BMI
BMI stands for Body Mass Index and was first introduced July 1972[1] . It's a calculation used to assess body fat levels based on an individual's weight and height.
It can be a preliminary indicator on whether an individual should consider losing weight which is important because overweight is one of the main triggers for many Non-Communicable Diseases — the leading cause of death worldwide. [2]
BMI is not perfect, but it is a reliable measure of relative obesity. It's a good screening tool. If you have a high BMI, you should investigate why.
Calculation
It is calculated by dividing a person's weight in kilograms by their height in meters squared (kg/m²).
The formula is written:
Where:
- W is Weight in KG
- H is Height in meters
BMI is used by the World Health Organization in their official definition of overweight and obesity:
The diagnosis of overweight and obesity is made by measuring people’s weight and height and by calculating the body mass index (BMI): weight (kg)/height² (m²). The body mass index is a surrogate marker of fatness and additional measurements, such as the waist circumference, can help the diagnosis of obesity. For adults, WHO defines overweight and obesity as follows: Overweight is a BMI greater than or equal to 25; and Obesity is a BMI greater than or equal to 30. [3]
BMI Categories
The full range of categories:
| Category | BMI (kg/m²) |
|---|---|
| Underweight (Severe thinness) | < 16.0 |
| Underweight (Moderate thinness) | 16.0 – 16.9 |
| Underweight (Mild thinness) | 17.0 – 18.4 |
| Normal range | 18.5 – 24.9 |
| Overweight (Pre-obese) | 25.0 – 29.9 |
| Obese (Class I) | 30.0 – 34.9 |
| Obese (Class II) | 35.0 – 39.9 |
| Obese (Class III) | ≥ 40.0 |
Graphing this out, you can visualize the different categories and quite easily figure out which category you're in, as long as you know your weight and height.
However, BMI does not directly measure body fat percentage and may not accurately reflect the body composition of those with high muscle mass.
Limitations
BMI does not directly measure body fat percentage
BMI is calculated using only weight and height, which means it does not differentiate between body fat and lean mass (such as muscle and bone). This can lead to misclassifications of body fat levels, particularly in individuals with high muscle mass, such as athletes or bodybuilders, who may be incorrectly categorized as overweight or obese based on their BMI alone.
BMI may not be accurate for certain populations
BMI categories are based on average body compositions, which can vary among different racial and ethnic groups.
For instance, the NHS claims[4] that if you have an Asian, Chinese, Middle Eastern, Black African or African-Caribbean family background you'll need to use a lower BMI score to measure overweight and obesity:
- 23 to 27.4 – you're in the overweight range
- 27.5 or above – you're in the obese range
BMI does not account for fat distribution
The location of body fat is important in assessing health risks. Visceral fat (fat stored around the organs in the abdominal area) is more strongly associated with health problems than subcutaneous fat (fat stored beneath the skin). BMI does not distinguish between these types of fat.
BMI is a screening tool, not a diagnostic tool
While BMI can be useful as a preliminary indicator of potential weight-related health risks, it should not be used as the sole diagnostic tool. Additional assessments, such as waist circumference measurements, body fat percentage tests, and other health markers, should be considered for a more comprehensive evaluation.
BMI may not reflect overall health
A person with a "normal" BMI can still have an unhealthy body composition or poor health due to factors like poor diet, lack of physical activity, or underlying health conditions. Conversely, an individual with a higher BMI may be healthy if they have a high proportion of lean mass and engage in healthy lifestyle behaviors.
References
Keys, Ancel, Flaminio Fidanza, Martti J. Karvonen, Noboru Kimura, and Henry L. Taylor. "Indices of Relative Weight and Obesity." Journal of Chronic Diseases 25, no. 6 (July 1972): 329-343. doi:10.1016/0021-9681(72)90027-6. ↩︎
World Health Organization, "Noncommunicable diseases," World Health Organization, last modified September 16, 2023, https://www.who.int/news-room/fact-sheets/detail/noncommunicable-diseases. ↩︎
World Health Organization. "Obesity and Overweight." Last modified March 1, 2024. https://www.who.int/news-room/fact-sheets/detail/obesity-and-overweight. ↩︎
NHS. "Obesity." February 15, 2023. https://www.nhs.uk/conditions/obesity/. ↩︎