Baby Weight Percentile Calculator | WHO Growth Standards

What is a baby weight percentile?

A baby weight percentile shows how a baby's weight compares with other babies of the same age and sex. A baby in the 30th percentile weighs more than 30% of babies that age and less than the other 70%. This calculator uses the World Health Organization (WHO) Child Growth Standards to work out that percentile from a baby's weight, age, and sex, for ages from birth to 24 months.

Percentiles do not grade a baby as good or bad. They describe one point on a range of normal sizes. A baby who stays near the same percentile over several months is usually growing well, even if that percentile is far from 50.

How to use the calculator

1. Choose the baby's sex. Boys and girls have separate WHO reference tables, so this changes the result.
2. Enter the weight, in kilograms or pounds.
3. Enter the age, in months or weeks.
4. Read the percentile and the growth chart that appears below it.

The calculator covers ages from birth up to 24 months, or about 104 weeks. It has no data for older children, so an age entered beyond that range shows an error instead of a percentile.

Baby weight percentile formula

WHO does not publish a simple lookup table of percentiles. It publishes three numbers, called L, M, and S, for each sex and each month of age from birth to 24 months:

• L controls the skew of the weight distribution at that age.
• M is the median weight at that age.
• S is roughly how spread out weights are around the median.

These three numbers convert a weight into a z-score, a measure of how many standard deviations the weight is from the median:

$z = \frac{(weight / M)^L - 1}{L \times S}$

(When L is 0, the formula simplifies to z = ln(weight / M) / S.)

The percentile then comes from the standard normal cumulative distribution function, written Φ (the Greek letter phi):

$percentile = 100 \times \Phi(z)$

Φ(z) gives the share of a standard normal distribution that falls below z. A z-score of 0 sits at the 50th percentile; a z-score of about −1.88 sits at the 3rd percentile; a z-score of about 1.88 sits at the 97th.

WHO publishes L, M, and S for each whole month of age. If the age falls between two published months, or is entered in weeks, the calculator converts weeks to months (dividing by 4.348, the average number of weeks in a month) and straight-line interpolates L, M, and S between the two nearest monthly rows before computing the z-score.

Worked example

A 6-month-old boy weighs 7.5 kg. WHO's boys' table gives, at 6 months: L = 0.1257, M = 7.934 kg, S = 0.10958.

weight / M = 7.5 / 7.934 = 0.9453

z = (0.9453^0.1257 − 1) / (0.1257 × 0.10958) ≈ −0.51

percentile = 100 × Φ(−0.51) ≈ 30

So this baby is at about the 30th percentile: he weighs less than about 70% of 6-month-old boys and more than about 30% of them.

For weight entered in pounds, the calculator first converts to kilograms by dividing by 2.20462, then runs the same formula.

Understanding the result

The chart plots the WHO reference curves for the 3rd, 10th, 25th, 50th, 75th, 90th, and 97th percentiles, alongside the entered weight as a single point. The calculator also labels the result:

• Below the 5th percentile: may indicate underweight
• 5th to 24th percentile: below average weight for age
• 25th to 75th percentile: normal weight for age
• 76th to 95th percentile: above average weight for age
• Above the 95th percentile: may indicate overweight

These labels flag a value worth discussing with a doctor. They are not a diagnosis. A baby who has tracked the 10th percentile since birth is often just small-framed. A baby who drops from the 75th percentile to the 25th over a few months is a more useful signal, because the pattern changed, even though 25 is still inside the "normal" band.

WHO standards, briefly

Before 2006, most growth charts described how children in one country actually grew, often based mainly on formula-fed children in the United States. The WHO Child Growth Standards, published in 2006, took a different approach: they describe how children grow under good conditions, using measurements from breastfed infants in six countries (Brazil, Ghana, India, Norway, Oman, and the United States). The American Academy of Pediatrics recommends WHO standards for children under 2, switching to a different (CDC) reference for older children. This calculator implements only the WHO weight-for-age standard, for ages 0 to 24 months; it does not use CDC data.

Frequently asked questions

What does the 50th percentile mean? It is the median: half of babies that age and sex weigh more, half weigh less. It is not a target weight.

Should I worry if my baby is in a low percentile? Not automatically. Genetics matters — small parents often have babies who track a low percentile from birth without any problem. A sudden drop across two or more percentile bands is more worth mentioning to a doctor than a low but steady percentile.

Why does my baby's percentile change between visits? Home and clinic scales can differ by over 100 grams. Weighing right after a feeding versus before one shifts the reading. Growth spurts in length can also temporarily lower the weight percentile before weight catches up.

Does this calculator use CDC growth charts? No. It uses only the WHO weight-for-age standard, which covers birth to 24 months. CDC charts are used for children over 2 and are not implemented here.

What age range does this calculator support? Birth to 24 months (about 0 to 104 weeks). It does not compute a percentile outside that range.

Should I use adjusted age for a premature baby? Enter the baby's age counted from the original due date, not the birth date, until about 2 to 3 years old. This calculator does not do that adjustment automatically; subtract the number of weeks premature from the current age before entering it.

References

1. World Health Organization. (2006). WHO Child Growth Standards: Length/height-for-age, weight-for-age, weight-for-length, weight-for-height and body mass index-for-age: Methods and development. Geneva: World Health Organization.
2. de Onis, M., Garza, C., Victora, C. G., Onyango, A. W., Frongillo, E. A., & Martines, J. (2004). The WHO Multicentre Growth Reference Study: Planning, study design, and methodology. Food and Nutrition Bulletin, 25(1 Suppl), S15–26.
3. Grummer-Strawn, L. M., Reinold, C., & Krebs, N. F. (2010). Use of World Health Organization and CDC growth charts for children aged 0–59 months in the United States. MMWR Recommendations and Reports, 59(RR-9), 1–15.