Body Mass Index (BMI) Calculator
Use this calculator to determine your Body Mass Index (BMI) and understand your weight category.
function updateLabels() {
var unitSystem = document.querySelector('input[name="unitSystem"]:checked').value;
if (unitSystem === 'metric') {
document.getElementById('weightLabel').innerText = 'Weight (kg):';
document.getElementById('heightLabel').innerText = 'Height (cm):';
document.getElementById('weightInput').placeholder = 'e.g., 70';
document.getElementById('heightInput').placeholder = 'e.g., 175';
} else {
document.getElementById('weightLabel').innerText = 'Weight (lbs):';
document.getElementById('heightLabel').innerText = 'Height (inches):';
document.getElementById('weightInput').placeholder = 'e.g., 150';
document.getElementById('heightInput').placeholder = 'e.g., 69';
}
// Clear previous results and errors when units change
document.getElementById('bmiResult').innerText = ";
document.getElementById('bmiCategory').innerText = ";
document.getElementById('errorMessage').innerText = ";
}
function calculateBMI() {
var weight = parseFloat(document.getElementById('weightInput').value);
var height = parseFloat(document.getElementById('heightInput').value);
var unitSystem = document.querySelector('input[name="unitSystem"]:checked').value;
var bmi;
var category;
var errorMessage = document.getElementById('errorMessage');
errorMessage.innerText = "; // Clear previous errors
if (isNaN(weight) || isNaN(height) || weight <= 0 || height <= 0) {
errorMessage.innerText = 'Please enter valid positive numbers for weight and height.';
document.getElementById('bmiResult').innerText = '';
document.getElementById('bmiCategory').innerText = '';
return;
}
if (unitSystem === 'metric') {
var heightMeters = height / 100; // Convert cm to meters
bmi = weight / (heightMeters * heightMeters);
} else { // Imperial
// BMI = (weight in lbs / (height in inches)^2) * 703
bmi = (weight / (height * height)) * 703;
}
if (bmi = 18.5 && bmi = 25.0 && bmi <= 29.9) {
category = 'Overweight';
} else {
category = 'Obese';
}
document.getElementById('bmiResult').innerText = bmi.toFixed(2);
document.getElementById('bmiCategory').innerText = category;
}
// Initialize labels on page load
window.onload = updateLabels;
.body-weight-calculator-container {
font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
background-color: #f9f9f9;
padding: 25px;
border-radius: 10px;
box-shadow: 0 4px 12px rgba(0, 0, 0, 0.1);
max-width: 600px;
margin: 20px auto;
border: 1px solid #e0e0e0;
}
.body-weight-calculator-container h2 {
color: #333;
text-align: center;
margin-bottom: 20px;
font-size: 1.8em;
}
.body-weight-calculator-container p {
color: #555;
text-align: center;
margin-bottom: 25px;
line-height: 1.6;
}
.calculator-form .input-group {
margin-bottom: 18px;
display: flex;
flex-direction: column;
}
.calculator-form .input-group label {
margin-bottom: 8px;
color: #444;
font-weight: 600;
font-size: 1em;
}
.calculator-form .input-group input[type="number"] {
padding: 12px 15px;
border: 1px solid #ccc;
border-radius: 6px;
font-size: 1em;
width: calc(100% – 30px);
box-sizing: border-box;
transition: border-color 0.3s ease;
}
.calculator-form .input-group input[type="number"]:focus {
border-color: #007bff;
outline: none;
box-shadow: 0 0 0 3px rgba(0, 123, 255, 0.25);
}
.calculator-form .input-group input[type="radio"] {
margin-right: 8px;
}
.calculator-form .input-group label[for="unitSystemMetric"],
.calculator-form .input-group label[for="unitSystemImperial"] {
display: inline-flex;
align-items: center;
margin-right: 20px;
font-weight: normal;
cursor: pointer;
}
.calculator-form button {
background-color: #007bff;
color: white;
padding: 12px 25px;
border: none;
border-radius: 6px;
font-size: 1.1em;
cursor: pointer;
transition: background-color 0.3s ease, transform 0.2s ease;
width: 100%;
box-sizing: border-box;
margin-top: 10px;
}
.calculator-form button:hover {
background-color: #0056b3;
transform: translateY(-2px);
}
.calculator-form button:active {
transform: translateY(0);
}
.result-area {
margin-top: 30px;
padding: 20px;
background-color: #e9f7ff;
border: 1px solid #b3e0ff;
border-radius: 8px;
text-align: center;
}
.result-area h3 {
color: #0056b3;
margin-bottom: 10px;
font-size: 1.5em;
}
.result-area p {
color: #333;
font-size: 1.1em;
margin-bottom: 0;
}
.result-area span {
font-weight: 700;
color: #007bff;
}
#errorMessage {
font-weight: 600;
margin-top: 15px;
text-align: center;
}
Understanding Your Body Mass Index (BMI)
The Body Mass Index (BMI) is a simple numerical measure that is commonly used to classify whether a person's weight is healthy in relation to their height. It's a widely accepted screening tool for weight categories that may lead to health problems.
How BMI is Calculated
The calculation for BMI depends on the unit system used:
- Metric System: BMI = weight (kg) / [height (m)]²
- Imperial System: BMI = [weight (lbs) / height (inches)²] * 703
For example, if you weigh 70 kg and are 1.75 meters (175 cm) tall, your BMI would be 70 / (1.75 * 1.75) = 70 / 3.0625 = 22.86.
If you weigh 150 lbs and are 69 inches tall, your BMI would be (150 / (69 * 69)) * 703 = (150 / 4761) * 703 = 0.0315 * 703 = 22.15.
BMI Categories
The World Health Organization (WHO) defines the following standard BMI categories for adults:
- Underweight: Less than 18.5
- Normal weight: 18.5 – 24.9
- Overweight: 25.0 – 29.9
- Obese: 30.0 or greater
These categories provide a general guide to assess potential health risks associated with weight.
Limitations of BMI
While BMI is a useful screening tool, it has limitations:
- Muscle Mass: BMI does not distinguish between muscle and fat. Athletes or individuals with high muscle mass may have a high BMI but low body fat, incorrectly classifying them as overweight or obese.
- Age and Sex: BMI ranges are generally the same for men and women, but body composition changes with age and can vary between sexes.
- Body Composition and Distribution: It doesn't account for fat distribution. For instance, abdominal fat (around the waist) is considered a higher health risk than fat distributed elsewhere.
- Ethnicity: Different ethnic groups may have different health risks at the same BMI.
Why BMI is Still Useful
Despite its limitations, BMI remains a valuable tool for several reasons:
- Simplicity: It's easy to calculate and understand, requiring only height and weight.
- Population Health: It's excellent for tracking weight trends and health risks across large populations.
- Initial Screening: For most adults, it provides a good initial indicator of whether their weight is in a healthy range.
For a more comprehensive assessment of health risks related to weight, healthcare professionals often consider BMI alongside other factors such as waist circumference, body fat percentage, diet, physical activity levels, and family history.