# Baby Growth Percentile Calculator

> Enter your baby's weight, length or head circumference and date of birth to see their percentile on the WHO growth charts, the charts US pediatricians use from birth to age 2.

- Interactive calculator: https://www.leoella.com/tools/baby-percentile-calculator
- Plain-text version: https://www.leoella.com/tools/baby-percentile-calculator.md
- Category: Baby & Toddler
- Publisher: Leo&Ella (https://www.leoella.com)
- Last reviewed: October 19, 2026

## How it works

The calculator works out the baby's age from the birth date and the measurement date (today by default), at 30.4375 days a month, and reads the WHO L, M and S values for that sex, measure and age from the CDC's WHO data files, interpolating between whole months from birth to 24 months. Each measurement (weight in kilograms, length or head circumference in centimeters, converted from pounds, ounces or inches) becomes a z-score with the LMS formula, z = ((value / M) to the power L, minus 1) / (L x S), with the WHO's adjustment beyond 3 standard deviations for weight, and the z-score becomes a percentile through the normal distribution. Results below the 2.3rd or above the 97.7th percentile are flagged, following the CDC.

## Key facts

- The CDC recommends that US clinicians use the 2006 WHO growth standards for children under 24 months and the CDC growth charts from 2 to 19 years. [2]
- With the WHO charts, the CDC recommends screening with the 2.3rd and 97.7th percentiles, two standard deviations either side of the median, printed on the charts as the 2nd and 98th. [2]
- The CDC notes that the 2.3rd and 97.7th percentile cutoffs on the WHO charts are statistical and not based on health outcomes, so a baby outside them needs a closer look, not a diagnosis. [2]
- The WHO growth standards were built from healthy, breastfed babies in six countries: Brazil, Ghana, India, Norway, Oman and the United States. [2]
- The CDC notes that slower growth among breastfed infants between about 3 and 18 months is normal on the WHO charts, which describe how breastfed babies grow. [2]
- The CDC publishes the WHO charts' L, M and S values for each month from birth to 24 months, for weight-for-age, length-for-age and head circumference-for-age. [1]

## Reference data

The 2nd and 98th percentile columns are the lines at 2 standard deviations below and above the median (the 2.3rd and 97.7th percentiles), the cutoffs the CDC recommends with the WHO charts.

### Weight-for-age, girls (WHO, birth to 24 months)

| Age | 2nd (2.3rd) percentile | 50th percentile | 98th (97.7th) percentile |
| --- | --- | --- | --- |
| Birth | 2.4 kg (5 lb 4 oz) | 3.2 kg (7 lb 2 oz) | 4.2 kg (9 lb 5 oz) |
| 1 month | 3.2 kg (7 lb 0 oz) | 4.2 kg (9 lb 4 oz) | 5.5 kg (12 lb 1 oz) |
| 2 months | 3.9 kg (8 lb 11 oz) | 5.1 kg (11 lb 5 oz) | 6.6 kg (14 lb 10 oz) |
| 4 months | 5.0 kg (11 lb 1 oz) | 6.4 kg (14 lb 3 oz) | 8.2 kg (18 lb 2 oz) |
| 6 months | 5.7 kg (12 lb 10 oz) | 7.3 kg (16 lb 1 oz) | 9.3 kg (20 lb 9 oz) |
| 9 months | 6.5 kg (14 lb 4 oz) | 8.2 kg (18 lb 2 oz) | 10.5 kg (23 lb 4 oz) |
| 12 months | 7.0 kg (15 lb 8 oz) | 8.9 kg (19 lb 12 oz) | 11.5 kg (25 lb 6 oz) |
| 18 months | 8.1 kg (17 lb 12 oz) | 10.2 kg (22 lb 9 oz) | 13.2 kg (29 lb 2 oz) |
| 24 months | 9.0 kg (19 lb 15 oz) | 11.5 kg (25 lb 5 oz) | 14.8 kg (32 lb 12 oz) |

