Body Mass Index Body Fat Calculator

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BMI & Body Fat Calculator

Male Female

Understanding BMI and Body Fat

Maintaining a healthy body composition is crucial for overall well-being and longevity. Body Mass Index (BMI) and Body Fat Percentage are two key metrics used to assess an individual's health status relative to their height and weight, and the distribution of fat in their body. This calculator helps you determine both, providing insights into your current health and potential risks.

What is BMI?

Body Mass Index (BMI) is a simple numerical index calculated from a person's weight and height. It's a widely used screening tool to identify individuals who may be at a higher risk for health problems associated with being underweight or overweight.

How is BMI Calculated?

The formula for BMI is:

  • Metric Units: BMI = Weight (kg) / (Height (m))^2
  • Since height is often measured in centimeters, the formula can be adjusted: BMI = Weight (kg) / (Height (cm) / 100)^2 which simplifies to BMI = (Weight (kg) * 10000) / (Height (cm))^2

BMI Categories:

  • Underweight: BMI < 18.5
  • Normal weight: 18.5 – 24.9
  • Overweight: 25 – 29.9
  • Obesity: BMI ≥ 30

It's important to note that BMI does not account for body composition (muscle vs. fat) and may not be accurate for athletes or very muscular individuals.

What is Body Fat Percentage?

Body Fat Percentage is the total weight of a person's fat divided by their total body weight. This metric provides a more nuanced view of health than BMI, as it distinguishes between fat mass and lean body mass (muscle, bone, organs). Excess body fat, especially visceral fat (around the organs), is linked to numerous health issues, including heart disease, type 2 diabetes, and certain cancers.

How is Body Fat Percentage Calculated (using measurements)?

This calculator uses the U.S. Navy Body Fat Formula, which is a common method utilizing circumference measurements. The specific formulas differ slightly for men and women:

  • For Men:
    Body Fat % = 495 / (1.0324 - 0.19077 * log10(Waist + Neck - Hip)) + 457 / (1.0324 - 0.19077 * log10(Waist + Neck - Hip)) - 414.15 Simplified for easier programming with given inputs (Waist, Neck): Body Fat % = 86.010 * log10(Waist - Neck) - 70.041 * log10(Height) + 30.308
  • For Women:
    Body Fat % = 495 / (1.29579 - 0.35004 * log10(Waist + Hip - Neck)) + 163.454 / (1.29579 - 0.35004 * log10(Waist + Hip - Neck)) - 414.15 Simplified for easier programming with given inputs (Waist, Hip, Neck): Body Fat % = 163.205 * log10(Waist + Hip - Neck) - 97.684 * log10(Height) - 78.387

Note: The U.S. Navy formula is an estimation and can be less accurate for individuals with very high or very low body fat percentages. The "hip" measurement is typically used for women; for men, it's often ignored or set to zero in variations of the formula, or sometimes it's assumed to be 37 inches (approx 94 cm) if not provided. Here we use a version that accounts for hip for women if provided. For men, the hip measurement is not used in the common U.S. Navy formula.

This calculator uses a common implementation of the U.S. Navy formula.

Interpreting Your Results

Use these results as a starting point for understanding your health. Consult with a healthcare professional for personalized advice.

Example Calculation:

Let's consider a 35-year-old male, weighing 80 kg, standing 180 cm tall, with a waist circumference of 90 cm and a neck circumference of 40 cm.

  • BMI Calculation: BMI = (80 * 10000) / (180 * 180) = 800000 / 32400 = 24.69 This falls into the "Normal weight" category.
  • Body Fat % Calculation (Male): log10(Waist – Neck) = log10(90 – 40) = log10(50) ≈ 1.699 log10(Height) = log10(180) ≈ 2.255 Body Fat % = 86.010 * 1.699 – 70.041 * 2.255 + 30.308 Body Fat % ≈ 146.13 – 157.94 + 30.308 ≈ 18.5

The results indicate a normal BMI and an estimated body fat percentage of approximately 18.5% for this individual.

