function toggleHipInput() {
var gender = document.getElementById("bf_gender").value;
var hipBox = document.getElementById("hip_container");
if (gender === "female") {
hipBox.style.display = "block";
} else {
hipBox.style.display = "none";
}
}
function calculateBodyFat() {
var gender = document.getElementById("bf_gender").value;
var height = parseFloat(document.getElementById("bf_height").value);
var neck = parseFloat(document.getElementById("bf_neck").value);
var waist = parseFloat(document.getElementById("bf_waist").value);
var hips = parseFloat(document.getElementById("bf_hips").value);
var resultContainer = document.getElementById("bf_result_container");
var percentageDisplay = document.getElementById("bf_percentage_display");
var categoryDisplay = document.getElementById("bf_category_display");
if (isNaN(height) || isNaN(neck) || isNaN(waist) || (gender === "female" && isNaN(hips))) {
alert("Please fill in all fields with valid numbers.");
return;
}
var bodyFat = 0;
if (gender === "male") {
// US Navy Formula for Men (Metric)
bodyFat = 495 / (1.0324 – 0.19077 * Math.log10(waist – neck) + 0.15456 * Math.log10(height)) – 450;
} else {
// US Navy Formula for Women (Metric)
bodyFat = 495 / (1.29579 – 0.35004 * Math.log10(waist + hips – neck) + 0.22100 * Math.log10(height)) – 450;
}
if (bodyFat < 0) bodyFat = 0;
var category = "";
if (gender === "male") {
if (bodyFat < 6) category = "Essential Fat";
else if (bodyFat < 14) category = "Athlete";
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 = "Athlete";
else if (bodyFat < 25) category = "Fitness";
else if (bodyFat < 32) category = "Average";
else category = "Obese";
}
percentageDisplay.innerHTML = bodyFat.toFixed(1) + "%";
categoryDisplay.innerHTML = "Category: " + category;
resultContainer.style.display = "block";
}
Understanding Body Fat Percentage
Body fat percentage is a much more accurate health metric than Body Mass Index (BMI). While BMI only considers your weight and height, body fat percentage measures how much of your total body weight is actually fat versus lean mass (muscle, bone, and organs).
How the Navy Method Works
This calculator uses the U.S. Navy Body Fat Formula, which is considered one of the most reliable "circumference-based" methods. It requires measurements of the neck, waist (at the navel for men, or narrowest point for women), and hips (for women only). By comparing these measurements to your height, the algorithm can estimate your body density and fat composition.
Body Fat Categories for Men and Women
Men and women have different biological requirements for fat. Women require higher fat levels for reproductive health and hormonal balance.
| Category |
Men Range |
Women Range |
| Essential Fat |
2 – 5% |
10 – 13% |
| Athletes |
6 – 13% |
14 – 20% |
| Fitness |
14 – 17% |
21 – 24% |
| Average |
18 – 24% |
25 – 31% |
| Obese |
25% + |
32% + |
Tips for Accuracy
- Be Consistent: Take measurements at the same time of day, preferably in the morning before eating.
- Measuring the Waist: For men, measure at the level of the navel. For women, measure at the narrowest part of the natural waistline.
- Don't Pull Too Tight: The tape should be snug against the skin but not compressing it.
- Height: Measure while standing straight against a wall without shoes.
Real-World Example
Consider a male who is 180 cm tall, with a neck circumference of 40 cm and a waist circumference of 90 cm. When these values are entered into the Navy formula, the result is approximately 18.7%. This puts the individual in the "Average" to "Fitness" transition zone, suggesting a healthy body composition for most active adults.