function toggleUnits() {
var system = document.getElementById("unitSystem").value;
var wLabel = document.getElementById("weightLabel");
var hLabel = document.getElementById("heightLabel");
if (system === "metric") {
wLabel.innerText = "Weight (kg)";
hLabel.innerText = "Height (cm)";
} else {
wLabel.innerText = "Weight (lbs)";
hLabel.innerText = "Height (inches)";
}
}
function calculateBMI() {
var system = document.getElementById("unitSystem").value;
var ageY = parseFloat(document.getElementById("ageYears").value) || 0;
var ageM = parseFloat(document.getElementById("ageMonths").value) || 0;
var weight = parseFloat(document.getElementById("weight").value);
var height = parseFloat(document.getElementById("height").value);
var totalMonths = (ageY * 12) + ageM;
if (!weight || !height || ageY < 2) {
alert("Please enter a valid weight, height, and age (minimum 2 years).");
return;
}
var bmi = 0;
if (system === "metric") {
bmi = weight / ((height / 100) * (height / 100));
} else {
bmi = (weight / (height * height)) * 703;
}
var roundedBMI = bmi.toFixed(1);
document.getElementById("bmiOutput").innerText = roundedBMI;
var status = "";
var color = "";
var description = "";
// Generic BMI-for-age classification logic (CDC standards)
// Note: In real pediatric medicine, this is mapped to a specific percentile curve.
// These thresholds represent the general interpretation for children/teens.
if (bmi = 14 && bmi = 22 && bmi < 27) {
status = "Overweight Range";
color = "#f1c40f";
} else {
status = "Obese Range";
color = "#c0392b";
}
var statusEl = document.getElementById("statusOutput");
statusEl.innerText = status;
statusEl.style.color = color;
document.getElementById("statusDescription").innerHTML = "For children and teens, BMI is age- and sex-specific and is often referred to as BMI-for-age. A BMI of " + roundedBMI + " for a " + ageY + "-year-old must be compared against growth charts to determine the exact percentile. Generally, a result in the '" + status + "' indicates how your child's BMI compares to other children of the same age and sex.";
document.getElementById("resultsArea").style.display = "block";
document.getElementById("resultsArea").scrollIntoView({ behavior: 'smooth' });
}
Understanding Pediatric BMI
Body Mass Index (BMI) for children and adolescents is a specialized screening tool used to determine if a child's weight is appropriate for their height, age, and biological sex. Unlike adults, where BMI categories are fixed, pediatric BMI must be interpreted relative to other children of the same demographic because body fat composition changes significantly as a child grows.
How Pediatric BMI is Calculated
The math behind the initial BMI value is identical to the adult formula:
Metric Units: Weight (kg) / [Height (m)]²
Imperial Units: [Weight (lb) / Height (in)²] x 703
However, once the number is produced, it is plotted on a CDC Growth Chart to find the child's percentile. The percentile tells you where the child stands compared to 100 peers of the same age and sex.
Weight Status Categories for Children
Percentile Range
Weight Status
Less than the 5th percentile
Underweight
5th percentile to less than the 85th
Healthy weight
85th percentile to less than the 95th
Overweight
95th percentile or greater
Obese
Important Examples
Case 1: 10-Year-Old Boy
If a 10-year-old boy has a BMI of 23, he is in the 95th percentile, which classifies him in the "Obese" category for his age group.
Case 2: 15-Year-Old Girl
If a 15-year-old girl has the same BMI of 23, she is in the 75th percentile, which is considered a "Healthy Weight" for her age.
Why Age and Gender Matter
Children's body fatness changes as they mature. Furthermore, boys and girls differ in their body fat distribution as they go through various stages of puberty. This is why a simple BMI number isn't enough; the BMI-for-age approach ensures that growth spurts and developmental changes are accounted for. This calculator provides the raw BMI and a general classification, but you should always consult with a pediatrician to review growth charts for a clinical diagnosis.