What is Body Mass Index

Body Mass Index

What is Body Mass Index

Body Mass Index (BMI) is a gauge of standard weight and height ratios. Research studies in large groups of people have shown that the BMI can be classified into ranges associated with health risk. There are four categories of BMI ranges.

• Underweight (BMI less than 18.5);


• Standardweight (BMIs 18.5 to 24.9);


• Overweight (BMIs 25 to 29.9), and


• Obese (BMI 30 and over).

BMI Formula

BMI (kg/m²) = weight in kilograms / height in meters²

Low or High BMI Associations

Most adults with a high BMI (overweight or obese) have a high percentage of body fat. Extra body fat is associated with increased risk of health problems such as diabetes, heart disease, high blood pressure, gallbladder disease and various forms of cancer. This is a preventable condition.

A low BMI (underweight) is associated with health problems such as osteoporosis, undernutrition and eating disorders.

The risk of developing weight-related health problems increases the more one's BMI falls outside the normal weight category. It is important to note that sudden or considerable weight gains or weight losses may also indicate health risk, even if this occurs within the normal weight BMI category.

BMI Classification Limitations

The classification system may underestimate or overestimate health risks in certain adults, such as, highly muscular adults, adults who naturally have a very lean body build, young adults who have not reached full growth, and adults over 65 years of age.

Very muscular adults, such as athletes, may have a low percentage of body fat but a large amount of muscle tissue. This can result in a BMI in the overweight range which may over estimate the risk of developing health problems.

For people who naturally have a very lean body build or for young adults who have not attained their full growth, a BMI just below the 'normal weight' range may not indicate an increased health risk.

For adults over age 65, more research is needed to determine if the cut-off points for the 'normal weight' range differ in any way from those for younger adults.

It is also important to note that BMI is only one part of a health risk assessment. To further clarify risk, other factors shoud be considered.