Body Mass Index (BMI) is a numerical value derived from a person's weight and height. It's a simple, non-invasive screening tool used to categorize a person's weight status into underweight, normal weight, overweight, or obesity. While BMI doesn't directly measure body fat, it is a widely recognized indicator that correlates with body fat percentage, especially in populations.
How is BMI Calculated?
The formula for BMI is:
BMI = (weight in kilograms) / (height in meters)2
In our calculator, we use convenient units of kilograms (kg) for weight and centimeters (cm) for height. The calculator automatically converts height from centimeters to meters for the calculation (by dividing by 100).
The calculation performed is:
BMI = weight_kg / (height_cm / 100)2
BMI Categories
The World Health Organization (WHO) and other health bodies generally use the following BMI ranges:
Underweight: Less than 18.5
Normal weight: 18.5 – 24.9
Overweight: 25 – 29.9
Obesity Class I: 30 – 34.9
Obesity Class II: 35 – 39.9
Obesity Class III: 40 or greater
Why is BMI Important?
Maintaining a healthy weight is crucial for overall health. BMI is used as a primary indicator to identify potential weight-related health risks, such as:
Increased risk of heart disease
Type 2 diabetes
High blood pressure
Osteoarthritis
Certain types of cancer
It's important to remember that BMI is a screening tool, not a diagnostic one. Factors like muscle mass, bone density, and body composition can influence BMI. For a comprehensive assessment of your health and weight status, it's always best to consult with a healthcare professional.
function calculateBMI() {
var weightInput = document.getElementById("weight");
var heightInput = document.getElementById("height");
var resultDiv = document.getElementById("result");
var bmiValueSpan = document.getElementById("bmiValue");
var bmiCategorySpan = document.getElementById("bmiCategory");
var weight = parseFloat(weightInput.value);
var heightCm = parseFloat(heightInput.value);
// Clear previous results
resultDiv.style.display = "none";
bmiValueSpan.textContent = "";
bmiCategorySpan.textContent = "";
// Validate inputs
if (isNaN(weight) || isNaN(heightCm) || weight <= 0 || heightCm <= 0) {
alert("Please enter valid positive numbers for weight and height.");
return;
}
// Convert height from cm to meters
var heightM = heightCm / 100;
// Calculate BMI
var bmi = weight / (heightM * heightM);
// Round BMI to two decimal places
var roundedBMI = bmi.toFixed(2);
var bmiCategory = "";
if (bmi = 18.5 && bmi = 25 && bmi = 30 && bmi = 35 && bmi <= 39.9) {
bmiCategory = "Obese (Class II)";
} else {
bmiCategory = "Obese (Class III)";
}
// Display the result
bmiValueSpan.textContent = roundedBMI;
bmiCategorySpan.textContent = "Category: " + bmiCategory;
resultDiv.style.display = "block";
}