### Weight-for-age, boys (WHO, birth to 24 months)

| Age | 2nd (2.3rd) percentile | 50th percentile | 98th (97.7th) percentile |
| --- | --- | --- | --- |
| Birth | 2.5 kg (5 lb 7 oz) | 3.3 kg (7 lb 6 oz) | 4.4 kg (9 lb 12 oz) |
| 1 month | 3.4 kg (7 lb 8 oz) | 4.5 kg (9 lb 14 oz) | 5.8 kg (12 lb 13 oz) |
| 2 months | 4.3 kg (9 lb 8 oz) | 5.6 kg (12 lb 4 oz) | 7.1 kg (15 lb 10 oz) |
| 4 months | 5.6 kg (12 lb 4 oz) | 7.0 kg (15 lb 7 oz) | 8.7 kg (19 lb 5 oz) |
| 6 months | 6.4 kg (14 lb 0 oz) | 7.9 kg (17 lb 8 oz) | 9.8 kg (21 lb 11 oz) |
| 9 months | 7.1 kg (15 lb 12 oz) | 8.9 kg (19 lb 10 oz) | 11.0 kg (24 lb 5 oz) |
| 12 months | 7.7 kg (17 lb 1 oz) | 9.6 kg (21 lb 4 oz) | 12.0 kg (26 lb 7 oz) |
| 18 months | 8.8 kg (19 lb 5 oz) | 10.9 kg (24 lb 2 oz) | 13.7 kg (30 lb 2 oz) |
| 24 months | 9.7 kg (21 lb 5 oz) | 12.2 kg (26 lb 13 oz) | 15.3 kg (33 lb 11 oz) |

### Length-for-age, girls (WHO, birth to 24 months)

| Age | 2nd (2.3rd) percentile | 50th percentile | 98th (97.7th) percentile |
| --- | --- | --- | --- |
| Birth | 45.4 cm (17.9 in) | 49.1 cm (19.3 in) | 52.9 cm (20.8 in) |
| 1 month | 49.8 cm (19.6 in) | 53.7 cm (21.1 in) | 57.6 cm (22.7 in) |
| 2 months | 53.0 cm (20.9 in) | 57.1 cm (22.5 in) | 61.1 cm (24.1 in) |
| 4 months | 57.8 cm (22.7 in) | 62.1 cm (24.4 in) | 66.4 cm (26.1 in) |
| 6 months | 61.2 cm (24.1 in) | 65.7 cm (25.9 in) | 70.3 cm (27.7 in) |
| 9 months | 65.3 cm (25.7 in) | 70.1 cm (27.6 in) | 75.0 cm (29.5 in) |
| 12 months | 68.9 cm (27.1 in) | 74.0 cm (29.1 in) | 79.2 cm (31.2 in) |
| 18 months | 74.9 cm (29.5 in) | 80.7 cm (31.8 in) | 86.5 cm (34.1 in) |
| 24 months | 80.0 cm (31.5 in) | 86.4 cm (34.0 in) | 92.9 cm (36.6 in) |

### Length-for-age, boys (WHO, birth to 24 months)

| Age | 2nd (2.3rd) percentile | 50th percentile | 98th (97.7th) percentile |
| --- | --- | --- | --- |
| Birth | 46.1 cm (18.1 in) | 49.9 cm (19.6 in) | 53.7 cm (21.1 in) |
| 1 month | 50.8 cm (20.0 in) | 54.7 cm (21.5 in) | 58.6 cm (23.1 in) |
| 2 months | 54.4 cm (21.4 in) | 58.4 cm (23.0 in) | 62.4 cm (24.6 in) |
| 4 months | 59.7 cm (23.5 in) | 63.9 cm (25.2 in) | 68.0 cm (26.8 in) |
| 6 months | 63.3 cm (24.9 in) | 67.6 cm (26.6 in) | 71.9 cm (28.3 in) |
| 9 months | 67.5 cm (26.6 in) | 72.0 cm (28.3 in) | 76.5 cm (30.1 in) |
| 12 months | 71.0 cm (28.0 in) | 75.7 cm (29.8 in) | 80.5 cm (31.7 in) |
| 18 months | 76.9 cm (30.3 in) | 82.3 cm (32.4 in) | 87.7 cm (34.5 in) |
| 24 months | 81.7 cm (32.2 in) | 87.8 cm (34.6 in) | 93.9 cm (37.0 in) |

### Head circumference-for-age, girls (WHO, birth to 24 months)

| Age | 2nd (2.3rd) percentile | 50th percentile | 98th (97.7th) percentile |
| --- | --- | --- | --- |
| Birth | 31.5 cm (12.4 in) | 33.9 cm (13.3 in) | 36.2 cm (14.3 in) |
| 1 month | 34.2 cm (13.5 in) | 36.5 cm (14.4 in) | 38.9 cm (15.3 in) |
| 2 months | 35.8 cm (14.1 in) | 38.3 cm (15.1 in) | 40.7 cm (16.0 in) |
| 4 months | 38.1 cm (15.0 in) | 40.6 cm (16.0 in) | 43.1 cm (17.0 in) |
| 6 months | 39.6 cm (15.6 in) | 42.2 cm (16.6 in) | 44.8 cm (17.6 in) |
| 9 months | 41.2 cm (16.2 in) | 43.8 cm (17.3 in) | 46.5 cm (18.3 in) |
| 12 months | 42.2 cm (16.6 in) | 44.9 cm (17.7 in) | 47.6 cm (18.7 in) |
| 18 months | 43.5 cm (17.1 in) | 46.2 cm (18.2 in) | 49.0 cm (19.3 in) |
| 24 months | 44.4 cm (17.5 in) | 47.2 cm (18.6 in) | 50.0 cm (19.7 in) |

