Body Mass Index (BMI) is a simple and widely used metric to assess an individual's body weight relative to their height. It provides a general indication of whether a person is underweight, has a healthy weight, is overweight, or obese. While not a direct measure of body fat, BMI serves as a useful screening tool for weight categories that may lead to health problems.
How BMI is Calculated
The formula for BMI is straightforward:
For metric units (kilograms and meters): BMI = Weight (kg) / (Height (m))^2
For imperial units (pounds and inches): BMI = (Weight (lb) / (Height (in))^2) * 703
In this calculator, we handle conversions for you. If you input weight in kilograms and height in centimeters, the calculator first converts centimeters to meters before applying the metric formula. If you input weight in pounds and height in inches, it uses the imperial formula directly. If you choose the feet and inches option, the total height in inches is calculated first.
BMI Categories and Health Implications
The calculated BMI value is then compared against standard categories established by health organizations:
Underweight: BMI less than 18.5
Healthy Weight: BMI between 18.5 and 24.9
Overweight: BMI between 25.0 and 29.9
Obese: BMI of 30.0 or greater
It's important to note that these categories are general guidelines. BMI may not be accurate for everyone, including athletes with high muscle mass, pregnant women, the elderly, and children, as it doesn't distinguish between muscle and fat. For a comprehensive health assessment, it is always best to consult with a healthcare professional.
Use Cases for the BMI Calculator
Personal Health Tracking: Monitor your weight status over time.
General Health Screening: Get a quick estimate of your weight category.
Fitness Goal Setting: Help inform your approach to weight management.
Informational Tool: Understand the basic principles of BMI calculation.
This calculator is designed to be accurate by performing necessary unit conversions and applying the correct BMI formulas. Use it as a starting point for conversations about your health with your doctor.
function calculateBMI() {
var weightInput = document.getElementById('weight');
var weightUnitSelect = document.getElementById('weightUnit');
var heightInput = document.getElementById('height');
var heightUnitSelect = document.getElementById('heightUnit');
var feetInput = document.getElementById('feet');
var inchesInput = document.getElementById('inches');
var weight = parseFloat(weightInput.value);
var height = parseFloat(heightInput.value);
var weightUnit = weightUnitSelect.value;
var heightUnit = heightUnitSelect.value;
var bmiResultElement = document.getElementById('bmiResult');
var bmiCategoryElement = document.getElementById('bmiCategory');
var bmiExplanationElement = document.getElementById('bmiExplanation');
// Clear previous results
bmiResultElement.innerText = '–';
bmiCategoryElement.innerText = '–';
bmiExplanationElement.innerText = '–';
// — Input Validation —
if (isNaN(weight) || weight <= 0) {
alert('Please enter a valid weight.');
return;
}
if (isNaN(height) || height <= 0) {
alert('Please enter a valid height.');
return;
}
var feet = 0;
var inches = 0;
var totalHeightInInches = 0;
var heightInMeters = 0;
// — Height Unit Handling —
if (heightUnit === 'cm') {
heightInMeters = height / 100;
} else if (heightUnit === 'in') {
heightInMeters = height * 0.0254; // Convert inches to meters
} else if (heightUnit === 'ftin') {
var feetValue = parseFloat(feetInput.value);
var inchesValue = parseFloat(inchesInput.value);
if (isNaN(feetValue) || feetValue < 0) {
alert('Please enter a valid number of feet.');
return;
}
if (isNaN(inchesValue) || inchesValue = 12) {
alert('Please enter a valid number of inches (0-11.99).');
return;
}
totalHeightInInches = (feetValue * 12) + inchesValue;
heightInMeters = totalHeightInInches * 0.0254;
}
// — Weight Unit Handling —
var weightInKg = weight;
if (weightUnit === 'lb') {
weightInKg = weight * 0.453592; // Convert pounds to kilograms
}
// — BMI Calculation —
var bmi = 0;
if (heightInMeters > 0) {
bmi = weightInKg / (heightInMeters * heightInMeters);
} else if (heightUnit === 'in' && !isNaN(height)) { // Imperial calculation with inches
bmi = (weight / (height * height)) * 703;
} else if (heightUnit === 'ftin' && totalHeightInInches > 0) { // Imperial calculation with feet & inches
bmi = (weight / (totalHeightInInches * totalHeightInInches)) * 703;
}
// — Display Results —
var bmiValue = bmi.toFixed(1);
bmiResultElement.innerText = bmiValue;
var category = ";
var explanation = ";
if (bmi = 18.5 && bmi = 25 && bmi = 30
category = 'Obese';
explanation = 'Your BMI is in the obese range. It is strongly recommended to consult a healthcare professional for guidance and support.';
}
bmiCategoryElement.innerText = category;
bmiExplanationElement.innerText = explanation;
// Reset color if it was changed for healthy weight
if (category !== 'Healthy Weight') {
bmiResultElement.style.color = 'var(–primary-blue)';
}
}
// Toggle visibility for feet and inches input groups based on height unit selection
document.getElementById('heightUnit').addEventListener('change', function() {
var selectedUnit = this.value;
var feetGroup = document.getElementById('feetInputGroup');
var inchesGroup = document.getElementById('inchesInputGroup');
var heightInput = document.getElementById('height');
if (selectedUnit === 'ftin') {
feetGroup.style.display = 'flex';
inchesGroup.style.display = 'flex';
heightInput.style.display = 'none'; // Hide the single height input
document.getElementById('feet').value = "; // Clear values
document.getElementById('inches').value = ";
document.getElementById('height').value = ";
} else {
feetGroup.style.display = 'none';
inchesGroup.style.display = 'none';
heightInput.style.display = 'flex'; // Show the single height input
document.getElementById('feet').value = "; // Clear values
document.getElementById('inches').value = ";
document.getElementById('height').value = ";
}
});
// Initial check for display on page load if 'ftin' is default or previously selected
document.addEventListener('DOMContentLoaded', function() {
var selectedUnit = document.getElementById('heightUnit').value;
var feetGroup = document.getElementById('feetInputGroup');
var inchesGroup = document.getElementById('inchesInputGroup');
var heightInput = document.getElementById('height');
if (selectedUnit === 'ftin') {
feetGroup.style.display = 'flex';
inchesGroup.style.display = 'flex';
heightInput.style.display = 'none';
} else {
feetGroup.style.display = 'none';
inchesGroup.style.display = 'none';
heightInput.style.display = 'flex';
}
});