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Baby Growth Percentile Calculator

Find your baby's weight, length, and head circumference percentiles from birth to 24 months using the WHO growth standards

By Waqar Mushtaq · Last updated:

Girl
US (lb/oz, in)

A growth percentile compares a baby's measurement with those of other babies of the same age and sex. A baby at the 40th percentile for weight weighs more than 40% of babies their age and less than 60%. This calculator uses the World Health Organization (WHO) Child Growth Standards, which the CDC recommends for all children in the U.S. from birth to 24 months. It gives percentiles and z-scores for weight, length, and head circumference, in metric or US units (pounds and ounces), and plots your baby on the same percentile curves a pediatrician uses. The age can be entered in months and days, and every measurement is optional, so you can check just one.

How Growth Percentiles Are Calculated (WHO LMS Method)

z=(X/M)L−1L⋅Sz = \frac{(X / M)^{L} - 1}{L \cdot S}

X = measurement · L, M, S = WHO parameters for the baby's age and sex (skewness, median, coefficient of variation)

Percentile=Φ(z)×100\text{Percentile} = \Phi(z) \times 100

Φ = standard normal cumulative distribution function

The WHO describes each growth curve with three parameters at every age: M, the median measurement; S, the spread (coefficient of variation); and L, a power that corrects for skew. (Baby weights are skewed toward heavier values, so weight has L ≠ 1; length and head circumference are close to symmetric, so L = 1.) The formula above turns a measurement into a z-score, the number of standard deviations from the median. The standard normal distribution then turns the z-score into a percentile.

Example: a 6-month-old girl weighing 7.3 kg. At 6 months the WHO girls' weight parameters are L = −0.0756, M = 7.297 kg, S = 0.12204. Her z-score is ((7.3 ÷ 7.297)^−0.0756 − 1) ÷ (−0.0756 × 0.12204) ≈ 0.003, so she's at the 50th percentile, right on the median.

Details this calculator handles:

  • Age between months. The WHO parameters are published for each completed month; for ages in between (e.g. 4 months 15 days), they're interpolated linearly.
  • Very high or very low weights. Beyond ±3 SD, the WHO's "restricted application" replaces the curved LMS tail with a straight-line extension. This keeps extreme weights from getting distorted z-scores, and this calculator applies it.
  • Screening cutoffs. For the WHO charts, the CDC recommends the 2.3rd and 97.7th percentiles (±2 SD) as the points for a closer look, rather than the 5th/95th used on CDC charts for older children. Results outside that range are highlighted.
  • Implausible values. Measurements far outside the WHO range (for example, pounds entered as kilograms) are flagged as likely errors.

The parameters come from the CDC's published WHO data files, and the tests check that each published percentile value gives back exactly that percentile.

Use Cases

Between Pediatric Visits

Weigh and measure at home and see where your baby falls on the same WHO curves the pediatrician uses. Home measurements, especially length, are less precise than clinical ones, so expect some difference.

Understanding a Well-Child Visit

Enter the weight, length, and head circumference from your baby's visit summary to see the percentiles and z-scores behind "she's doing great" -- and what the numbers mean.

Tracking a Trend Over Time

Percentiles are most useful as a series. Check each visit's measurements to see whether your baby is tracking along a steady percentile line, which matters more than any single number.

Converting Between Pounds/Ounces and Kilograms

Switch between US and metric units to compare a U.S. visit record with international growth charts or guidance, without converting by hand.

Frequently Asked Questions

What percentile is normal for a baby?

There isn't a single "normal" percentile. A healthy baby can be at the 5th percentile or the 95th. What pediatricians watch is the trend: a baby who stays near the same percentile line over time is usually growing well. The CDC suggests a closer look below the 2.3rd or above the 97.7th percentile on WHO charts, or when a baby crosses two or more major percentile lines.

Why does this use WHO charts instead of CDC charts?

The CDC recommends the WHO growth standards for children under 2 in the U.S. The WHO standards describe how healthy breastfed babies grow under good conditions, which makes them a better reference for infants. CDC growth charts are recommended from age 2 onward.

What is a z-score?

A z-score is how many standard deviations a measurement is above or below the median for the baby's age and sex. A z-score of 0 is the 50th percentile, −2 is about the 2.3rd percentile, and +2 is about the 97.7th. Clinicians often prefer z-scores because they keep spreading out at the extremes, where percentiles bunch up near 0 and 100.

Should I correct for prematurity?

For babies born early, pediatricians usually plot growth using corrected (adjusted) age until about 2 years: chronological age minus the number of weeks born early. For example, a 6-month-old born 8 weeks early has a corrected age of about 4 months. Enter the corrected age here if that's what your pediatrician uses.

Is this a substitute for a pediatrician's assessment?

No. Percentiles are a screening tool. A pediatrician interprets them alongside the baby's growth history, feeding, development, the parents' sizes, and a physical exam. If a result worries you, especially one outside the highlighted range or a big jump between visits, bring it up at your next appointment.

Methodology

The formula above is implemented as a standalone module covered by unit tests, so its worked examples are re-checked on every change to the site. It is verified against the authoritative sources listed below.

Not medical advice. This calculator is a general informational tool and cannot account for your individual circumstances. See the terms of use.

Maintained by Waqar Mushtaq. Spotted a result that looks wrong? Report it and it becomes a failing test.

Sources

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