While many rely solely on BMI (Body Mass Index), it fails to distinguish between muscle mass and fat mass. The US Navy method is widely recognized as one of the most accurate "tape measure" techniques for estimating body composition without the need for expensive DEXA scans or hydrostatic weighing.
Measurement Best Practices
Height: Measure without shoes, standing straight against a wall.
Neck: Measure below the larynx, sloping slightly downward to the front.
Waist: For men, measure at the navel. For women, measure at the narrowest point of the natural waistline.
Hips (Women only): Measure at the widest part of the buttocks/hips.
Body Fat Categories for Adults
Category
Women
Men
Essential Fat
10-13%
2-5%
Athletes
14-20%
6-13%
Fitness
21-24%
14-17%
Average
25-31%
18-24%
Obese
32%+
25%+
function toggleHipInput() {
var gender = document.querySelector('input[name="gender"]:checked').value;
var hipContainer = document.getElementById('hip-container');
if (gender === 'female') {
hipContainer.style.display = 'block';
} else {
hipContainer.style.display = 'none';
}
}
function calculateBodyFat() {
var gender = document.querySelector('input[name="gender"]:checked').value;
var height = parseFloat(document.getElementById('height').value);
var weight = parseFloat(document.getElementById('weight').value);
var neck = parseFloat(document.getElementById('neck').value);
var waist = parseFloat(document.getElementById('waist').value);
var resultBox = document.getElementById('result-box');
var percentageDisplay = document.getElementById('fat-percentage');
var categoryDisplay = document.getElementById('fat-category');
var fatMassDisplay = document.getElementById('fat-mass');
var leanMassDisplay = document.getElementById('lean-mass');
if (!height || !weight || !neck || !waist || (gender === 'female' && !document.getElementById('hip').value)) {
alert("Please enter all required measurements.");
return;
}
var bodyFatPercentage = 0;
if (gender === 'male') {
// US Navy Formula for Men (Metric)
// %BF = 495 / (1.0324 – 0.19077 * log10(waist – neck) + 0.15456 * log10(height)) – 450
bodyFatPercentage = 495 / (1.0324 – 0.19077 * Math.log10(waist – neck) + 0.15456 * Math.log10(height)) – 450;
} else {
var hip = parseFloat(document.getElementById('hip').value);
// US Navy Formula for Women (Metric)
// %BF = 495 / (1.29579 – 0.35004 * log10(waist + hip – neck) + 0.22100 * log10(height)) – 450
bodyFatPercentage = 495 / (1.29579 – 0.35004 * Math.log10(waist + hip – neck) + 0.22100 * Math.log10(height)) – 450;
}
if (isNaN(bodyFatPercentage) || bodyFatPercentage <= 0) {
alert("Please check your measurements. The formula could not calculate a valid result.");
return;
}
var fatMass = (weight * (bodyFatPercentage / 100)).toFixed(1);
var leanMass = (weight – fatMass).toFixed(1);
percentageDisplay.innerText = bodyFatPercentage.toFixed(1) + "%";
fatMassDisplay.innerText = fatMass + " kg";
leanMassDisplay.innerText = leanMass + " kg";
var category = "";
var bgColor = "";
if (gender === 'male') {
if (bodyFatPercentage < 6) { category = "Essential Fat"; bgColor = "#3498db"; }
else if (bodyFatPercentage < 14) { category = "Athlete"; bgColor = "#27ae60"; }
else if (bodyFatPercentage < 18) { category = "Fitness"; bgColor = "#2ecc71"; }
else if (bodyFatPercentage < 25) { category = "Average"; bgColor = "#f1c40f"; }
else { category = "Obese"; bgColor = "#e74c3c"; }
} else {
if (bodyFatPercentage < 14) { category = "Essential Fat"; bgColor = "#3498db"; }
else if (bodyFatPercentage < 21) { category = "Athlete"; bgColor = "#27ae60"; }
else if (bodyFatPercentage < 25) { category = "Fitness"; bgColor = "#2ecc71"; }
else if (bodyFatPercentage < 32) { category = "Average"; bgColor = "#f1c40f"; }
else { category = "Obese"; bgColor = "#e74c3c"; }
}
categoryDisplay.innerText = category;
categoryDisplay.style.backgroundColor = bgColor;
resultBox.style.display = 'block';
resultBox.style.backgroundColor = bgColor + "11"; // Light tint of category color
resultBox.scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}