### Head circumference-for-age, boys (WHO, birth to 24 months)

| Age | 2nd (2.3rd) percentile | 50th percentile | 98th (97.7th) percentile |
| --- | --- | --- | --- |
| Birth | 31.9 cm (12.6 in) | 34.5 cm (13.6 in) | 37.0 cm (14.6 in) |
| 1 month | 34.9 cm (13.8 in) | 37.3 cm (14.7 in) | 39.6 cm (15.6 in) |
| 2 months | 36.8 cm (14.5 in) | 39.1 cm (15.4 in) | 41.5 cm (16.3 in) |
| 4 months | 39.2 cm (15.5 in) | 41.6 cm (16.4 in) | 44.0 cm (17.3 in) |
| 6 months | 40.9 cm (16.1 in) | 43.3 cm (17.1 in) | 45.8 cm (18.0 in) |
| 9 months | 42.5 cm (16.7 in) | 45.0 cm (17.7 in) | 47.5 cm (18.7 in) |
| 12 months | 43.5 cm (17.1 in) | 46.1 cm (18.1 in) | 48.6 cm (19.1 in) |
| 18 months | 44.7 cm (17.6 in) | 47.4 cm (18.7 in) | 50.0 cm (19.7 in) |
| 24 months | 45.5 cm (17.9 in) | 48.3 cm (19.0 in) | 51.0 cm (20.1 in) |

## Frequently asked questions

### What does my baby's percentile mean?

It compares your baby with healthy babies of the same age and sex on the WHO growth charts. A baby at the 40th percentile for weight weighs more than about 40 in 100 of them and less than about 60. It isn't a grade: the 15th and the 85th percentile can both be perfectly healthy.

### Which growth chart does this calculator use?

The WHO Child Growth Standards for birth to 24 months, the charts the CDC recommends US clinicians use for children under 2. From age 2, pediatricians switch to the CDC growth charts. The calculator uses the WHO data files the CDC publishes.

### What percentile is too low or too high?

On the WHO charts, the CDC recommends the 2.3rd and 97.7th percentiles (2 standard deviations from the median, printed as the 2nd and 98th) as the lines for a closer look, rather than the 5th and 95th. Being outside them isn't a diagnosis; your pediatrician looks at the trend over several visits, your baby's health and the family's heights.

### Why is my breastfed baby's percentile dropping?

The WHO charts were built from breastfed babies, so they reflect how breastfed babies normally grow. The CDC notes that slower growth among breastfed infants between about 3 and 18 months is normal. If your baby crosses several percentile lines, ask your pediatrician.

### Should I use my preemie's adjusted age?

The AAP advises using a premature baby's corrected age during the first 2 years to judge where they should be. Ask your pediatrician which age they plot growth by. To see percentiles by adjusted age here, enter your baby's original due date as the date of birth.

### How do I measure my baby at home?

Weigh yourself holding your baby and subtract your own weight, or use a baby scale. Length is measured lying flat, from the top of the head to the heel, which is hard to do alone, and head circumference with a soft tape around the widest part of the head. Measurements from a checkup are the most reliable.

## Sources

1. [CDC: WHO growth chart data files (L, M and S values)](https://www.cdc.gov/growthcharts/who-data-files.htm)
2. [CDC MMWR 2010;59(RR-9): Use of WHO and CDC Growth Charts for Children Aged 0–59 Months in the United States](https://www.cdc.gov/mmwr/preview/mmwrhtml/rr5909a1.htm)
3. [World Health Organization: Child Growth Standards](https://www.who.int/tools/child-growth-standards)

## About this page

This is general information for parents, not medical advice; a doctor, midwife or pediatrician who knows the family should guide individual decisions. The calculator runs in the visitor's browser and stores nothing they enter. When quoting this page, cite "Baby Growth Percentile Calculator, Leo&Ella" and link https://www.leoella.com/tools/baby-percentile-calculator.