# BMI calculator: kids and teens 2 to 19

Works out a child’s or teen’s BMI, its BMI-for-age percentile on the CDC growth charts (with the CDC extended method above the 95th percentile) and the CDC weight category, for ages 2 to 19.

- Page: https://www.acalculator.org/health/bmi-calculator-for-kids
- JSON spec: https://www.acalculator.org/health/bmi-calculator-for-kids.json
- Version: c2cc8a2ae086

## Default answer

Example with the default inputs (Sex Boy, Based on Age, Years 10, Months 0, Height 4 ft 7 in, Weight 70 lb) on the example date Wednesday, September 30, 2026: At 10 years, a BMI of 16.3 is at percentile 42.6: Healthy weight.

## 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. |
| height | Height | The child’s height without shoes. |
| weight | Weight | The child’s weight. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| category | Weight category | The CDC category: underweight below the 5th percentile, healthy weight to under the 85th, overweight to under the 95th, obesity at or above the 95th, severe obesity at 120% of the 95th percentile or a BMI of 35 or more. |
| bmi | BMI | Body mass index: weight in kilograms divided by height in metres squared, to one decimal place. |
| percentile | BMI-for-age percentile | The share of children of the same sex and age with a lower BMI, rounded to one decimal place and shown between 0.1 and 99.9. |
| z | Z-score | The BMI-for-age z-score: the standard normal value of the percentile. |
| ofP95 | Share of the 95th percentile | The BMI as a percent of the 95th percentile BMI for the sex and age, to one decimal place. |
| healthyFrom | Healthy weight from | The lowest weight at this height whose percentile shows as 5.0. |
| healthyTo | Healthy weight up to | The weight at this height whose percentile shows as 85.0 (just under). |
| age | Age | The child’s age in years and months. |

## Method

BMI = weight ÷ height²; at or below the 95th percentile BMI (P95), z = ((BMI ÷ M)^L − 1) ÷ (L × S) and percentile = Φ(z) × 100; above P95, percentile = 90 + 10 × Φ((BMI − P95) ÷ sigma) and z = Φ⁻¹(percentile ÷ 100) (CDC extended BMI-for-age charts).

## Assumptions

- L, M and S come from the CDC 2000 BMI-for-age table for the sex, read on a straight line between table ages; P95 is the BMI at z = 1.6449; sigma comes from the CDC regression on age in years.
- Age in months is 12 × years + months, or the days from birth to the measurement ÷ 30.4375. The categories are for ages 2 through 19.
- Categories are read from the percentile rounded half up to one decimal place. Severe obesity: BMI at least 120% of P95 (one decimal place) or a BMI of 35.0 or more.
- BMI is a screening measure for children. It does not measure body fat and is not a diagnosis.

## Worked examples

1. sex = female, from = age, years = 9, months = 6.5, height = 1.4, weight = 41.552 gives bmi = 21.2, z = 1.421501, percentile = 92.2, category = Overweight, ofP95 = 94.7%. Source: CDC, Extended BMI-for-age growth charts data files: girl, 114.5 months, BMI 21.2, z = 1.4215, percentile 92.2 (L −2.257782149, M 16.57626713, S 0.132796819).
2. sex = male, from = age, years = 4, months = 2.5, height = 1, weight = 22.6 gives bmi = 22.6, z = 2.831446, percentile = 99.8, ofP95 = 126.8%, category = Severe obesity. Source: CDC, Extended BMI-for-age growth charts data files: boy, 50.5 months, BMI 22.6, P95 17.8219, sigma 2.398288, percentile 99.7683, z 2.83.
3. sex = female, from = age, years = 12, months = 0, height = 1.524, weight = 61.688562 gives bmi = 26.6, percentile = 96, category = Obesity. Source: CDC, Child and Teen BMI Categories: a 12-year-old girl, 5 feet, 136 pounds, BMI 26.6, 96th percentile, obesity.
4. sex = female, from = age, years = 6, months = 0.5, height = 1.15, weight = 17 gives bmi = 12.9, percentile = 0.8, category = Underweight, z = -2.390365. Source: CDC bmiagerev.csv, girls, 72.5 months.
5. sex = male, from = age, years = 19, months = 0, height = 1.8, weight = 113.4 gives bmi = 35, percentile = 97.8, ofP95 = 117.9%, z = 2.008654, category = Severe obesity. Source: CDC Child and Teen BMI Categories (severe obesity: 120% of P95, or a BMI of 35 or more); CDC bmiagerev.csv, boys, 228 months.

## FAQ

### Why is BMI different for children?

Body fat changes a lot as children grow, and differs between boys and girls. So a child’s BMI is compared with other children of the same sex and age, as a percentile, instead of with fixed adult cut-offs such as 25 and 30.

### What are the BMI categories for children?

The CDC uses the BMI-for-age percentile: underweight below the 5th percentile, healthy weight from the 5th to below the 85th, overweight from the 85th to below the 95th, and obesity at the 95th or above. Severe obesity is a BMI of 120% of the 95th percentile or more, or a BMI of 35 or more.

### What is the extended BMI-for-age method?

The 2000 CDC charts are not reliable far above the 95th percentile. In 2022 the CDC published extended charts that use a different method there (Wei and others, 2020): above the 95th percentile, the percentile is 90 + 10 × Φ((BMI − P95) ÷ σ). Below the 95th percentile nothing changes. This calculator uses the extended method above the 95th percentile.

### What does "percent of the 95th percentile" mean?

It is the child’s BMI divided by the 95th percentile BMI for the age and sex. A value of 120% or more is the CDC’s severe obesity cut-off, and the American Academy of Pediatrics splits severe obesity into class 2 (120% to under 140%) and class 3 (140% or more).

### At what age should I use the adult BMI calculator instead?

From the 20th birthday. The CDC child and teen categories are for ages 2 through 19. Under 2, doctors use weight-for-length on the WHO charts instead of BMI.

### Is BMI a good measure for my child?

BMI is a screening measure. It does not measure body fat directly, and muscular or tall children can have a high BMI. A pediatrician looks at BMI with growth over time, family history and other checks.

## Sources

- Centers for Disease Control and Prevention, National Center for Health Statistics. CDC growth charts data files: BMI-for-age, 2 to 20 years (bmiagerev.csv). https://www.cdc.gov/growthcharts/cdc-data-files.htm
- Centers for Disease Control and Prevention. Extended BMI-for-age growth charts: data files, with the formulas and worked examples for percentiles above the 95th and the sigma regression equations. https://www.cdc.gov/growthcharts/extended-bmi-data-files.htm
- Centers for Disease Control and Prevention. Child and Teen BMI Categories (June 28, 2024). https://www.cdc.gov/bmi/child-teen-calculator/bmi-categories.html
- Wei R, Ogden CL, Parsons VL, Freedman DS, Hales CM. A method for calculating BMI z-scores and percentiles above the 95th percentile of the CDC growth charts. Ann Hum Biol. 2020;47(6):514-521. https://doi.org/10.1080/03014460.2020.1808065
- Flegal KM, Cole TJ. Construction of LMS parameters for the Centers for Disease Control and Prevention 2000 growth charts. National Health Statistics Reports no. 63. Hyattsville, MD: NCHS; 2013. https://www.cdc.gov/nchs/data/nhsr/nhsr063.pdf