Baby Growth Percentile Calculator
Compare your baby's weight, height, or head circumference with the official WHO Child Growth Standards — the same curves your pediatrician uses.
How the Percentile Is Calculated
We use the official WHO LMS method — the exact same math behind pediatric growth charts:
z = ((value / M)^L − 1) / (L × S) → percentile
Exact age in days
Growth changes fast: a 3-week-old and a 7-week-old have very different expected weights. We compute your baby's exact age in days from the birth date and interpolate the WHO reference for that precise day.
WHO LMS parameters
For each age and sex, the WHO publishes three parameters (L, M, S) describing the healthy distribution of weight, length, and head circumference, based on 8,440 healthy breastfed children across 6 countries.
From z-score to percentile
Your baby's measurement is converted to a z-score, then to a percentile. P40 means your baby is above 40% of healthy babies of the same age and sex — and less than 60%.
How to Read a Growth Percentile
Percentiles are one of the most misunderstood numbers in parenting. Here is what they actually mean — and what they don't.
- Percentiles are not grades. P85 is not "better" than P30. Both are healthy positions on the curve — they just describe where your baby sits compared to the reference population.
- The trend beats the number. A baby steadily tracking along P15 is doing great. A baby dropping from P75 to P25 in a few weeks deserves a closer look, even though both numbers are "normal".
- P3 to P97 is the typical range. By definition, 3% of perfectly healthy babies are below P3 and 3% are above P97 — being outside the band is a signal to check, not a diagnosis.
- Genetics set the baseline. Tall parents tend to have babies tracking high on length; petite parents the opposite. Your pediatrician always interprets the curve in family context.
When to Talk to Your Pediatrician
Most percentile worries resolve with a simple conversation at the next check-up. These patterns are worth mentioning:
- Crossing two or more percentile lines (for example P75 → P25) in a short period, in either direction.
- Weight that stays flat or drops for several weeks in a young infant, especially alongside fewer wet diapers or low energy.
- Head circumference jumping upward across percentile lines between visits.
- A clear drop in feeding paired with a falling weight curve.
Context matters: growth spurts, illness recovery, starting solids, and even switching from breast milk to formula all cause temporary wobbles in the curve. One odd measurement is rarely meaningful on its own — the pattern over weeks is what counts.
Getting an Accurate Measurement at Home
- 1Always use the same scale. Differences between scales can easily exceed a week of expected weight gain.
- 2Weigh your baby naked or in a dry diaper only. A wet diaper and clothes can add 100–300 g — enough to shift the percentile.
- 3Measure at the same time of day, ideally before a feeding. A full stomach adds real weight.
- 4Write every measurement down with the date. The curve over weeks is what your pediatrician actually reads — single points mislead.
- 5Trust the official measurements from check-ups for decisions. Home scales are for spotting trends between visits, not replacing them.
Sources & Limitations
The percentiles on this page are computed from the official WHO Child Growth Standards (2006): the published LMS tables for weight-for-age, length/height-for-age and head circumference-for-age, boys and girls, from birth to 5 years. Those standards come from the WHO Multicentre Growth Reference Study, which followed around 8,500 healthy children across six countries — they describe how children grow in good conditions, not how one particular population happens to grow. Several national charts are built on the same data, such as the UK-WHO charts used in the British child health record. Two limits are worth knowing: values between two published ages are interpolated, so a result can sit a fraction of a percentile away from a chart plotted by hand, and babies born preterm should be plotted on corrected age, which this calculator does not apply.
- WHO — Child Growth Standards: the official 2006 standards, birth to 5 years
- WHO — Weight-for-age: the published percentile and z-score tables for boys and girls
- WHO — Multicentre Growth Reference Study: how the standards were built (about 8,500 children, six countries, 1997–2003)
- RCPCH — UK-WHO growth charts 0–4 years: the same standards as used in a national child health record
Reference values, not a diagnosis, and not medical advice. A percentile is a position among healthy babies of the same age and sex, not a grade: on its own, a single measurement — low or high — diagnoses nothing. What a doctor follows is the trend across several visits and the whole picture (feeding, development, the parents' build). How the measurement is taken matters too: a bathroom scale, a wriggling baby or a length measured single-handed can easily move the result by several percentiles. For babies born preterm, use corrected age. Any concern, and any sharp change of direction in the curve, belongs with your pediatrician or child health clinic.
Average weight by age
Just want the typical numbers for a specific month? These reference pages list the full WHO percentile ranges — weight, length and head circumference, boys and girls.
Average Weight for a 4-Month-Old (WHO Percentiles)
WHO percentiles P3–P97, with a quick weight check.
Average Weight for a 6-Month-Old (WHO Percentiles)
WHO percentiles P3–P97, with a quick weight check.
Average Weight for a 12-Month-Old (WHO Percentiles)
WHO percentiles P3–P97, with a quick weight check.
Track the whole curve, not just one point
Doudou logs feedings, sleep, diapers, and growth — and draws your baby's WHO percentile curve automatically. Built for both parents, synced in real time.
Related Tools
Frequently Asked Questions
- What is a baby growth percentile?
- A percentile compares your baby's measurement with the WHO reference population of healthy babies of the same age and sex. If your baby's weight is at the 60th percentile, they weigh more than 60% of healthy babies that age — and less than 40%. It describes position, not health: every percentile between P3 and P97 is considered typical.
- Is a low percentile bad?
- No. A baby consistently tracking along the 10th percentile is usually just a naturally small, healthy baby — especially with petite parents. What pediatricians watch for is a sharp change: a baby who drops across two or more percentile lines in a short time. The trend on the curve matters far more than the number itself.
- Which growth standard does this calculator use?
- The WHO Child Growth Standards (2006), based on the Multicentre Growth Reference Study of 8,440 healthy breastfed children in Brazil, Ghana, India, Norway, Oman, and the USA. They describe how children should grow in optimal conditions, and are the reference recommended in most countries — including for the curves in French carnets de santé.
- Why is my baby's percentile different from the one at the doctor's office?
- Three common reasons: a different scale (home scales vary), clothes or a wet diaper (100–300 g), and a different reference (some countries or pediatric software still use CDC or national curves instead of the WHO standards). Small differences of a few percentile points are completely normal.
- Do percentiles work for premature babies?
- Not directly. For babies born before 37 weeks, pediatricians use the corrected age — the age counted from the due date rather than the birth date — usually until age 2. Enter the corrected age's equivalent birth date if you want an approximation, and rely on your pediatric team for the real assessment.
- How often should I weigh my baby?
- Outside medical advice to do otherwise: roughly weekly in the first month, then monthly is plenty. Daily weighing amplifies meaningless fluctuations (feeds, diapers, digestion can move the number by 200 g within a day) and creates stress without adding information.