Baby Growth Percentile Calculator (WHO, 0–24 months)

Understanding the baby growth percentile calculator — concept and principle

This calculator compares a baby's weight, length and head circumference with the WHO Child Growth Standards for ages 0–24 months. It gives the percentile and z-score for each measurement, the median weight and length at that age, and whether all values lie within the usual range of z −2 to +2. Parents and health workers use it between check-ups.

Compared with WHO growth standards — with the default inputs: 48th percentile.

How it is calculated

The WHO standards give L, M (median) and S values for each sex and month
z-score = ((measurement ÷ M)^L − 1) ÷ (L × S)
Percentile = share of the reference population below that z-score (normal distribution) × 100
Values between whole months are interpolated; beyond z ±3 for weight, the WHO restricted method is used

The principle

The WHO Child Growth Standards were built from healthy breastfed children raised in good conditions in six countries, so they describe how babies grow when their needs are met. The LMS method handles the skewed spread of measurements at each age. A z-score of 0 means exactly the median; each unit is one standard deviation. The 50th percentile means half of babies in the reference are smaller. Doctors look at how a baby follows its own curve over time more than at a single point.

Worked example

Defaults: boy, 6 months, 7.9 kg, 67.6 cm, head 43.3 cm.

Median weight at 6 months = 7.93 kg; weight z = -0.04, percentile 48.4.
Median length = 67.6 cm; length z = -0.01, percentile 49.6.
Head z = -0.03, percentile 49.
All z-scores between −2 and +2: yes.

Each point is the calculator re-run with only “Baby’s weight (kg)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Baby’s weight (kg)	Weight-for-age percentile
1.98	0
3.95	0
5.92	0.4
7.9	48.4
9.88	97.9
11.8	100
15.8	100

How to read the result

Where does your result fall?

This baby is almost exactly at the median for all three measurements. Any percentile between the 3rd and 97th can be healthy; what matters more is steady growth along a similar curve. Z-scores below −2 or above +2 are the WHO cut-offs that prompt a closer look, as does crossing two major percentile lines.

Common mistakes

Measuring length standing instead of lying down for under-2s.
Using a preterm baby's chronological age instead of corrected age.
Weighing with clothes or a full nappy.
Worrying about one low or high reading without looking at the trend.

Keep in mind: Not a diagnosis. Discuss growth concerns with your doctor or health visitor, who will measure carefully and look at the whole child.

Frequently asked questions

What is a normal percentile for a baby?

A wide range is normal; consistent growth along a curve matters more than the exact percentile.

Why use WHO standards?

They describe healthy growth of breastfed babies in good conditions and are recommended for ages 0–2 in many countries.

What does a z-score of −2 mean?

About 2.3% of the reference babies are below it; WHO uses it as a cut-off for a closer look.

Sources: WHO Multicentre Growth Reference Study Group, WHO Child Growth Standards (2006): weight-for-age, length-for-age and head circumference-for-age

Checked 9 Nov 2026. WHO Multicentre Growth Reference Study Group, WHO Child Growth Standards (2006): weight-for-age, length-for-age and head circumference-for-age, LMS tables for 0–24 months, and the restricted method for weight z-scores beyond ±3 (WHO Anthro technical report). Tables as distributed in the pygrowup package.

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