{
  "slug": "growth-percentile-calculator",
  "title": "Child Growth Percentile Calculator",
  "heading": "Child Growth Percentile Calculator",
  "category": "fitness-health",
  "url": "https://www.revenuelab.fyi/toolbox/growth-percentile-calculator",
  "summary": "Estimate your child's weight/height percentile using CDC-style z-scores.",
  "description": "Pediatricians plot weight and height (and weight-for-length under age 2) against CDC or WHO growth charts to see where a child falls relative to peers of the same age and sex, since percentile tracking over time matters more than any single measurement. This calculator uses simplified normative mean and standard deviation approximations by age band to estimate a z-score and convert it to a percentile, giving you a ballpark read between well-child visits — it is an estimate, not a diagnostic substitute for the actual CDC/WHO chart your pediatrician uses.",
  "formula": "z-score = (measured value − reference mean for age/sex) ÷ reference standard deviation; percentile = normal CDF(z-score) × 100.",
  "dateModified": "2026-09-30",
  "run_url": "https://www.revenuelab.fyi/api/public/calc?tool=growth-percentile-calculator",
  "inputs": [
    {
      "id": "sex",
      "label": "Sex",
      "kind": "select",
      "hint": null,
      "default": "female",
      "options": [
        {
          "value": "female",
          "label": "Female"
        },
        {
          "value": "male",
          "label": "Male"
        }
      ]
    },
    {
      "id": "ageMonths",
      "label": "Age",
      "kind": "number",
      "hint": null,
      "default": 24,
      "unit": "months",
      "min": 0,
      "max": 240
    },
    {
      "id": "measurement",
      "label": "Weight",
      "kind": "number",
      "hint": null,
      "default": 27,
      "unit": "lb",
      "min": 2,
      "max": 250
    },
    {
      "id": "referenceMean",
      "label": "Reference average weight for age (lb)",
      "kind": "number",
      "hint": null,
      "default": 27.5,
      "unit": "lb",
      "min": 1,
      "max": null
    },
    {
      "id": "referenceSD",
      "label": "Reference standard deviation (lb)",
      "kind": "number",
      "hint": null,
      "default": 3.2,
      "unit": "lb",
      "min": 0.1,
      "max": null
    }
  ],
  "outputs": [
    {
      "id": "percentile",
      "label": "Estimated percentile",
      "format": "decimal",
      "hint": null,
      "primary": true
    },
    {
      "id": "zScore",
      "label": "z-score",
      "format": "decimal",
      "hint": null,
      "primary": false
    }
  ],
  "worked_example": {
    "inputs": [
      "Sex: Female",
      "Age: 24 months",
      "Weight: 27 lb",
      "Reference average weight for age (lb): 27.5 lb",
      "Reference standard deviation (lb): 3.2 lb"
    ],
    "outputs": [
      "Estimated percentile: 43.8",
      "z-score: -0.16"
    ]
  },
  "how_to": {
    "title": "How to use this",
    "steps": [
      "Enter sex.",
      "Enter age (months).",
      "Enter weight (lb).",
      "Enter reference average weight for age (lb) (lb).",
      "Enter reference standard deviation (lb) (lb).",
      "Read your estimated percentile on the right — it updates as you type.",
      "Hit Share to keep the scenario or send it to someone."
    ]
  },
  "scenarios": [
    {
      "name": "Conservative",
      "description": "Lower-end numbers — what if things land soft?",
      "values": {
        "sex": "female",
        "ageMonths": 14,
        "measurement": 16.2,
        "referenceMean": 16.5,
        "referenceSD": 1.92
      }
    },
    {
      "name": "Typical",
      "description": "Defaults — the most common real-world setup.",
      "values": {
        "sex": "female",
        "ageMonths": 24,
        "measurement": 27,
        "referenceMean": 27.5,
        "referenceSD": 3.2
      }
    },
    {
      "name": "Ambitious",
      "description": "Higher-end numbers — what if things really pop?",
      "values": {
        "sex": "female",
        "ageMonths": 38,
        "measurement": 43.2,
        "referenceMean": 44,
        "referenceSD": 5.120000000000001
      }
    }
  ],
  "limitations": [
    "Population formulas describe averages; individual bodies vary, especially at the extremes of height, age, or muscle mass.",
    "Results do not account for medical conditions, medication, or pregnancy.",
    "Use this as a directional reference, not a diagnosis — talk to a clinician before acting on it."
  ],
  "faq": [
    {
      "q": "Where do I find the reference mean and standard deviation for my child's age?",
      "a": "The CDC publishes LMS parameter tables by exact age in months and sex for weight, height, head circumference, and BMI, available free on cdc.gov/growthcharts. This calculator uses those inputs directly so results track the same statistical method pediatricians use, without hardcoding a single lookup table."
    },
    {
      "q": "Is a low or high percentile a problem?",
      "a": "Not by itself — a consistently tracked 10th percentile child growing steadily along that line is generally healthy, since percentile reflects where a child falls in the population distribution, not a pass/fail grade. What concerns pediatricians is a percentile that crosses two or more major percentile lines in either direction, which can signal a growth or nutrition issue worth investigating."
    },
    {
      "q": "Why do doctors use weight-for-length instead of BMI under age 2?",
      "a": "BMI as a ratio behaves oddly in infants and toddlers because body proportions change rapidly with age, so the CDC/WHO charts use weight-for-length up to 24 months and switch to BMI-for-age afterward. Make sure you're using the right chart type for your child's age."
    }
  ],
  "related": [
    "https://www.revenuelab.fyi/toolbox/wake-window-planner",
    "https://www.revenuelab.fyi/toolbox/car-seat-replacement-timeline"
  ],
  "license": "CC-BY-4.0",
  "citation": "RevenueLab — Child Growth Percentile Calculator (https://www.revenuelab.fyi/toolbox/growth-percentile-calculator)"
}