What is BMI?

What is BMI?

Diet and weight, young woman sitting on her haunches on a scale

Body Mass Index (BMI) is a standard measure of body fat. It is calculated based on height and weight. The formula to calculate BMI is mass (kg) divided by height (m) squared. Although BMI does not directly measure of body fat, previous research has demonstrated a strong correlation between BMI and direct body fat measurements. Because BMI is inexpensive and easy to calculate, health professionals often use BMI measurements to gauge an individual’s risk for diseases that are linked to excess weight. In general, the higher the BMI then the higher the risk for type 2 diabetes, hypertension, and heart disease. It is critical to remember that BMI is not a diagnostic tool, but rather a screening tool that should be used in combination with medical assessments including a detailed family history, evaluations of diet and lifestyle, waist circumference and other relevant health screens. You should pay attention to your BMI because it is a quick and easy indicator of your weight status, and in some cases it may suggest that it is time to adjust your diet and activity level in order to achieve a healthy weight.


BMI measurements for adults

BMI recommendations are the same for both men and women over the age of 20 years old. You may think that BMI recommendations should be different for men and women since you probably expect that the healthy weight for a woman of a specific height would be lower than the healthy weight for man of that same height. However, research has shown that the medical risks that are associated with a BMI greater than 25 are the same for both men and women. For example, the risk of death due to heart disease is strikingly similar for men and women as BMI increases. Similar findings were reported for the risk of death due to complications associated with diabetes. The table below shows how people are placed into various weight status categories based on their BMI.


Weight Status BMI
Underweight Less than 18.5
Normal weight 18.5 to 24.9
Overweight 25 to 29.9
Obese 30 or greater

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Limitations of BMI measurements

Despite the fact that there is a strong correlation between BMI measurements and body fat, factors such as race, age, sex, and muscle mass may also influence the reliability of BMI measurements. Because the BMI calculation does not differentiate between the sexes, it does not take into account that women tend to have more body fat that men. Therefore, if a man and a woman have the same BMI, the woman will likely have more body fat that the man. Similarly, athletes tend to be very muscular, and as such their BMI may be higher and may overestimate their body fat. On the other hand, since muscle mass tends to decrease with age, the BMI of an older person may underestimate their body fat. With regards to race, the BMI is not a reliable measure of body fat in African American men and women that are middle-aged and older. Compared to their Hispanic and Caucasian counterparts, African Americans had less body fat at the same BMI. Despite the limitations associated with BMI, it is a good screening tool when used in conjunction with other medical assessments.