Body Mass Index (BMI) and Body Fat Percentage are two crucial metrics for assessing your overall health and fitness level. While BMI provides a quick estimation of body weight relative to height, body fat percentage offers a more nuanced view of your body composition, distinguishing between fat mass and lean mass. This calculator helps you compute both and provides insights based on your inputs.
What is BMI?
BMI is a value derived from the mass (weight) and height of a person. It's a screening tool used to indicate the range of weight categories that may increase the risk of health problems. The most commonly used formula for BMI is:
BMI = Weight (kg) / (Height (m))^2
To use this calculator, please enter your weight in kilograms and your height in centimeters. The calculator will automatically convert your height to meters for the calculation.
What is Body Fat Percentage?
Body fat percentage is the total weight of fat in your body divided by your total body weight, expressed as a percentage. This metric is often considered a better indicator of health than BMI because it accounts for muscle mass. High body fat percentage is linked to increased risks of cardiovascular disease, type 2 diabetes, certain cancers, and other health issues.
Calculating body fat percentage accurately can be complex and often requires specialized equipment (like DEXA scans or bioelectrical impedance analysis). However, several estimation formulas exist using measurements like waist circumference, hip circumference, neck circumference, and height.
This calculator uses the U.S. Navy's body fat estimation formula for men and a modified version for women, which utilizes waist, hip (for women), and neck circumferences, along with height.
Note: The accuracy of these formulas can vary based on individual body types and measurement precision. The 'log10' represents the base-10 logarithm.
Interpreting Your Results
Once you have your BMI and body fat percentage, you can compare them to standard charts to understand your health status.
BMI Categories (WHO):
Below 18.5: Underweight
18.5 – 24.9: Normal weight
25.0 – 29.9: Overweight
30.0 and above: Obesity
General Body Fat Percentage Guidelines (approximate):
Men:
Athletes: 6-13%
Fitness: 14-17%
Average: 18-24%
Obese: 25%+
Women:
Athletes: 14-20%
Fitness: 21-24%
Average: 25-31%
Obese: 32%+
Remember, these are general guidelines. Consulting with a healthcare professional or a certified fitness trainer is always recommended for personalized advice.
function calculateBmiAndBodyFat() {
var weightKg = parseFloat(document.getElementById("weight").value);
var heightCm = parseFloat(document.getElementById("height").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 neckCm = parseFloat(document.getElementById("neck").value);
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = "; // Clear previous results
// — Input Validation —
if (isNaN(weightKg) || isNaN(heightCm) || isNaN(age) || isNaN(waistCm) || isNaN(neckCm) || (gender === 'female' && isNaN(hipCm)) ) {
resultDiv.innerHTML = 'Error: Please enter valid numbers for all required fields.';
return;
}
if (heightCm <= 0 || weightKg <= 0 || waistCm <= 0 || neckCm <= 0 || (gender === 'female' && hipCm <= 0) || age <= 0) {
resultDiv.innerHTML = 'Error: All measurements and age must be positive values.';
return;
}
// — BMI Calculation —
var heightM = heightCm / 100; // Convert cm to meters
var bmi = weightKg / (heightM * heightM);
var bmiCategory = "";
if (bmi = 18.5 && bmi = 25 && bmi <= 29.9) {
bmiCategory = "Overweight";
} else {
bmiCategory = "Obese";
}
// — Body Fat Percentage Calculation (U.S. Navy Method) —
var bodyFatPercentage = 0;
var bodyFatCategory = "";
// Helper function for logarithm
function log10(value) {
return Math.log(value) / Math.log(10);
}
if (gender === "male") {
if (waistCm <= neckCm) { // Waist should be larger than neck for the formula
resultDiv.innerHTML = 'Error: Waist circumference must be greater than neck circumference for males.';
return;
}
var bfNumerator = 495;
var bfDenominator = 1.0324 – (0.19077 * log10(waistCm – neckCm)) + (0.15457 * log10(heightCm));
if (bfDenominator === 0) { // Avoid division by zero
resultDiv.innerHTML = 'Error: Calculation error. Check your input values.';
return;
}
bodyFatPercentage = (bfNumerator / bfDenominator) – 450;
} else { // Female
if (hipCm <= 0) { // Hip circumference is required for females
resultDiv.innerHTML = 'Error: Please enter Hip Circumference for females.';
return;
}
var bfNumerator = 495;
var bfDenominator = 1.29579 – (0.13704 * log10(hipCm + waistCm – neckCm)) + (0.05264 * log10(heightCm));
if (bfDenominator === 0) { // Avoid division by zero
resultDiv.innerHTML = 'Error: Calculation error. Check your input values.';
return;
}
bodyFatPercentage = (bfNumerator / bfDenominator) – 450;
}
// Clamp body fat percentage to realistic values (e.g., 1% to 99%)
bodyFatPercentage = Math.max(1, Math.min(99, bodyFatPercentage));
// — Determine Body Fat Category —
if (gender === "male") {
if (bodyFatPercentage = 6 && bodyFatPercentage = 14 && bodyFatPercentage = 18 && bodyFatPercentage <= 24) bodyFatCategory = "Average";
else bodyFatCategory = "Obese";
} else { // Female
if (bodyFatPercentage = 14 && bodyFatPercentage = 21 && bodyFatPercentage = 25 && bodyFatPercentage <= 31) bodyFatCategory = "Average";
else bodyFatCategory = "Obese";
}
// — Display Results —
var resultHTML = 'Your Results:';
resultHTML += 'BMI: ' + bmi.toFixed(1) + ' (' + bmiCategory + ')';
resultHTML += 'Body Fat %: ' + bodyFatPercentage.toFixed(1) + '% (' + bodyFatCategory + ')';
resultDiv.innerHTML = resultHTML;
}