Calculate metabolic energy expenditure per unit of body mass.
Kilograms (kg)
Pounds (lb)
Grams (g)
kcal / day
Watts (J/s)
ml O₂ / hour
Weight Specific Metabolic Rate
0.00 kcal / kg / day
Also expressed as: 0.00 W/kg
Total Estimated Metabolic Rate: 0 kcal/day
function toggleBmrInput() {
var radios = document.getElementsByName('calcMode');
var inputDiv = document.getElementById('bmrInputSection');
var selected = 'estimate';
for (var i = 0; i < radios.length; i++) {
if (radios[i].checked) {
selected = radios[i].value;
break;
}
}
if (selected === 'manual') {
inputDiv.style.display = 'flex';
} else {
inputDiv.style.display = 'none';
}
}
function calculateSpecificMR() {
// 1. Get Inputs
var weightInput = parseFloat(document.getElementById('wsmrWeight').value);
var weightUnit = document.getElementById('wsmrWeightUnit').value;
var modeRadios = document.getElementsByName('calcMode');
var mode = 'estimate';
for (var i = 0; i < modeRadios.length; i++) {
if (modeRadios[i].checked) {
mode = modeRadios[i].value;
break;
}
}
// Error Handling
if (isNaN(weightInput) || weightInput <= 0) {
alert("Please enter a valid body mass greater than zero.");
return;
}
// 2. Normalize Weight to KG
var weightKg = weightInput;
if (weightUnit === 'lb') {
weightKg = weightInput * 0.453592;
} else if (weightUnit === 'g') {
weightKg = weightInput / 1000;
}
// 3. Determine Total BMR (kcal/day)
var totalBMRKcal = 0;
if (mode === 'estimate') {
// Using Kleiber's Law: BMR (kcal/day) approx 70 * Mass(kg)^0.75
// Note: 70 is the standard coefficient for placental mammals.
totalBMRKcal = 70 * Math.pow(weightKg, 0.75);
} else {
var bmrInput = parseFloat(document.getElementById('wsmrTotalBMR').value);
var bmrUnit = document.getElementById('wsmrBmrUnit').value;
if (isNaN(bmrInput) || bmrInput L/day
// L/day = (ml/hr * 24) / 1000
// kcal/day = L/day * 4.8
var litersPerDay = (bmrInput * 24) / 1000;
totalBMRKcal = litersPerDay * 4.8;
}
}
// 4. Calculate Weight Specific BMR
// Formula: Specific Rate = Total Rate / Mass
var specificKcalPerKg = totalBMRKcal / weightKg;
// Convert Specific Rate to Watts/kg
// 1 kcal/day = 0.048426 Watts
var specificWattsPerKg = specificKcalPerKg * 0.048426;
// 5. Display Results
document.getElementById('resKcalKg').innerHTML = specificKcalPerKg.toFixed(2) + ' kcal / kg / day';
document.getElementById('resWattsKg').innerHTML = specificWattsPerKg.toFixed(4);
document.getElementById('resTotal').innerHTML = totalBMRKcal.toFixed(0);
var resultBox = document.getElementById('wsmrResult');
resultBox.style.display = 'block';
resultBox.scrollIntoView({ behavior: 'smooth' });
}
How to Calculate Weight Specific Metabolic Rate
Weight-specific metabolic rate (or mass-specific metabolic rate) is a physiological measure used to express the energy expenditure of an organism relative to its body mass. Unlike total metabolic rate, which typically increases as organisms get larger, the weight-specific metabolic rate decreases as body size increases. This phenomenon implies that one gram of tissue in a mouse consumes significantly more energy than one gram of tissue in an elephant.
The Formula
To calculate the weight-specific metabolic rate, you divide the total metabolic rate (BMR) by the total body mass. The basic equation is:
Specific BMR = Total BMR / Body Mass
Where:
Total BMR is the total energy used by the organism per unit of time (e.g., kcal/day or Watts).
Body Mass is the weight of the organism (e.g., kg or g).
Understanding Scaling Laws (Kleiber's Law)
Why do smaller animals have higher specific metabolic rates? This is described by Kleiber's Law. In the 1930s, Max Kleiber discovered that an animal's metabolic rate scales to the ¾ power of the animal's mass ($Mass^{0.75}$).
When we convert this to a weight-specific basis, we divide by mass ($Mass^{1.0}$), resulting in a scaling exponent of $-0.25$.
Total BMR ∝ $Mass^{0.75}$
Weight-Specific BMR ∝ $Mass^{-0.25}$
This negative exponent explains why a shrew must eat nearly its own body weight in food daily to survive, while a large mammal needs only a small fraction of its body weight in food.
Example Calculation
Let's look at a practical example comparing a human and a mouse.
Parameter
Human (70 kg)
Mouse (0.03 kg)
Total BMR
~1,700 kcal/day
~5 kcal/day
Math
1700 / 70
5 / 0.03
Weight Specific Rate
~24.3 kcal/kg/day
~166.7 kcal/kg/day
As shown in the table, even though the human burns far more total energy, the mouse burns energy roughly 7 times faster per unit of body mass.
Why is this important?
Calculating weight-specific metabolic rate is crucial in pharmacology, comparative physiology, and veterinary medicine. It explains why drug dosages cannot simply be scaled linearly by weight; smaller animals metabolize compounds much faster than larger ones relative to their size. It also helps researchers understand thermal regulation, lifespan potential, and cellular efficiency across different species.