What is my baby’s percentile?
Enter your baby’s sex, age and weight, and the length and head size if you have them, to see their percentiles on the WHO child growth standards.
- Weight percentile
- 39.8
At 6 months, the weight is at percentile 39.8 on the WHO growth standard.
2.3rd to 97.7th percentile
- Below the 2.3rd percentile
- 2.3rd to 97.7th percentile
- Above the 97.7th percentile
- Weight z-score
- -0.26
- Length percentile
- 44.2
- Length z-score
- -0.15
- Head percentile
- 45.1
- Head z-score
- -0.12
- Age
- 6 months
Answer for the example date Wednesday, September 30, 2026. It changes to today's date when the page loads.
Weight percentile: 39.8. At 6 months, the weight is at percentile 39.8 on the WHO growth standard.
Where does the weight fall?
Where does the length fall?
Where does the head size fall?
This calculator gives general information only. It is not medical advice and does not replace a doctor or other health care provider. Talk with a professional about your results. Terms of use
How to calculate
Finds a baby’s weight, length and head circumference percentiles and z-scores from birth to 24 months on the WHO child growth standards.
Example with the default inputs (Sex Boy, Based on Age, Years 0, Months 6, Weight 17 lb, Length 26.5 in, Head circumference 17 in) on the example date Wednesday, September 30, 2026: At 6 months, the weight is at percentile 39.8 on the WHO growth standard.
Method: z = ((x ÷ M)^L − 1) ÷ (L × S) for each measurement x; percentile = Φ(z) × 100, with L, M and S for the sex and age from the WHO tables, read on a straight line between whole months.
- The WHO 2006 growth standards describe healthy breastfed babies; the CDC advises them for all US children under 2.
- Age in months is 12 × years + months, or the days from birth to the measurement ÷ 30.4375.
- Each percentile is rounded half up to one decimal place and shown between 0.1 and 99.9. The chart bands mark the 2.3rd and 97.7th percentiles, the screening cut-offs the CDC advises with the WHO charts.
- A percentile compares a baby with a reference group. It is not a diagnosis; a doctor looks at growth over time.
Worked examples
Each example is checked against the calculator on every build.
- Sex Boy, Based on Age, Years 0, Months 6, Weight 17.49 lb, Length 25.54 in, Head circumference 17.85 in gives Weight percentile 50, Weight z-score 0, Length percentile 10, Length z-score -1.281552, Head percentile 95, Head z-score 1.644857.Source: WHO boys month 6 (CDC copies of the WHO tables): weight 50th percentile 7.934 kg, length 10th 64.88071 cm, head circumference 95th 45.33835 cm
- Sex Girl, Based on Birth date, Birth date 2025-12-01, Measured on 2026-09-01, Weight 17.64 lb, Length 27.56 in, Head circumference 16.93 in gives Weight percentile 41, Weight z-score -0.228768, Length percentile 47.6, Length z-score -0.060537, Head percentile 26.7, Head z-score -0.620877, Age 9 months.Source: WHO girls, 274 days = 9.002 months, between the month 9 and month 10 rows
- Sex Boy, Based on Age, Years 1, Months 6, Weight 19.84 lb gives Weight percentile 4, Weight z-score -1.750726.Source: WHO boys weight-for-age, month 18: L 0.0211, M 10.9385, S 0.11119
How it works
1. Age in months. Enter the age as years and months (months may have decimals, so 2 weeks is about 0.5), or as a birth date and a measurement date:
- age in months = 12 × years + months, or
- age in months = (measurement date − birth date, in days) ÷ 30.4375.
No answer if the measurement date is before the birth date, or if the age is over 24 months.
2. The tables. The WHO weight-for-age (kg), length-for-age (cm) and head circumference-for-age (cm) standards, each with one row per whole month from 0 to 24 and L, M and S for boys and for girls, used exactly as published.
3. L, M and S at the age. At a whole month the row is used as it is. Between months a₀ and a₀ + 1, each of L, M and S is read on a straight line: L = L₀ + (a − a₀) × (L₁ − L₀), and the same for M and S.
4. Each percentile. For each measurement x entered (weight in kg, length and head circumference in cm):
- z = ((x ÷ M)^L − 1) ÷ (L × S), or z = ln(x ÷ M) ÷ S if L is 0;
- percentile = Φ(z) × 100, where Φ is the standard normal cumulative distribution.
