# What is my baby’s percentile?

Finds a baby’s weight, length and head circumference percentiles and z-scores from birth to 24 months on the WHO child growth standards.

- Page: https://www.acalculator.org/health/baby-percentile-calculator
- JSON spec: https://www.acalculator.org/health/baby-percentile-calculator.json
- Version: 839f87cd972f

## Default answer

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.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| sex | Sex | The child’s sex. Boys and girls have separate growth charts. |
| from | Based on | Enter the age in years and months, or the birth date and the date of the measurement. |
| years | Years | The age in whole years. |
| months | Months | Months past the whole years, 0 to under 12. Decimals are allowed: 2 weeks is about 0.5. |
| born | Birth date | The child’s date of birth. |
| on | Measured on | The date of the measurement. Today, unless you change it. |
| weight | Weight | The baby’s weight. |
| length | Length | The baby’s length, measured lying down. |
| head | Head circumference | Around the head, above the eyebrows and ears. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| weightPct | Weight percentile | The share of babies of the same sex and age with a smaller weight, rounded to one decimal place and shown between 0.1 and 99.9. |
| weightZ | Weight z-score | How many standard deviations the weight is from the WHO median for the age. |
| lengthPct | Length percentile | The share of babies of the same sex and age with a smaller length, rounded to one decimal place and shown between 0.1 and 99.9. |
| lengthZ | Length z-score | How many standard deviations the length is from the WHO median for the age. |
| headPct | Head percentile | The share of babies of the same sex and age with a smaller head, rounded to one decimal place and shown between 0.1 and 99.9. |
| headZ | Head z-score | How many standard deviations the head is from the WHO median for the age. |
| age | Age | The baby’s age in years and months. |

## 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.

## Assumptions

- 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

1. sex = male, from = age, years = 0, months = 6, weight = 7.934, length = 0.648807, head = 0.453384 gives weightPct = 50, weightZ = 0, lengthPct = 10, lengthZ = -1.281552, headPct = 95, headZ = 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.
2. sex = female, from = dates, born = 2025-12-01, on = 2026-09-01, weight = 8, length = 0.7, head = 0.43 gives weightPct = 41, weightZ = -0.228768, lengthPct = 47.6, lengthZ = -0.060537, headPct = 26.7, headZ = -0.620877, age = 9 months. Source: WHO girls, 274 days = 9.002 months, between the month 9 and month 10 rows.
3. sex = male, from = age, years = 1, months = 6, weight = 9 gives weightPct = 4, weightZ = -1.750726. Source: WHO boys weight-for-age, month 18: L 0.0211, M 10.9385, S 0.11119.

## FAQ

### 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.

## Sources

- Centers for Disease Control and Prevention. WHO growth charts data files: weight-for-age, length-for-age and head circumference-for-age, birth to 24 months (CDC copies of the WHO tables). https://www.cdc.gov/growthcharts/who-data-files.htm
- WHO Multicentre Growth Reference Study Group. 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, 2006. https://www.who.int/publications/i/item/924154693X
- WHO Multicentre Growth Reference Study Group. WHO Child Growth Standards: head circumference-for-age, arm circumference-for-age, triceps skinfold-for-age and subscapular skinfold-for-age. Methods and development. Geneva: World Health Organization, 2007. https://www.who.int/publications/i/item/9789241547185
- Grummer-Strawn LM, Reinold C, Krebs NF. Use of World Health Organization and CDC growth charts for children aged 0-59 months in the United States. MMWR Recomm Rep. 2010;59(RR-9):1-15. https://www.cdc.gov/mmwr/preview/mmwrhtml/rr5909a1.htm