function toggleHipInput() {
var gender = document.getElementById('gender').value;
var hipContainer = document.getElementById('hip-container');
if (gender === 'female') {
hipContainer.style.display = 'block';
} else {
hipContainer.style.display = 'none';
}
}
function calculateBodyFat() {
var gender = document.getElementById('gender').value;
var height = parseFloat(document.getElementById('height').value);
var neck = parseFloat(document.getElementById('neck').value);
var waist = parseFloat(document.getElementById('waist').value);
var hip = parseFloat(document.getElementById('hip').value);
var resultBox = document.getElementById('bf-result-box');
var bfDisplay = document.getElementById('bf-percentage');
var catDisplay = document.getElementById('bf-category');
if (!height || !neck || !waist || (gender === 'female' && !hip)) {
alert('Please enter all required measurements.');
return;
}
var bodyFat = 0;
if (gender === 'male') {
// US Navy Formula for Men: 495 / (1.0324 – 0.19077 * log10(waist – neck) + 0.15456 * log10(height)) – 450
bodyFat = 495 / (1.0324 – 0.19077 * Math.log10(waist – neck) + 0.15456 * Math.log10(height)) – 450;
} else {
// US Navy Formula for Women: 495 / (1.29579 – 0.35004 * log10(waist + hip – neck) + 0.22100 * log10(height)) – 450
bodyFat = 495 / (1.29579 – 0.35004 * Math.log10(waist + hip – neck) + 0.22100 * Math.log10(height)) – 450;
}
if (isNaN(bodyFat) || bodyFat 60) {
alert('The measurements provided resulted in an unrealistic calculation. Please check your inputs.');
return;
}
bfDisplay.innerHTML = bodyFat.toFixed(1) + "%";
var category = "";
if (gender === 'male') {
if (bodyFat < 6) category = "Essential Fat";
else if (bodyFat < 14) category = "Athletes";
else if (bodyFat < 18) category = "Fitness";
else if (bodyFat < 25) category = "Average";
else category = "Obese";
} else {
if (bodyFat < 14) category = "Essential Fat";
else if (bodyFat < 21) category = "Athletes";
else if (bodyFat < 25) category = "Fitness";
else if (bodyFat < 32) category = "Average";
else category = "Obese";
}
catDisplay.innerHTML = "Category: " + category;
resultBox.style.display = "block";
}
Understanding Body Fat Percentage
Body fat percentage is a much more accurate metric for health and fitness than simple body weight or Body Mass Index (BMI). While BMI only considers height and weight, body fat percentage distinguishes between lean muscle mass and adipose tissue (fat).
The U.S. Navy Body Fat Method
This calculator uses the U.S. Navy Circumference Method, which is one of the most widely used and scientifically validated equations for estimating body composition without expensive clinical equipment. By measuring the circumference of the neck, waist, and hips (for women), the formula can estimate body density with a high degree of correlation to hydrostatic weighing.
Body Fat Categories
Healthy body fat ranges vary significantly between men and women due to physiological differences and reproductive requirements. Here is a general breakdown:
Description
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%+
How to Get Accurate Measurements
Height: Measure without shoes, standing straight against a flat wall.
Neck: Measure just below the larynx (Adam's apple), with the tape sloping slightly downward to the front.
Waist (Men): Measure at the level of the navel.
Waist (Women): Measure at the narrowest part of the torso (above the navel).
Hips (Women only): Measure at the widest point of the buttocks.
Why Track Body Fat?
If you are on a fitness journey, "weight" can be misleading. As you gain muscle and lose fat, your weight might stay the same even though your health and appearance are improving significantly. Tracking your body fat percentage every 4 weeks allows you to ensure that the weight you are losing is actually fat, not valuable muscle mass.