function calculateBmiAndBodyFat() { var gender = document.getElementById("gender").value; var age = parseFloat(document.getElementById("age").value); var weight = parseFloat(document.getElementById("weight").value); var heightCm = parseFloat(document.getElementById("height").value); var waist = parseFloat(document.getElementById("waist").value); var hip = parseFloat(document.getElementById("hip").value); var neck = parseFloat(document.getElementById("neck").value); var bmiResultElement = document.getElementById("bmiResult"); var bodyFatResultElement = document.getElementById("bodyFatResult"); // Clear previous results bmiResultElement.innerHTML = ""; bodyFatResultElement.innerHTML = ""; var isValid = true; if (isNaN(weight) || weight <= 0) { bmiResultElement.innerHTML = "Please enter a valid weight."; isValid = false; } if (isNaN(heightCm) || heightCm <= 0) { bmiResultElement.innerHTML += "Please enter a valid height."; isValid = false; } if (isNaN(age) || age <= 0) { bodyFatResultElement.innerHTML = "Please enter a valid age."; isValid = false; } if (isNaN(waist) || waist <= 0) { bodyFatResultElement.innerHTML += "Please enter a valid waist circumference."; isValid = false; } if (isNaN(neck) || neck <= 0) { bodyFatResultElement.innerHTML += "Please enter a valid neck circumference."; isValid = false; } if (gender === "female" && (isNaN(hip) || hip <= 0)) { bodyFatResultElement.innerHTML += "Please enter a valid hip circumference for females."; isValid = false; } if (!isValid) { return; } // BMI Calculation var heightM = heightCm / 100; var bmi = weight / (heightM * heightM); bmi = bmi.toFixed(2); var bmiCategory = ""; if (bmi = 18.5 && bmi = 25 && bmi <= 29.9) { bmiCategory = "Overweight"; } else { bmiCategory = "Obese"; } bmiResultElement.innerHTML = "Your BMI is: " + bmi + " (" + bmiCategory + ")"; // Body Fat Percentage Calculation (U.S. Navy Method) var bodyFat = 0; var bodyFatCategory = ""; // Check for valid circumference values for calculation var canCalculateBodyFat = true; if (isNaN(waist) || waist <= 0) canCalculateBodyFat = false; if (isNaN(neck) || neck <= 0) canCalculateBodyFat = false; if (gender === "female" && (isNaN(hip) || hip <= 0)) canCalculateBodyFat = false; if (isNaN(heightCm) || heightCm <= 0) canCalculateBodyFat = false; // Height is used in some Navy formula variations if (canCalculateBodyFat) { if (gender === "male") { // U.S. Navy formula for men using Waist and Neck // Simplified version often cited: BF% = 495 / (1.0324 – 0.19077 * log10(Waist – Neck)) – 450 // Another common implementation: 86.010 * log10(Waist – Neck) – 70.041 * log10(Height) + 30.308 var logWaistMinusNeck = Math.log10(waist – neck); var logHeight = Math.log10(heightCm); bodyFat = 86.010 * logWaistMinusNeck – 70.041 * logHeight + 30.308; } else { // Female // U.S. Navy formula for women using Waist, Hip, and Neck // Simplified version often cited: BF% = 495 / (1.29579 – 0.35004 * log10(Waist + Hip – Neck)) – 450 // Another common implementation: 163.205 * log10(Waist + Hip – Neck) – 97.684 * log10(Height) – 78.387 var logWaistHipMinusNeck = Math.log10(waist + hip – neck); var logHeight = Math.log10(heightCm); bodyFat = 163.205 * logWaistHipMinusNeck – 97.684 * logHeight – 78.387; } bodyFat = bodyFat.toFixed(2); // Body Fat Categories (Approximate, can vary by source) var minMaleFat = 0, maxMaleFat = 0, minFemaleFat = 0, maxFemaleFat = 0; if (gender === "male") { if (age < 20) { minMaleFat = 8; maxMaleFat = 20; } else if (age < 40) { minMaleFat = 11; maxMaleFat = 22; } else if (age < 60) { minMaleFat = 13; maxMaleFat = 25; } else { minMaleFat = 14; maxMaleFat = 27; } if (bodyFat = minMaleFat && bodyFat maxMaleFat && bodyFat <= maxMaleFat + 6) bodyFatCategory = "Overweight"; else bodyFatCategory = "Obese"; } else { // Female if (age < 20) { minFemaleFat = 11; maxFemaleFat = 22; } else if (age < 40) { minFemaleFat = 14; maxFemaleFat = 25; } else if (age < 60) { minFemaleFat = 16; maxFemaleFat = 28; } else { minFemaleFat = 17; maxFemaleFat = 30; } if (bodyFat = minFemaleFat && bodyFat maxFemaleFat && bodyFat <= maxFemaleFat + 6) bodyFatCategory = "Overweight"; else bodyFatCategory = "Obese"; } bodyFatResultElement.innerHTML = "Estimated Body Fat: " + bodyFat + "% (" + bodyFatCategory + ")"; } else { bodyFatResultElement.innerHTML = "Could not calculate body fat due to missing measurements."; } }

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