Basal Metabolic Rate (BMR) represents the number of calories your body needs to accomplish its most basic (basal) life-sustaining functions. Even when you are completely at rest, your body burns calories to perform functions such as breathing, circulation, nutrient processing, and cell production.
The Mifflin-St Jeor Equation
This calculator uses the Mifflin-St Jeor equation, widely considered the most accurate formula for estimating BMR in clinical settings today. It was introduced in 1990 and has largely replaced the older Harris-Benedict equation.
The formulas are as follows:
For Men:
BMR = (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) + 5
For Women:
BMR = (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) – 161
How to Use the BMR Formula
To perform the calculation manually, follow these steps:
Step 1: Convert your weight to kilograms (Divide lbs by 2.20462).
Step 2: Convert your height to centimeters (Multiply inches by 2.54).
Step 3: Multiply your weight (kg) by 10.
Step 4: Multiply your height (cm) by 6.25.
Step 5: Multiply your age (years) by 5.
Step 6: Add the results of Step 3 and Step 4, then subtract the result of Step 5.
Step 7: If you are male, add 5. If you are female, subtract 161.
Why is BMR Important?
Your BMR accounts for approximately 60% to 75% of the total calories you burn each day. It is the starting point for calculating your Total Daily Energy Expenditure (TDEE). Once you know your BMR, you can estimate your total caloric needs based on your activity level:
Sedentary: BMR × 1.2
Lightly Active: BMR × 1.375
Moderately Active: BMR × 1.55
Very Active: BMR × 1.725
Factors Affecting BMR
Several variables influence your metabolic rate, which is why the formula requires specific inputs:
Muscle Mass: Muscle tissue burns more calories at rest than fat tissue. This is why strength training can increase your BMR.
Age: Metabolism naturally slows down as you age, often due to a loss of muscle mass and hormonal changes.
Gender: Men typically have a higher BMR than women because they tend to have more lean muscle mass and a larger body size.
Genetics: Some individuals naturally have a faster or slower metabolism.
function switchUnit(unit) {
var metricInputs = document.getElementById('metricInputs');
var imperialInputs = document.getElementById('imperialInputs');
var btnMetric = document.getElementById('btnMetric');
var btnImperial = document.getElementById('btnImperial');
var currentUnitInput = document.getElementById('currentUnit');
if (unit === 'metric') {
metricInputs.style.display = 'block';
imperialInputs.style.display = 'none';
btnMetric.classList.add('active');
btnImperial.classList.remove('active');
currentUnitInput.value = 'metric';
} else {
metricInputs.style.display = 'none';
imperialInputs.style.display = 'block';
btnMetric.classList.remove('active');
btnImperial.classList.add('active');
currentUnitInput.value = 'imperial';
}
// Hide result on switch to avoid confusion
document.getElementById('resultBox').style.display = 'none';
}
function calculateBMR() {
// Get common inputs
var age = parseInt(document.getElementById('age').value);
var genderRadios = document.getElementsByName('gender');
var gender = 'male';
for (var i = 0; i < genderRadios.length; i++) {
if (genderRadios[i].checked) {
gender = genderRadios[i].value;
break;
}
}
var weight = 0; // in kg
var height = 0; // in cm
var unit = document.getElementById('currentUnit').value;
// Validation & Conversion
if (unit === 'metric') {
var wKg = parseFloat(document.getElementById('weightKg').value);
var hCm = parseFloat(document.getElementById('heightCm').value);
if (isNaN(wKg) || isNaN(hCm) || isNaN(age)) {
alert("Please enter valid numbers for Age, Weight, and Height.");
return;
}
weight = wKg;
height = hCm;
} else {
var wLbs = parseFloat(document.getElementById('weightLbs').value);
var hFt = parseFloat(document.getElementById('heightFt').value);
var hIn = parseFloat(document.getElementById('heightIn').value);
// Handle incomplete height input gracefully
if (isNaN(hIn)) hIn = 0;
if (isNaN(wLbs) || isNaN(hFt) || isNaN(age)) {
alert("Please enter valid numbers for Age, Weight, and Height.");
return;
}
// Convert to Metric
weight = wLbs / 2.20462;
height = (hFt * 30.48) + (hIn * 2.54);
}
// Apply Mifflin-St Jeor Equation
// Men: (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) + 5
// Women: (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) – 161
var bmr = (10 * weight) + (6.25 * height) – (5 * age);
if (gender === 'male') {
bmr += 5;
} else {
bmr -= 161;
}
// Display Result
var resultBox = document.getElementById('resultBox');
var bmrResult = document.getElementById('bmrResult');
// Round to nearest whole number
bmrResult.innerHTML = Math.round(bmr).toLocaleString() + " kcal/day";
resultBox.style.display = 'block';
}