# What is my child’s weight percentile?

Finds a child’s weight-for-age percentile and z-score from birth to 20 years, on the WHO growth standards under 24 months and the CDC growth charts from 2 years.

- Page: https://www.acalculator.org/health/weight-percentile-calculator
- JSON spec: https://www.acalculator.org/health/weight-percentile-calculator.json
- Version: e4a97753e68b

## Default answer

Example with the default inputs (Sex Boy, Based on Age, Years 4, Months 0, Weight 35 lb) on the example date Wednesday, September 30, 2026: At 4 years, a weight of 35 lb is at percentile 42.6 on the CDC weight-for-age (2 to 20 years) chart.

## 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 child’s weight. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| percentile | Weight percentile | The share of children of the same sex and age who weigh less, rounded to one decimal place and shown between 0.1 and 99.9. |
| z | Z-score | How many standard deviations the weight is above (+) or below (−) the median for the age. |
| median | Median weight at this age | The 50th percentile weight for the sex and age. |
| low | 5th percentile weight | The weight at the 5th percentile for the sex and age. |
| high | 95th percentile weight | The weight at the 95th percentile for the sex and age. |
| range | Screening range | Where the percentile falls against the screening cut-offs: 2.3rd and 97.7th on the WHO charts, 5th and 95th on the CDC charts. |
| age | Age | The child’s age in years and months. |
| chart | Growth chart | The growth chart used for this age. |

## Method

z = ((weight ÷ M)^L − 1) ÷ (L × S); percentile = Φ(z) × 100, with L, M and S for the sex and age from the WHO table under 24 months and the CDC table from 24 months, read on a straight line between table ages.

## Assumptions

- Under 24 months the page uses the WHO 2006 growth standards; from 24 months to 20 years, the CDC 2000 growth charts, as the CDC advises for US children.
- Age in months is 12 × years + months, or the days from birth to the measurement ÷ 30.4375.
- The percentile is rounded half up to one decimal place and shown between 0.1 and 99.9.
- A percentile compares a child 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 = 6.611702 gives percentile = 5, z = -1.644853, median = 7.934, low = 6.611702, high = 9.481939, range = Between the 2.3rd and 97.7th percentiles, chart = WHO weight-for-age (birth to 24 months). Source: WHO weight-for-age, boys, month 6 (CDC copy of the WHO table): L 0.1257, M 7.934, S 0.10958.
2. sex = female, from = age, years = 9, months = 6.5, weight = 40.933155 gives percentile = 90, z = 1.281552, median = 31.040322, range = Between the 5th and 95th percentiles, chart = CDC weight-for-age (2 to 20 years). Source: CDC wtage.csv, girls, 114.5 months: L −0.902197638, M 31.0403221, S 0.191041375, P90 40.9331551 kg.
3. sex = male, from = dates, born = 2024-01-15, on = 2026-07-15, weight = 13.607771 gives percentile = 53.2, z = 0.080793, age = 2 years 6 months. Source: CDC wtage.csv, boys, between 29.5 and 30.5 months: 912 days ÷ 30.4375 = 29.963 months.

## FAQ

### What does a weight percentile mean?

It is the share of children of the same sex and age in the reference group who weigh less. A boy at the 70th percentile weighs more than 70 of every 100 boys his age on the chart, and less than 30.

### What is a normal weight percentile?

There is no single "normal" percentile. Healthy children sit anywhere on the chart. Doctors look at whether a child follows their own curve over time. As a screening guide, the CDC suggests the 2.3rd and 97.7th percentiles on the WHO charts (under 2) and the 5th and 95th on the CDC charts.

### Why does the calculator switch charts at 2 years?

The CDC advises the WHO growth standards for children under 24 months, because they describe healthy breastfed babies, and the CDC growth charts from 2 to 20 years. The calculator follows that advice, so a child of 23 months and one of 25 months are compared with different charts.

### What is a z-score?

A z-score says how many standard deviations a weight is from the median weight for the age. 0 is the median, +1 is one standard deviation above it and −2 is two below. A z-score of −2 is about the 2.3rd percentile and +2 about the 97.7th.

### Is weight percentile the best measure of a healthy weight?

No. From age 2, doctors use BMI-for-age, which relates weight to height. A tall child can have a high weight percentile and a healthy BMI. Use the BMI calculator for kids for that.

### How do I enter a baby’s age in weeks?

Choose "Birth date" and enter the birth date and the date of the weighing, or enter the months with decimals: 3 weeks is about 0.7 months (21 ÷ 30.4375).

## Sources

- Centers for Disease Control and Prevention, National Center for Health Statistics. CDC growth charts data files: weight-for-age, 2 to 20 years (wtage.csv), with the LMS instructions. https://www.cdc.gov/growthcharts/cdc-data-files.htm
- Centers for Disease Control and Prevention. WHO growth charts data files: weight-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
- Kuczmarski RJ, Ogden CL, Guo SS, et al. 2000 CDC Growth Charts for the United States: methods and development. Vital Health Stat 11. 2002;(246):1-190. https://www.cdc.gov/nchs/data/series/sr_11/sr11_246.pdf
- 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
- Cole TJ, Green PJ. Smoothing reference centile curves: the LMS method and penalized likelihood. Stat Med. 1992;11(10):1305-1319. https://doi.org/10.1002/sim.4780111005