Baby Weight Percentile Calculator

About the Baby Weight Percentile Calculator

Weighing is one of the most regular things that happens to a baby, and the number that comes back is usually a percentile. It is also one of the most misunderstood. A percentile is not a score, and a higher one is not better. It simply says where a baby sits among healthy babies of the same age and sex.

This baby weight percentile calculator uses the WHO Child Growth Standards, the charts used for babies in the UK and recommended by the CDC for under-twos in the United States. It looks up the WHO reference for your baby's exact day of age, works out the z-score and percentile, and shows the median and the normal range beside it. For babies born early, it uses corrected age.

How to Use the Baby Weight Percentile Calculator

Choose metric or imperial, and your baby's sex — boys and girls grow differently, so they have separate charts.

Enter the date of birth and the date of the weighing (leave it blank for today).

Enter the weight in kilograms, or in pounds and ounces.

If your baby was born before 37 weeks, enter how many weeks early: 40 minus the weeks of pregnancy at birth. A baby born at 34 weeks is 6 weeks early.

How the Percentile Is Worked Out

Every modern growth chart describes each day of age with three numbers, known as L, M and S:

  M  =  the median weight for that age and sex
  S  =  how spread out weights are (coefficient of variation)
  L  =  a power that corrects for skew

  z  =  ((weight ÷ M)^L − 1) ÷ (L × S)
  percentile  =  the share of a normal distribution below z

Weight is skewed — heavy babies are further above the median than light babies are below it — and the L value straightens that out so the z-score behaves like a normal distribution. For very large or very small values, WHO adds a further adjustment beyond three standard deviations, which the calculator applies.

Step-by-Step Example

A girl weighed on her first birthday at 9.00 kg.

  Age:          365 days
  WHO median:   8.95 kg for girls of this age
  z-score:      0.05
  Percentile:   52nd

About 52 in 100 girls of the same age weigh less. The normal range, from the 3rd to the 97th percentile, runs from about 7.1 to 11.3 kg — a wide band, which is the point: healthy babies vary a great deal.

Why Exact Age Matters

In the first weeks, a baby's weight changes fast and not in a straight line. It usually dips after birth, then climbs steeply. The WHO publishes its reference by day of age for this reason.

While building this calculator, the monthly WHO tables were tested against the daily ones: interpolating between months was out by up to a quarter of a standard deviation in the first weeks — enough to move a baby several percentile points. So the calculator uses the daily reference throughout.

Corrected Age for Premature Babies

A baby born six weeks early is, at three months old, developmentally closer to a six-week-old. Plotting them by age from birth would make them look small for no real reason.

So growth is plotted by corrected age: age from birth minus the weeks born early. This is normally continued until about two years, by which time most premature babies have caught up. Babies still before their corrected due date are plotted on specialist preterm charts instead, which this calculator does not cover.

The Trend Matters More Than the Number

A single percentile tells you where a baby is. The pattern across several weighings tells you how they are growing, and that is what health professionals look at.

Following a curve — staying roughly on the same percentile — is what healthy growth usually looks like, at any percentile.

Small moves are normal, especially in the first weeks and around weaning.

Crossing two of the main percentile lines (for example, from above the 50th to below the 9th) is a common prompt for a review of feeding and health.

Below the 0.4th percentile is also usually reviewed.

Different scales, clothing and a full or empty stomach all shift a single reading, so the most useful comparisons are measurements taken the same way.

Understanding Your Result

Weight percentile is where your baby sits among healthy babies of the same age and sex.

What it means puts that into words.

For comparison gives the median weight for the age and the 3rd to 97th percentile range.

z-score is the underlying measure, in standard deviations from the median.

Age used shows the age the calculator used — corrected, if you entered weeks early.

Worth knowing covers newborn weight loss, the trend, and when a review is usual.

When Should You Use This Calculator?

After a weighing, to understand the number in the red book or app.

To see where a home weighing sits, bearing in mind home scales vary.

For a premature baby, to see the corrected-age percentile.

Not as a substitute for your health visitor's or doctor's assessment.

Common Mistakes

Treating a higher percentile as better. Any percentile in the normal range is fine.

Comparing single readings on different scales. Measure the same way.

Using age from birth for a premature baby. Use corrected age.

Worrying about newborn weight loss. Losing up to about 10% and regaining it by two weeks is normal.

Mixing charts. WHO and older national charts give different percentiles.

If you are worried about your baby's feeding or growth, your health visitor, midwife or doctor is the right person to talk to. Nothing here is medical advice.