{
  "id": "bmi-kids",
  "version": "c2cc8a2ae086",
  "status": "published",
  "name": "BMI Calculator for Kids",
  "question": "BMI calculator: kids and teens 2 to 19",
  "summary": "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.",
  "category": "health",
  "subcategory": "body",
  "url": "https://www.acalculator.org/health/bmi-calculator-for-kids",
  "markdown": "https://www.acalculator.org/health/bmi-calculator-for-kids.md",
  "kind": "function",
  "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."
  ],
  "inputs": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "sex": {
        "title": "Sex",
        "description": "The child’s sex. Boys and girls have separate growth charts.",
        "type": "string",
        "enum": [
          "male",
          "female"
        ]
      },
      "from": {
        "title": "Based on",
        "description": "Enter the age in years and months, or the birth date and the date of the measurement.",
        "type": "string",
        "enum": [
          "age",
          "dates"
        ]
      },
      "years": {
        "title": "Years",
        "description": "The age in whole years.",
        "type": "integer",
        "minimum": 2,
        "maximum": 19
      },
      "months": {
        "title": "Months",
        "description": "Months past the whole years, 0 to under 12. Decimals are allowed: 2 weeks is about 0.5.",
        "type": "number",
        "minimum": 0,
        "exclusiveMaximum": 12
      },
      "born": {
        "title": "Birth date",
        "description": "The child’s date of birth.",
        "type": "string",
        "format": "date"
      },
      "on": {
        "title": "Measured on",
        "description": "The date of the measurement. Today, unless you change it.",
        "type": "string",
        "format": "date"
      },
      "height": {
        "title": "Height",
        "description": "The child’s height without shoes.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "length",
        "minimum": 0.5,
        "maximum": 2.3
      },
      "weight": {
        "title": "Weight",
        "description": "The child’s weight.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "mass",
        "minimum": 5,
        "maximum": 250
      }
    }
  },
  "outputs": {
    "category": {
      "label": "Weight category",
      "description": "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.",
      "format": "text"
    },
    "bmi": {
      "label": "BMI",
      "description": "Body mass index: weight in kilograms divided by height in metres squared, to one decimal place.",
      "format": "number"
    },
    "percentile": {
      "label": "BMI-for-age percentile",
      "description": "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.",
      "format": "number"
    },
    "z": {
      "label": "Z-score",
      "description": "The BMI-for-age z-score: the standard normal value of the percentile.",
      "format": "number"
    },
    "ofP95": {
      "label": "Share of the 95th percentile",
      "description": "The BMI as a percent of the 95th percentile BMI for the sex and age, to one decimal place.",
      "format": "percent"
    },
    "healthyFrom": {
      "label": "Healthy weight from",
      "description": "The lowest weight at this height whose percentile shows as 5.0.",
      "format": "quantity",
      "unit": "lb"
    },
    "healthyTo": {
      "label": "Healthy weight up to",
      "description": "The weight at this height whose percentile shows as 85.0 (just under).",
      "format": "quantity",
      "unit": "lb"
    },
    "age": {
      "label": "Age",
      "description": "The child’s age in years and months.",
      "format": "text"
    }
  },
  "defaultAnswer": {
    "inputs": {
      "sex": "male",
      "from": "age",
      "years": 10,
      "months": 0,
      "born": "2016-09-30",
      "on": "2026-09-30",
      "height": 1.397,
      "weight": 31.751465900000003
    },
    "outputs": {
      "category": "Healthy weight",
      "bmi": 16.3,
      "percentile": 42.6,
      "z": -0.18581614505985405,
      "ofP95": 73.6,
      "healthyFrom": 27.714712388103163,
      "healthyTo": 37.763859281826825,
      "age": "10 years"
    },
    "text": "At 10 years, a BMI of 16.3 is at percentile 42.6: Healthy weight."
  },
  "examples": [
    {
      "given": {
        "sex": "female",
        "from": "age",
        "years": 9,
        "months": 6.5,
        "height": 1.4,
        "weight": 41.552
      },
      "expect": {
        "bmi": 21.2,
        "z": 1.421501177104806,
        "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); Python 3 cross-check in docs/progress/WP-54.md"
    },
    {
      "given": {
        "sex": "male",
        "from": "age",
        "years": 4,
        "months": 2.5,
        "height": 1,
        "weight": 22.6
      },
      "expect": {
        "bmi": 22.6,
        "z": 2.8314464831728046,
        "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; Python 3 cross-check in docs/progress/WP-54.md"
    },
    {
      "given": {
        "sex": "female",
        "from": "age",
        "years": 12,
        "months": 0,
        "height": 1.524,
        "weight": 61.68856232
      },
      "expect": {
        "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"
    },
    {
      "given": {
        "sex": "female",
        "from": "age",
        "years": 6,
        "months": 0.5,
        "height": 1.15,
        "weight": 17
      },
      "expect": {
        "bmi": 12.9,
        "percentile": 0.8,
        "category": "Underweight",
        "z": -2.39036471655681
      },
      "source": "CDC bmiagerev.csv, girls, 72.5 months; hand calculation in content.mdx"
    },
    {
      "given": {
        "sex": "male",
        "from": "age",
        "years": 19,
        "months": 0,
        "height": 1.8,
        "weight": 113.4
      },
      "expect": {
        "bmi": 35,
        "percentile": 97.8,
        "ofP95": 117.9,
        "z": 2.0086538620176504,
        "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"
    }
  ],
  "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"
  ],
  "related": [
    "bmi",
    "growth-chart",
    "weight-percentile"
  ],
  "changelog": []
}