function toggleHipField() {
var gender = document.getElementById('gender').value;
var hipField = document.getElementById('hipField');
hipField.style.display = (gender === 'female') ? 'block' : 'none';
}
function updateLabels() {
var units = document.getElementById('units').value;
var suffix = (units === 'imperial') ? '(in)' : '(cm)';
var weightSuffix = (units === 'imperial') ? '(lbs)' : '(kg)';
document.getElementById('heightLabel').innerText = 'Height ' + suffix;
document.getElementById('weightLabel').innerText = 'Weight ' + weightSuffix;
document.getElementById('neckLabel').innerText = 'Neck Circumference ' + suffix;
document.getElementById('waistLabel').innerText = 'Waist Circumference ' + suffix;
document.getElementById('hipLabel').innerText = 'Hip Circumference ' + suffix;
}
function calculateBodyFat() {
var gender = document.getElementById('gender').value;
var units = document.getElementById('units').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 hip = parseFloat(document.getElementById('hip').value) || 0;
if (!height || !weight || !neck || !waist || (gender === 'female' && !hip)) {
alert('Please fill in all fields correctly.');
return;
}
var hVal = height;
var nVal = neck;
var wVal = waist;
var hiVal = hip;
// Convert to metric for formula if imperial
if (units === 'imperial') {
hVal = height * 2.54;
nVal = neck * 2.54;
wVal = waist * 2.54;
hiVal = hip * 2.54;
}
var bf = 0;
if (gender === 'male') {
bf = 495 / (1.0324 – 0.19077 * Math.log10(wVal – nVal) + 0.15456 * Math.log10(hVal)) – 450;
} else {
bf = 495 / (1.29579 – 0.35004 * Math.log10(wVal + hiVal – nVal) + 0.22100 * Math.log10(hVal)) – 450;
}
if (bf 100) bf = 100;
var resultArea = document.getElementById('resultArea');
var bfDisplay = document.getElementById('bfPercentage');
var categoryDisplay = document.getElementById('bfCategory');
var massDisplay = document.getElementById('fatMass');
resultArea.style.display = 'block';
bfDisplay.innerText = bf.toFixed(1) + '%';
var category = "";
var color = "";
if (gender === 'male') {
if (bf < 6) { category = "Essential Fat"; color = "#3498db"; }
else if (bf < 14) { category = "Athletes"; color = "#27ae60"; }
else if (bf < 18) { category = "Fitness"; color = "#f1c40f"; }
else if (bf < 25) { category = "Average"; color = "#e67e22"; }
else { category = "Obese"; color = "#e74c3c"; }
} else {
if (bf < 14) { category = "Essential Fat"; color = "#3498db"; }
else if (bf < 21) { category = "Athletes"; color = "#27ae60"; }
else if (bf < 25) { category = "Fitness"; color = "#f1c40f"; }
else if (bf < 32) { category = "Average"; color = "#e67e22"; }
else { category = "Obese"; color = "#e74c3c"; }
}
categoryDisplay.innerText = "Category: " + category;
bfDisplay.style.color = color;
var fatWeight = (weight * (bf / 100)).toFixed(1);
var leanWeight = (weight – fatWeight).toFixed(1);
var unitLabel = (units === 'imperial') ? ' lbs' : ' kg';
massDisplay.innerHTML = "Fat Mass: " + fatWeight + unitLabel + " | Lean Mass: " + leanWeight + unitLabel + "";
resultArea.scrollIntoView({ behavior: 'smooth' });
}
How the U.S. Navy Body Fat Calculator Works
The U.S. Navy Body Fat Calculator uses a formula developed by the Naval Health Research Center to estimate an individual's body fat percentage. Unlike a standard BMI (Body Mass Index) calculator, which only uses height and weight, the Navy method incorporates circumference measurements to better distinguish between fat tissue and lean muscle mass.
The Importance of Body Fat Percentage
Body fat percentage is often considered a superior health metric compared to total weight. Two people can weigh exactly the same, but if one is an athlete with high muscle mass and the other has a sedentary lifestyle, their health risks are vastly different. Excessive body fat is linked to cardiovascular diseases, type 2 diabetes, and various metabolic disorders.
How to Measure Accurately
For the most accurate results with this calculator, follow these measurement guidelines:
Height: Measure without shoes, standing straight against a flat surface.
Neck: Measure just below the larynx (Adam's apple), with the tape slanting slightly downward toward the front.
Waist (Men): Measure at the navel (belly button) level.
Waist (Women): Measure at the narrowest part of the torso (natural waistline).
Hips (Women only): Measure at the widest point of the buttocks.
Understanding Your Results
Category
Women (% Fat)
Men (% Fat)
Essential Fat
10-13%
2-5%
Athletes
14-20%
6-13%
Fitness
21-24%
14-17%
Average
25-31%
18-24%
Obese
32%+
25%+
Accuracy and Limitations
While the U.S. Navy formula is remarkably accurate for a tape-measure method (usually within 3-4% of DEXA scans), it remains an estimate. It may overestimate body fat in individuals with significant muscle mass in their necks or underestimate it in those with "stubborn" fat stores in areas not measured by the formula. For medical-grade accuracy, consider professional methods like hydrostatic weighing or DEXA scanning.