Every parent asks the same question at a checkup: "Is my baby growing the way they should?" A baby weight percentile answers it in one number. It compares your baby's weight to a large reference group of healthy boys and girls the exact same age, then reports where your baby sits on that curve. This calculator uses the official World Health Organization (WHO) Child Growth Standards for weight-for-age from birth to 36 months, the same charts pediatricians use in most of the world.
Enter your baby's sex, age in months, and current weight, and the tool returns the weight-for-age percentile plus the WHO z-score. A baby at the 50th percentile is exactly average; a baby at the 25th is heavier than 25% of reference babies and lighter than 75%. Percentiles are not report cards — they describe body size, not health, intelligence, or future athleticism.
Approximation for quick screening. This tool provides an approximate estimate only. Exact percentiles require WHO/CDC reference tables and browser-side interpolation; consult a clinician for medical decisions. The engine here models the WHO weight-for-age curves with a simplified LMS-type z-score approximation rather than running the official WHO Anthro software tables, so the reported percentile can differ by a few points from a pediatric clinic reading — especially near the extremes (under 3rd or over 97th). Use it to track your own baby's trend and to flag when to ask your pediatrician; never rely on it as a medical diagnosis.
How to Use
- Choose Baby's Sex — boy or girl. The WHO publishes separate growth charts for each sex because boys and girls follow slightly different weight curves.
- Enter Baby's Age in completed months. Type
0for a newborn and move up one month per month of age, up to 36 months. Use whole months; the engine rounds to the nearest published WHO data point. - Enter Baby's Weight in pounds. If your clinic uses ounces, convert them to decimal pounds first — for example, 7 lb 4 oz is 7.25 lb — or weigh your baby on a digital infant scale that reads decimals.
- Read the result card. It shows the converted weight in kilograms, the WHO z-score, the Weight-for-Age Percentile, and a plain-English interpretation.
What the numbers mean in plain English. A percentile of 50 means your baby weighs about as much as the average reference baby. Between 10 and 90 is the broad healthy middle. Below 2 means unusually light for age and is worth a call to the pediatrician; above 98 means unusually heavy. The z-score tells you how many standard deviations above (positive) or below (negative) the median your baby sits — z = 0 is exactly average, z = +1 is a bit above, z = -1 is a bit below. These are screening numbers, not a diagnosis.
The percentile is calculated with the LMS method used by the WHO. For each sex and month, the chart stores three numbers: M (the median weight), S (the coefficient of variation), and L (the skew). The engine turns your baby's weight into a z-score, then maps that z-score to a percentile on a normal curve. This calculator applies that method with WHO median reference values for each age and sex and approximates the same percentiles on the WHO chart; it is not the official WHO anthro software. The 50th percentile always sits at z = 0, the 2.3rd at z = -2, and the 97.7th at z = +2.
How Percentiles and z-Scores Work
A percentile is the share of reference babies your baby weighs the same as or more than. If your 6-month-old daughter is at the 30th percentile, 30% of reference 6-month-old girls weigh less and 70% weigh more.
The z-score is more precise than the percentile for extreme values. WHO bands are:
- Above the 98th percentile (z > +2): very high weight for age.
- 90th–98th (z between +1.28 and +2): high weight.
- 10th–90th (roughly z between -1.28 and +1.28): the healthy, broad middle. About 80% of all babies land here.
- 2nd–10th (z between -2 and -1.28): underweight — worth discussing with your pediatrician.
- Below the 2nd percentile (z < -2): severely underweight — prompt medical review is recommended.
Babies routinely move up and down the chart in the first two years. A newborn might drop to the 20th percentile after losing the expected 5-7% birth weight in the first week, then climb back to the 60th percentile by month four. Small fluctuations are normal; a sharp drop across two major percentile lines is what clinicians watch for.
Examples
Example 1 — The average 12-month-old boy. A 12-month-old boy weighing 21.2 lb converts to 9.62 kg, almost exactly the WHO median (about 9.6 kg). His z-score is about +0.02, which maps to the 50.8th percentile — right in the middle of the healthy range.
Example 2 — The average newborn. A full-term baby girl weighing 7.5 lb at birth (3.40 kg) sits a little above the birth median (about 3.2 kg), around the 70th percentile. A typical birth weight in the United States is about 7.5 lb, and roughly 5% of newborns weigh less than 5.5 lb at delivery.
Example 3 — A small 6-month-old. A 6-month-old girl weighing 15.0 lb (6.80 kg) has a z-score near -0.57, landing around the 28.4th percentile. That is comfortably inside the healthy 10th–90th band and nothing to worry about.
Example 4 — A heavier toddler. A 24-month-old boy weighing 28.0 lb (12.70 kg) sits just above the WHO median of about 12.2 kg, reaching about the 63rd percentile. Still solidly average; only above the 98th percentile would this trigger a conversation about growth rate rather than size.
FAQ
What is a normal baby weight percentile?
There is no single "normal" percentile. Anywhere between the 10th and 90th percentile is considered the healthy range, which covers about 80% of babies. The 50th percentile is simply the statistical average. What matters most is that your baby follows their own curve over time, not which exact line they land on.
Is the 50th percentile the best?
No. The 50th percentile means "average," not "optimal." A healthy baby at the 15th percentile and a healthy baby at the 85th percentile are both growing well. Parents often worry when a child is not on the 50th, but being average is not a goal — steady growth along a consistent curve is.
How do I convert ounces to pounds?
There are 16 ounces in a pound. Divide the ounces by 16 and add them to the pounds. A baby who weighs 8 lb 10 oz is 8 + 10/16 = 8.625 lb. Enter 8.625 in the weight box.
Why do boys' and girls' charts differ?
On average, boys weigh slightly more than girls from birth onward, and the two sexes follow different growth trajectories. The WHO built separate LMS tables so that a boy's weight is judged against other boys and a girl's against other girls, which gives a far fairer comparison.
What percentile should a newborn be?
Most full-term newborns cluster between the 25th and 75th percentile at birth, around 7.5 lb on average. Expect a temporary drop of 5-7% in the first week as newborns shed excess fluid, followed by steady regain over about two weeks.
What if my baby is below the 2nd or above the 98th percentile?
These are the WHO extreme bands. A weight below the 2nd percentile (z below -2) suggests the baby may be underweight; above the 98th (z above +2) suggests very high weight. Neither is automatically a problem, but both should be reviewed with your pediatrician, who will also look at length, head circumference, feeding, and overall trend.
Does this replace my pediatrician's growth chart?
No. This calculator is a quick reference for home use. Pediatricians plot weight, length, and head circumference together and compare them over many visits, which catches issues a single number cannot. Always bring real concerns to your child's doctor.
Related Calculators
Disclaimer
This baby weight percentile calculator is provided for informational purposes only and is not medical advice, diagnosis, or a substitute for professional pediatric care. Percentiles are estimates based on the WHO Child Growth Standards and the exact age, sex, and weight you enter; individual results vary. Never change a baby's feeding, diet, or medical care based on this tool alone, and always consult a qualified pediatrician or health-care provider with questions about your child's growth, nutrition, or weight.