Sedentary (Little or no exercise)
Lightly Active (Exercise 1-3 days/week)
Moderately Active (Exercise 3-5 days/week)
Very Active (Exercise 6-7 days/week)
Extra Active (Very hard exercise/physical job)
Your Metabolic Results
Basal Metabolic Rate (BMR):–
Total Daily Energy Expenditure (TDEE):–
Interpretation: Your BMR is the calories your body burns at complete rest. Your TDEE is the estimated calories you burn in a day including activity.
function updateLabels() {
var units = document.getElementById("units").value;
var weightLabel = document.getElementById("weightLabel");
var heightLabel = document.getElementById("heightLabel");
var heightInput = document.getElementById("height");
var weightInput = document.getElementById("weight");
if (units === "imperial") {
weightLabel.innerText = "Weight (lbs)";
weightInput.placeholder = "e.g. 150";
heightLabel.innerText = "Height (inches)";
heightInput.placeholder = "e.g. 69 (5ft 9in)";
document.getElementById("heightHelper").innerText = "Note: 5ft = 60in, 6ft = 72in";
} else {
weightLabel.innerText = "Weight (kg)";
weightInput.placeholder = "e.g. 70";
heightLabel.innerText = "Height (cm)";
heightInput.placeholder = "e.g. 175";
document.getElementById("heightHelper").innerText = "";
}
}
function calculateBMR() {
// 1. Get input values
var units = document.getElementById("units").value;
var gender = document.getElementById("gender").value;
var age = parseFloat(document.getElementById("age").value);
var weight = parseFloat(document.getElementById("weight").value);
var height = parseFloat(document.getElementById("height").value);
var activityMultiplier = parseFloat(document.getElementById("activity").value);
// 2. Validation
if (isNaN(age) || isNaN(weight) || isNaN(height) || age <= 0 || weight <= 0 || height <= 0) {
alert("Please enter valid positive numbers for age, weight, and height.");
return;
}
// 3. Convert to Metric for Calculation (Mifflin-St Jeor uses kg and cm)
var weightKg = weight;
var heightCm = height;
if (units === "imperial") {
// 1 lb = 0.453592 kg
weightKg = weight * 0.453592;
// 1 inch = 2.54 cm
heightCm = height * 2.54;
}
// 4. Calculate BMR using Mifflin-St Jeor Equation
// Formula: (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) + s
// s = +5 for men, -161 for women
var bmr = 0;
var s = (gender === "male") ? 5 : -161;
bmr = (10 * weightKg) + (6.25 * heightCm) – (5 * age) + s;
// 5. Calculate TDEE
var tdee = bmr * activityMultiplier;
// 6. Display Results
var resultDiv = document.getElementById("result");
var bmrDisplay = document.getElementById("bmrValue");
var tdeeDisplay = document.getElementById("tdeeValue");
bmrDisplay.innerText = Math.round(bmr).toLocaleString() + " kcal / day";
tdeeDisplay.innerText = Math.round(tdee).toLocaleString() + " kcal / day";
resultDiv.style.display = "block";
}
How Is Metabolic Rate Calculated?
Understanding how metabolic rate is calculated is the cornerstone of managing body weight, optimizing athletic performance, and ensuring general health. Your metabolic rate determines how much energy (measured in calories) your body requires to function. While accurate laboratory testing involves measuring gas exchange (calorimetry), most health professionals and individuals rely on predictive mathematical equations.
1. The Components of Metabolism
Before diving into the calculations, it is essential to understand what makes up your total energy expenditure. The final number, known as TDEE (Total Daily Energy Expenditure), is a sum of several factors:
Basal Metabolic Rate (BMR): The number of calories your body burns at complete rest to maintain vital functions like breathing, blood circulation, and cell production. This accounts for 60-75% of total calorie burn.
Thermic Effect of Food (TEF): The energy required to digest, absorb, and process nutrients. This typically accounts for about 10% of your daily energy expenditure.
Physical Activity Level (PAL): The calories burned during exercise and non-exercise activity thermogenesis (NEAT), such as walking to the car or fidgeting.
2. The Calculation Formulas
Several formulas have been developed over the last century to estimate BMR. The calculator above utilizes the Mifflin-St Jeor equation, which is currently considered the most accurate standard for the general population.
The Mifflin-St Jeor Equation
Introduced in 1990, this formula adjusts for gender, weight, height, and age. The logic calculates a base "score" from physical attributes and then applies a gender-specific constant.
Gender
Formula (Metric)
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
The Harris-Benedict Equation
Created in 1919 and revised in 1984, this was the standard for many years. It tends to overestimate calorie needs slightly in modern, more sedentary populations compared to Mifflin-St Jeor.
3. Adjusting for Activity (TDEE)
Once the BMR is calculated, it represents a comatose state. To find out how many calories you actually burn in a day, we multiply the BMR by an Activity Factor.
Activity Level
Multiplier
Description
Sedentary
1.2
Desk job, little to no exercise
Lightly Active
1.375
Light exercise/sports 1-3 days/week
Moderately Active
1.55
Moderate exercise/sports 3-5 days/week
Very Active
1.725
Hard exercise/sports 6-7 days/week
Extra Active
1.9
Very hard daily exercise or physical job
4. Factors That Influence the Calculation
While these calculators provide a strong baseline, individual metabolic rates can vary based on factors not always captured in standard formulas:
Muscle Mass: Muscle tissue burns more calories at rest than fat tissue. An individual with high muscle mass will have a higher actual BMR than calculated. The Katch-McArdle formula is often used for this if body fat percentage is known.
Age: Metabolism naturally slows down with age, primarily due to a loss of muscle mass and hormonal changes. The equations account for this by subtracting value based on age.
Genetics: Some individuals naturally have a faster or slower metabolism, which can vary the result by approximately 5-10%.
Hormonal Health: Conditions such as hypothyroidism or Cushing's syndrome can significantly lower metabolic rate.
Why This Calculation Matters
Knowing how your metabolic rate is calculated allows you to create an energy balance plan. To lose weight, you generally aim to eat fewer calories than your TDEE (a caloric deficit). To gain muscle, you aim for a slight surplus. Without a mathematical baseline, dietary changes are often just guesswork.