Body Fat Mass Index (BFMI), also known as the Body Fat Percentage (BFP), is a measure that estimates the amount of fat tissue you have relative to your total body mass. Unlike Body Mass Index (BMI), which only considers height and weight, BFMI often incorporates circumference measurements to provide a more refined estimate, especially when distinguishing between fat mass and lean mass (muscle).
This calculator uses a common formula that requires weight, height, age, gender, and specific circumference measurements (neck, waist, and hip for females). These measurements help to account for variations in body composition that BMI does not capture.
How the Calculation Works:
The calculation for Body Fat Percentage (BFP) can vary depending on the method used. A widely referenced formula, often attributed to the U.S. Navy, and adapted for this calculator, uses the following inputs:
Weight (W): In kilograms (kg).
Height (H): In centimeters (cm).
Age (A): In years.
Gender: Male or Female.
Neck Circumference (N): In centimeters (cm).
Waist Circumference (Wa): In centimeters (cm).
Hip Circumference (Hi): In centimeters (cm) – Primarily for females.
The formulas differ slightly for males and females to account for physiological differences.
For Men:
Body Fat % = 495 / (1.0324 – 0.19077 * log10(Wa – N) + 0.15457 * log10(H)) – 450 Note: For men, the hip measurement is not typically used in this specific formula. This calculator simplifies to use neck and waist.
Body fat percentage is a crucial indicator of health. Generally:
Athletes: Men typically 6-13%, Women 14-20%
Fitness: Men typically 14-17%, Women 21-24%
Average: Men typically 18-24%, Women 25-31%
Obese: Men 25%+, Women 32%+
It's important to remember that these are general guidelines. Factors like age, activity level, and individual health conditions can influence optimal body fat ranges. Consulting with a healthcare professional or certified fitness expert is recommended for personalized advice.
Why Use a Body Fat Mass Index Calculator?
BFMI offers a more nuanced view of body composition than BMI alone. It helps individuals:
Track progress in fitness programs more accurately.
Differentiate between muscle gain and fat gain.
Identify health risks associated with excess body fat.
Set realistic health and fitness goals.
function calculateBodyFat() {
var weight = parseFloat(document.getElementById("weight").value);
var heightCm = parseFloat(document.getElementById("height").value);
var age = parseFloat(document.getElementById("age").value);
var gender = document.getElementById("gender").value;
var neck = parseFloat(document.getElementById("neck").value);
var waist = parseFloat(document.getElementById("waist").value);
var hip = parseFloat(document.getElementById("hip").value);
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = "; // Clear previous results
// Input validation
if (isNaN(weight) || isNaN(heightCm) || isNaN(age) || isNaN(neck) || isNaN(waist) || (gender === 'female' && isNaN(hip))) {
resultDiv.innerHTML = "Please enter valid numbers for all required fields.";
return;
}
if (weight <= 0 || heightCm <= 0 || age <= 0 || neck <= 0 || waist <= 0 || (gender === 'female' && hip <= 0)) {
resultDiv.innerHTML = "All measurements must be positive values.";
return;
}
var bodyFatPercentage = 0;
var heightM = heightCm / 100; // Convert height to meters for potential other formulas, though not used in this log10 version
if (gender === "male") {
// US Navy Method for Men (adapted)
var logWaistMinusNeck = Math.log10(waist – neck);
var logHeight = Math.log10(heightCm); // Height in cm for this specific formula
if (isNaN(logWaistMinusNeck) || isNaN(logHeight) || logHeight === 0) {
resultDiv.innerHTML = "Calculation error. Check measurements.";
return;
}
bodyFatPercentage = (495 / (1.0324 – 0.19077 * logWaistMinusNeck + 0.15457 * logHeight)) – 450;
} else { // Female
// US Navy Method for Women (adapted)
var logWaistPlusHipMinusNeck = Math.log10(waist + hip – neck);
var logHeight = Math.log10(heightCm); // Height in cm for this specific formula
if (isNaN(logWaistPlusHipMinusNeck) || isNaN(logHeight) || logHeight === 0) {
resultDiv.innerHTML = "Calculation error. Check measurements.";
return;
}
bodyFatPercentage = (495 / (1.29579 – 0.35004 * logWaistPlusHipMinusNeck + 0.22100 * logHeight)) – 450;
}
// Ensure body fat percentage is not negative (can happen with extreme measurements)
if (bodyFatPercentage 90) {
bodyFatPercentage = 90;
}
var fatMassKg = (bodyFatPercentage / 100) * weight;
var leanBodyMassKg = weight – fatMassKg;
// Calculate BFMI (Body Fat Mass Index) – This is different from Body Fat Percentage
// BFMI = (Fat Mass in kg / (Height in meters)^2) * 100. Note: Sometimes just calculated as Fat Mass / Height^2
// Let's stick to the common definition of Body Fat Percentage first, as BFMI is often confused.
// If the user truly wants BFMI index, it's: Lean Body Mass (kg) / (Height (m))^2
// Let's provide both: Body Fat Percentage and a derived BFMI value.
var heightMeters = heightCm / 100;
var bfmiIndex = leanBodyMassKg / (heightMeters * heightMeters);
resultDiv.innerHTML = "Your Estimated Body Fat: " + bodyFatPercentage.toFixed(2) + "%" +
"Fat Mass: " + fatMassKg.toFixed(2) + " kg" +
"Lean Body Mass: " + leanBodyMassKg.toFixed(2) + " kg" +
"Body Fat Mass Index (BFMI): " + bfmiIndex.toFixed(2) + "";
}