Each percentile is rounded half up to one decimal place (the decimal value decides) and then kept between 0.1 and 99.9. Each z-score is shown to two decimal places. Weight is required; length and head circumference are optional, and their rows are left out when they are empty.
5. The chart bands mark the WHO screening cut-offs the CDC suggests, read from the shown percentile: below 2.3, from 2.3 to 97.7, and above 97.7. The age row shows the age in months rounded half up to one decimal place, as years and months.
Rounding. Every “rounded half up” on this page reads the decimal value: a result within 1e-9 (relative) of a half counts as the half. For example, 80 ÷ 1.6², which a computer stores as 31.249999999999993, rounds to 31.3.
Assumptions
- Φ is computed with Marsaglia’s series (J Stat Softw 2004), which matches the exact value to within 5e-15; percentiles beyond z = ±8 show as 0.1 or 99.9.
- The WHO standards describe babies who were breastfed and grew up in healthy conditions, in six countries.
- Measurements are without clothes or a nappy for weight, and lying down for length.
- One measurement is a snapshot; growth over several visits tells much more.
- This calculator gives general information. It is not medical advice or a diagnosis; talk to your pediatrician about your baby’s growth.
Worked examples by hand
A 6-month-old boy: 7.934 kg, 64.88071 cm long, head 45.33835 cm. These are the WHO table’s own 50th percentile weight, 10th percentile length and 95th percentile head circumference for boys at 6 months. Weight: M = 7.934, so x ÷ M = 1 and z = 0, the 50.0 percentile. Length (L = 1, M = 67.6236, S = 0.03165): z = (64.88071 ÷ 67.6236 − 1) ÷ 0.03165 = −1.2816, the 10.0 percentile. Head (L = 1, M = 43.3306, S = 0.02817): z = (45.33835 ÷ 43.3306 − 1) ÷ 0.02817 = 1.6449, the 95.0 percentile.
A girl born December 1, 2025, measured on September 1, 2026: 8 kg, 70 cm, head 43 cm. The dates are 274 days apart, 274 ÷ 30.4375 = 9.002 months, so each value is read 0.002 of the way from the month 9 row to the month 10 row. Weight: L = −0.150740, M = 8.225923, S = 0.121990; z = ((8 ÷ 8.225923)^−0.150740 − 1) ÷ (−0.150740 × 0.121990) = −0.2288, percentile 41.0. Length: M = 70.146248, S = 0.034440; z = (70 ÷ 70.146248 − 1) ÷ 0.034440 = −0.0605, percentile 47.6. Head: M = 43.830825, S = 0.030530; z = −0.6209, percentile 26.7. The age reads "9 months".
A boy of 1 year 6 months weighing 9 kg (19.8 lb). The WHO boys’ row for month 18 is L = 0.0211, M = 10.9385, S = 0.11119. z = ((9 ÷ 10.9385)^0.0211 − 1) ÷ (0.0211 × 0.11119) = −1.7507, and the percentile is 4.0: in the 2.3 to 97.7 band.
Other questions people ask
What does my baby’s percentile mean?
It is the share of babies of the same sex and age in the WHO standard who measure less. A baby at the 30th percentile for weight weighs more than 30 of every 100 babies of that age and sex, and less than 70.
Is a low or high percentile a problem?
Not by itself. Healthy babies are found across the chart; what matters is steady growth along their own curve. The CDC suggests that doctors look more closely at babies below the 2.3rd or above the 97.7th percentile on the WHO charts, or whose percentile moves a lot between visits.
Why WHO charts and not CDC charts?
The CDC advises the WHO growth standards for all US children under 2. They describe how healthy breastfed babies grow in good conditions, while the older CDC infant charts describe how a mix of US babies grew in the past.
How do I measure length and head circumference?
Length is measured with the baby lying flat, from the top of the head to the heels with the legs straight. Head circumference is the largest distance around the head, just above the eyebrows and ears. Your pediatrician’s measurements are the most reliable.
My baby was born early. Can I use this?
For a baby born before 37 weeks, doctors often use the age corrected for prematurity (the age minus the weeks born early) until about 2 years. Enter the corrected age to do the same. Ask your pediatrician which age to use.
Why are the percentiles so different for weight and length?
Each measurement is compared on its own chart. A long, lean baby can be at the 80th percentile for length and the 40th for weight. That can be completely healthy.