Difference Between BMI and Body Fat

As obesity has become an ever increasing problem all over the world, people start to pay attention to things like diet, exercise, and body weight. Obesity is a modifiable risk factor for diabetes, high blood pressure, heart diseases, and strokes. BMI and body fat are terms closely related to obesity. However, there are differences between the two terms which is discussed here in detail.


BMI is the abbreviation for body mass index. It is the basis of the medical definition of obesity and overweight. Weight alone does not say much because a higher weight would be normal for a taller person while it is not so for a shorter person. Weight is directly related to height. Therefore, weight should be normal for a height. Body mass index is calculated using height in meters and weight in Kilograms. The equation is as follows.

Body mass index = Weight (Kg) / Height2 (m2)

World Health Organization has published the international cut off table for adult underweight, overweight, and obesity according to the body mass index.

  • Underweight is defined as body mass index below 18.5 Kgm-2.
  • Severe underweight is body mass index below 16 Kgm-2.
  • Moderate underweight is body mass index between 16 – 17 Kgm-2.
  • Mild underweight is body mass index between 17 – 18.5 Kgm-2.
  • Normal range is between 18.5 – 25 Kgm-2.
  • Pre obese is body mass index between 25 – 30 Kgm-2.
  • Obesity is body mass index above 30 Kgm‑2.


Obesity is classified into three severities. Class 1 is between 30 – 35 Kgm-2. Class 2 is between 35 – 40 Kgm-2. Class 3 is above 40 Kgm-2. Body mass index in pre-obese and obese levels are directly related to increased risk of non-communicable diseases. It is important to understand that while body mass index is directly related to waist circumference and abdominal fat it is not a good indicator of total body fat.

Body Fat

Body fat is not limited to the area around the waist. Body fat can be divided into three components. They are storage fats, structural fats, and brown fat. Storage fats are fats in adipose tissue. These are formed with excess energy and commonly found around the waist, thighs, neck, buttocks, and omentum inside the abdomen. These tissues contain adipocytes filled with complex fats. These cells are hormone sensitive, and they contain two types of enzymes that break the fat down. They are hormone sensitive lipase and lipoprotein lipase. Action of these enzymes governs the amount of fat stored in these tissues. When energy intake is below the expenditure, these fats get broken down and used for energy production.

Structural fats are fats incorporated into the cell and tissue structures. The cell membranes and organelle membranes are made up of a compound of fats and phosphates called phospholipids. There are various types of fats to tissue architecture. These fats are not utilized for energy production.

Brown fats are most commonly found in children. Brown fats act as good heat generators due to uncoupled cellular chain reactions, channeling energy produced by glucose to heat generation. Adults also have a limited amount of brown fat. In essence, nobody can reach “zero percent body fat” literally, but it is only an expression of storage fats.

What is the difference between BMI and Body Fat?

• Body mass index is an indicator of the relationship between weight and height while body fat is a much broader concept which covers total body fat content.

• Body mass index is directly related to storage fats.

• Body fat content is not used to define obesity while body mass index is.