Body Mass Index (BMI) and Body Fat Percentage are two distinct but related metrics used to assess a person's health status concerning their weight and body composition. While BMI provides a general indication, body fat percentage offers a more precise view of how much of your body weight is fat versus lean mass.
What is BMI?
Body Mass Index (BMI) is a value derived from the mass (weight) and height of a person. It is calculated using a simple formula and is widely used as a screening tool to indicate the level of body fat and thus the risk of health problems associated with weight.
The standard formula for BMI is:
BMI = weight (kg) / [height (m)]^2
To use the calculator, your height is required in centimeters (cm) and weight in kilograms (kg). The calculator will automatically convert cm to meters for the calculation.
BMI Categories:
Underweight: BMI < 18.5
Normal weight: BMI 18.5–24.9
Overweight: BMI 25–29.9
Obesity: BMI ≥ 30
It's important to note that BMI does not differentiate between muscle and fat. Therefore, individuals with a high muscle mass (like athletes) might have a high BMI and be incorrectly categorized as overweight or obese.
What is Body Fat Percentage?
Body fat percentage is the measure of how much of your total body weight is composed of fat tissue. This metric is generally considered a more accurate indicator of health than BMI because it distinguishes between fat mass and lean body mass (muscles, bones, organs, water).
Several methods exist to estimate body fat percentage, ranging from simple measurements to more complex technologies:
Circumference Methods: These use measurements like waist, hip, and neck circumference, along with height and gender, to estimate body fat. The U.S. Navy method is a common example.
Bioelectrical Impedance Analysis (BIA): This method sends a weak electrical current through the body. Fat resists the current more than lean tissue, allowing for an estimation. Many home scales and fitness trackers use this.
Skinfold Calipers: A trained professional pinches the skin and underlying fat at specific body sites.
DEXA Scans (Dual-energy X-ray Absorptiometry): A medical imaging technique that provides precise body composition data, considered a gold standard but not easily accessible.
Our calculator primarily uses the U.S. Navy Circumference Method for body fat estimation, as it can be done with basic measurements readily available.
*Note: The hip circumference is generally used for women in this calculation. For men, it's sometimes used if they have a proportionally larger hip circumference than waist, but the waist-only calculation is more standard. The calculator prompts for hip circumference for women and provides a more general estimation if not provided for men.*
General Body Fat Percentage Guidelines:
Men (Age 20-39): Essential Fat (2-5%), Athletes (6-13%), Fitness (14-17%), Average (18-24%), Obese (≥25%)
Women (Age 20-39): Essential Fat (10-13%), Athletes (14-20%), Fitness (21-24%), Average (25-31%), Obese (≥32%)
These are general guidelines and can vary based on age and individual factors.
How to Use This Calculator:
Enter your height in centimeters, weight in kilograms, age, gender, and the relevant circumference measurements (waist is always needed; hip is crucial for women and beneficial for men). Click "Calculate" to get your BMI, BMI category, and an estimated body fat percentage.
This calculator is a tool for general information and health awareness. It is not a substitute for professional medical advice. Always consult with a healthcare provider for personalized health assessments and recommendations.
function calculateBmiAndBodyFat() {
var heightCm = parseFloat(document.getElementById("height").value);
var weightKg = parseFloat(document.getElementById("weight").value);
var age = parseInt(document.getElementById("age").value);
var gender = document.getElementById("gender").value;
var waistCm = parseFloat(document.getElementById("waist").value);
var hipCm = parseFloat(document.getElementById("hip").value);
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = ""; // Clear previous results
// — BMI Calculation —
var bmi = 0;
var bmiCategory = "";
if (!isNaN(heightCm) && !isNaN(weightKg) && heightCm > 0 && weightKg > 0) {
var heightM = heightCm / 100; // Convert cm to meters
bmi = weightKg / (heightM * heightM);
bmi = bmi.toFixed(1); // Round to one decimal place
if (bmi = 18.5 && bmi = 25 && bmi 0 && !isNaN(heightCm) && heightCm > 0) {
// U.S. Navy Method for Men
var bodyFatRaw = (495 / (1.0324 – 0.19077 * (waistCm / 40.7) + 0.15456 * (heightCm / 24.6))) – 450;
bodyFatPercentage = bodyFatRaw.toFixed(1);
bodyFatMessage = "Estimated Body Fat Percentage (Men): " + bodyFatPercentage + "%";
} else {
bodyFatMessage = "Cannot calculate body fat percentage without valid waist and height.";
}
} else { // Female
if (!isNaN(waistCm) && waistCm > 0 && !isNaN(hipCm) && hipCm > 0 && !isNaN(heightCm) && heightCm > 0) {
// U.S. Navy Method for Women
var bodyFatRaw = (495 / (1.29579 – 0.35004 * (waistCm / hipCm) + 0.22100 * (heightCm / 24.6))) – 450;
bodyFatPercentage = bodyFatRaw.toFixed(1);
bodyFatMessage = "Estimated Body Fat Percentage (Women): " + bodyFatPercentage + "%";
} else {
bodyFatMessage = "Cannot calculate body fat percentage for women without valid waist, hip, and height.";
}
}
// — Display Results —
var resultHtml = "
Your Health Metrics
";
resultHtml += "
BMI: " + bmi + "
";
resultHtml += "
Category: " + bmiCategory + "
";
if (bodyFatMessage) {
resultHtml += "
" + bodyFatMessage + "
";
} else {
resultHtml += "
Body Fat: Calculation not possible with provided inputs.