A tool called body mass index (BMI) can be used to help decide when adults may be at risk for developing health problems because they are underweight or overweight. BMI uses a chart based on weight and height. BMI is calculated by dividing people's weight (measured in kilograms) by their height (measured in metres squared). The result is looked up in the table to determine the risk of developing health problems.
Health risk and body mass index (BMI)
|Group||BMI (kg/m2)||Risk of developing health problems|
|underweight||less than 18.5||increased risk|
|normal weight||18.5–24.9||least risk|
obese class I
obese class II
obese class III
|30 or more
40 or more
very high risk
extremely high risk
Source: Health Canada
BMI is not used the same way for people under 18 or women who are pregnant or breastfeeding. Finding out a child’s BMI is different than for adults. Children and adolescents are still growing and their amount of body fat changes as they grow. BMI charts are also different for boys and girls. Talk to your doctor to find out if your child is underweight or overweight for their age.
Drawbacks of using BMI
BMI is not a perfect tool. There are some drawbacks to using it as a measure of obesity.
- BMI is an indirect measure of body fat.
- BMI does not necessarily reflect the changes that occur with age.
- BMI and body fat actually differ for men and women. Women often have a greater percentage of body fat, but this is not reflected in height and weight measurements.
- BMI may not reflect the difference between excess body fat and muscle mass. For example, lean people, such as athletes, with high muscle mass sometimes have high BMI scores.
- BMI may be based on self-reported height and weight, which can affect accuracy.