.9) รท 0.9 = 0.67 Percentile โ ฮฆ(0.67) ร 100 = 75th percentile This means the baby's weight is higher than roughly 75% of same-age, same-sex reference infants.
Frequently Asked Questions
A baby percentile calculator compares a baby's weight against reference growth data for babies of the same age and sex, expressing the result as a percentile โ the percentage of same-age babies that weigh less. It's based on World Health Organization (WHO) growth reference medians for children under 2.
Percentiles between roughly the 5th and 95th are generally considered within the typical range for healthy growth, though what matters most for an individual baby is a steady growth pattern over time rather than a single percentile snapshot.
The calculator compares your baby's weight to the WHO reference median weight for the same age and sex, converts the difference into a z-score using reference standard deviation data, and then converts that z-score into a percentile using the normal distribution.
Not necessarily โ some babies are naturally smaller or larger and track consistently along a lower or higher percentile line throughout infancy. A single percentile reading matters less than whether the trend over multiple checkups is stable or changing significantly.
Boys and girls have different average growth trajectories from birth, so weight-for-age reference data is published separately by sex to give an accurate comparison.
This tool covers babies from birth (0 months) to 24 months, matching the age range most commonly tracked on standard infant growth charts.
This calculator gives a quick reference estimate using simplified WHO reference points; your pediatrician's official growth chart plots your baby's exact measurements against detailed percentile curves and is the more precise and authoritative source.
The [Birth Weight Percentile Calculator](/birth-weight-percentile-calculator/) compares weight at the moment of birth against gestational age in weeks, while this calculator compares a baby's current weight against chronological age in months after birth.
This calculator uses chronological (actual) age rather than adjusted age, so for babies born prematurely, results may read lower than expected; many pediatricians recommend using adjusted age for growth tracking until around 2 years old.
They're similar but not identical โ the WHO reference is based on breastfed infants from multiple countries and is commonly used for children under 2, while the CDC chart is US-based and often used from age 2 onward; this calculator uses simplified WHO-style reference points.
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