Baby Percentile Calculator
Where your baby's weight, length and head circumference fall on the WHO growth standards for their age and sex, from birth to 5 years. A percentile is a place in line among 100 babies, not a score.
Your baby's percentiles will appear here
Enter sex, age and at least one measurement, then press Show Percentiles.
What a Baby Percentile Means
Line up 100 healthy babies of the same age and sex from lightest to heaviest. Your baby's weight percentile is their place in that line: at the 37th percentile, 37 babies weigh less and 63 weigh more. That is all the number says. It is not a mark out of 100, and the 50th percentile is the middle of the range, not the goal. A baby at the 10th percentile who has been near the 10th since birth is growing exactly as expected; the reading pediatricians care about is not the position but whether it holds steady over time.
This calculator uses the WHO Child Growth Standards (2006), built from about 8,500 healthy, breastfed children in Brazil, Ghana, India, Norway, Oman and the United States. Because those children grew under good conditions, the WHO curves describe how babies should grow, which is why the CDC recommends them for children under 2 in the United States.
Median Weight and Length by Age
For orientation, the WHO median (50th percentile) at common ages. Length is measured lying down until 2 years and standing from 2, which is why the standard shows a small step at 24 months.
Half of healthy babies measure below each of these figures, so a baby under the median is in the company of 50 of every 100 babies.
Worked Example
A 6-month-old girl who weighs 7.0 kg (15 lb 7 oz), measures 65 cm (25.6 in) and has a head circumference of 42 cm (16.5 in):
- Weight for age: 37th percentile (z-score -0.35)
- Length for age: 37th percentile (z-score -0.33)
- Head circumference for age: 44th percentile (z-score -0.16)
All three sit a little under the median and comfortably inside the WHO typical range. Nothing here needs action; a pediatrician would simply note the readings and compare them with the next visit.
Why Percentiles Change Between Visits
Babies do not grow along a ruler. In the first two years many drift toward the size their genes intend, so a large newborn may settle to a lower percentile and a small one climb, without anything being wrong. Measurement itself adds noise: a wriggling baby, clothes on the scale, a different tape or a feed an hour earlier can move a reading by a percentile band. What pediatricians look for is a sustained crossing of two major percentile lines in either direction, or a reading outside the WHO typical range (below -2 or above +2 standard deviations), and even then the first step is a careful re-measurement.
WHO or CDC?
The two charts answer slightly different questions. The WHO standards describe how healthy children grow under good conditions (a standard); the CDC charts describe how American children did grow in survey years (a reference). Since 2010 the CDC has recommended WHO for children under 2 and its own charts from 2 to 19. This tool stays on WHO from birth to 5, so after age 2 a US clinic printout can differ by a few percentile points; that gap is expected and not a sign that either number is wrong.
What This Tool Deliberately Does Not Do
It does not draw a growth curve or keep a history; one measurement cannot show a trend, and the trend is what matters. Members can send a reading to the Growth Tracker, which plots every measurement on the WHO percentile lines over time. It does not report BMI for age (that is the Baby BMI Calculator), compares head circumference only up to 36 months (the window in which it is tracked most closely; WHO publishes the standard to 5 years), does not correct for prematurity, and does not diagnose anything: no percentile can tell you whether a baby is feeding well or developing normally. Those are questions for your pediatrician, who sees the whole child.
Frequently Asked Questions
What does a baby percentile mean?
A percentile is a place in line among 100 healthy babies of the same age and sex. If your baby's weight is at the 37th percentile, 37 of 100 babies weigh less and 63 weigh more. It says where a measurement sits, not whether it is good or bad; there is no percentile a healthy baby is supposed to be at.
What does the 50th percentile mean?
The 50th percentile is the median: half of babies the same age and sex measure less and half measure more. It is the middle of the range, not a target. A baby at the 15th or the 85th percentile is just as likely to be growing well; what pediatricians watch is whether a child stays near their own line over time.
Is a low percentile a problem?
Not on its own. WHO defines the typical range as -2 to +2 standard deviations, roughly the 2nd to the 98th percentile, and about 2 in 100 healthy babies sit below that line simply because someone has to be smallest. What matters more than one number is the trend across visits and the whole picture: feeding, development, family size. A reading outside the typical range is a reason to have it checked by a pediatrician, not a diagnosis.
Why did my baby's percentile drop?
Common reasons are ordinary: a measurement taken differently (clothes on, a wriggling baby, a different scale), a recent illness, a change in feeding, or the normal shift in the first two years as babies settle toward the size their genes intend. Pediatricians look for a sustained crossing of two major percentile lines rather than a single dip. If a drop worries you, ask for a careful re-measurement at your next visit.
Should I use WHO or CDC growth charts?
In the United States the CDC recommends the WHO standards for children under 2 and the CDC charts from 2 to 19. The WHO standards describe how healthy, breastfed children grow under good conditions, which is why they are used as the reference for babies. This calculator uses WHO for birth to 5 years, so from age 2 a reading from your pediatrician in the US may differ a little; that difference is expected.
How do I measure my baby at home?
Weigh your baby naked or in a dry diaper on the same scale each time. For length under 2 years, lay your baby on a flat surface with the head against a fixed edge, gently straighten the legs and mark the heel. For head circumference, run a soft tape around the widest part of the head, just above the eyebrows and ears. Home measurements can differ from the clinic by a centimeter or more, so when you have clinic numbers, use those.
Does this calculator work for premature babies?
It calculates from the age you enter and does not adjust for prematurity. Pediatricians plot babies born before 37 weeks by corrected age (age counted from the due date) until about 2 years. The simple workaround is to enter the due date instead of the birth date. For very early or very small babies, specialised charts are used in the clinic; ask your care team which one.
Does this show BMI for age?
No, on purpose. Baby BMI-for-age has its own WHO standard and its own page here, the Baby BMI Calculator, which reports BMI, percentile and z-score for ages 0 to 5. This page covers weight, length or height, and head circumference.
Related Tools and Reading
- Growth Tracker: every measurement on WHO percentile charts over time (ages 0-5)
- Baby BMI Calculator: BMI for age with percentile and z-score (ages 0-5)
- Child Height Predictor: an estimated adult height range from today's height and both parents' heights (ages 1-17)
- Baby Formula Calculator: how much formula by age and weight
- Child Development Stages: From Birth to 18
Methodology and Sources
Percentiles and z-scores are calculated with the LMS method from the official WHO Child Growth Standards (2006) expanded tables for weight-for-age, length/height-for-age and head-circumference-for-age, birth to 60 months, for boys and girls. Age is counted in completed months, the way growth charts are read. The WHO typical range used on this page is -2 to +2 standard deviations, the cut-offs WHO itself uses. The engine is the same one behind the Growth Tracker, and every value it prints was checked against WHO's published tables before release. The calculator runs entirely in your browser; nothing you enter is sent anywhere unless you choose to save it to your account.
Important Considerations
A percentile describes a position among healthy children, not your child's health, and it cannot detect or rule out any condition. Babies born early, babies with a medical condition, and children over 5 fall outside what this page can read properly. For any concern about feeding, growth or development, the right first step is your pediatrician, who measures carefully, plots the trend and knows your child.