What Is Fat Mass and How to Calculate It

In today's world, the understanding of body composition, particularly fat mass, has become increasingly important. Fat mass, also known as body fat, is a significant concern due to its association with various health conditions and a decrease in life quality. Therefore, it is essential to understand what fat mass is, how it can be calculated, and the implications of having a high fat mass percentage.

What is Fat Mass?

Fat mass refers to the total amount of fats present in a human body. It is usually expressed in percentage points relating to the total body mass. There are two types of fat mass:

  1. Primary or Essential Fat: This type of fat is crucial for survival, overall health, and reproductive function. It is found in the central nervous system, bone marrow, mammary glands, around the kidneys, spleen, and some other tissues. The percentage of essential body fat is higher in women due to the demands of pregnancy and other hormonal functions. In males, the amount of essential fat should not be less than 3-5%, and for females, it should be more than 12%.

  2. Secondary or Storage Fat: This type of fat, accumulated in adipose tissue, represents the body's main energy reserve. It is found mainly at the subcutaneous level, but also in the visceral area. The proportion between the two varies depending on age, sex, ethnicity, and physical fitness. Normal values are around 12%.

How to Calculate Body Fat?

There are several methods to estimate or calculate fat mass, varying in complexity and accuracy:

  1. Body Mass Index (BMI): Although BMI doesn't measure body fat directly, it's often used as an indirect indicator. It's calculated by dividing your weight in kilograms by the square of your height in meters (kg/m²). However, it doesn't distinguish between weight from fat and muscle, making it less reliable for determining fat mass.
  2. Skinfold Measurements: This method involves measuring the thickness of skinfolds at specific body sites with calipers. These measurements are then used in equations to estimate body density and, subsequently, body fat percentage. From the body fat percentage, fat mass can be calculated by multiplying the body fat percentage by the total body weight.
  3. Bioelectrical Impedance Analysis (BIA): BIA devices pass a small electrical current through the body to estimate body composition, including body fat percentage. The resistance to the electrical current differs between fat, muscle, and water, allowing the device to estimate fat mass.
  4. Dual-Energy X-ray Absorptiometry (DEXA): DEXA is a more precise method that involves scanning the body with X-rays to measure bone density, lean body mass, and fat mass. It provides detailed body composition analysis, including regional fat distribution.
  5. Hydrostatic Weighing: Also known as underwater weighing, this method compares your normal body weight (outside the water) to your body weight while submerged. The difference is used to calculate body density, from which body fat percentage and fat mass can be estimated.
  6. Air Displacement Plethysmography (Bod Pod): Similar to hydrostatic weighing, this method measures body volume by displacing air in a sealed chamber. It then uses this volume along with body weight to calculate body density and, subsequently, body fat percentage and fat mass.

However, a simpler and more immediate method is calculating lean mass from anthropometric values such as height and some body circumferences, using the formula developed by Wilmore and Behnke. This formula uses height and body circumferences (e.g., chest, waist, hips, and limbs) to estimate body density, from which one can derive body fat percentage and subsequently lean body mass.

The exact formula can vary depending on the specific measurements available and may be different for men and women due to differences in body composition. Generally, the formula involves measurements of the circumference of various body parts and the individual's height. You can quickly find many online calculator sites by Googling "Wilmore and Behnke online calculator".

However, for the most accurate assessment, professional methods like DEXA scans or hydrostatic weighing are recommended. To accurately assess your body fat mass, you should consider consulting with one of the following professionals, depending on the method of assessment and the level of precision you're seeking:

  • Certified Personal Trainer or Fitness Coach
  • Registered Dietitian or Nutritionist
  • Exercise Physiologist
  • Sports Medicine Doctor

What is the Ideal Fat Mass Percentage?

The ideal fat mass percentage varies between men and women. The total "reference" fat mass should represent 15% in men and 25% in women. These values are lower in athletes, where levels close to the primary fat percentage are reached, and higher in obese individuals.

How to Reduce Fat Mass?

The reduction of fat mass, particularly secondary fat, requires establishing a negative caloric balance. This means consuming more energy than what is introduced through the diet, establishing what is technically defined as a low-calorie diet. However, a low-calorie diet does not necessarily mean a "drastic calorie cut". There are various ways to reduce energy intake, such as eliminating junk foods and alcohol, reducing portions, reducing the amount of seasoning oil, etc.

It is important to structure the dietary pattern carefully, especially in certain conditions like athletes, the elderly, or those with certain diseases. Therefore, to lose weight correctly, it is best to consult a dietician.


Understanding and managing body fat is essential for maintaining a healthy lifestyle. The ability to calculate and interpret fat mass can provide valuable insights into one's health and fitness level. It's important to remember that while having some body fat is necessary for bodily functions, too much can lead to health problems. Therefore, maintaining a balanced diet and regular exercise routine is key to managing body fat levels.

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