Understanding BMI: What the Numbers Mean and How They Affect Your Health
Healthcare professionals often use it as an initial screening tool to identify whether an adult may be underweight, within a healthy weight range, overweight, or living with obesity.
BMI does not provide a complete picture of someone’s health, but understanding the numbers can help start a useful conversation about nutrition, lifestyle, and possible health risks.
How Is BMI Calculated?
BMI is calculated using the following formula:
BMI = weight in kilograms ÷ height in metres²
For example, someone who weighs 70 kilograms and is 1.75 meters tall would have a BMI of approximately 22.9:
70 ÷ (1.75 × 1.75) = 22.9
When using pounds and inches, the formula is:
BMI = weight in pounds ÷ height in inches² × 703
BMI measures weight relative to height. It does not directly measure body fat.
For adults aged 20 and older, BMI is generally divided into the following categories:
These categories are screening ranges rather than diagnoses. A person’s BMI should be considered alongside other information, including medical history, blood pressure, cholesterol, blood glucose, physical activity, diet, sleep, and overall fitness.
Below 18.5
18.5–24.9
25.0–29.9
30.0–34.9
35.0–39.9
40.0 or higher
18.5
Underweight
18.5–24.9
Healthy weight
25.0–29.9
Overweight
30.0–34.9
Class 1 obesity
35.0–39.9
Class 2 obesity
40.0 or higher
Class 3, severe obesity
What Does a Low BMI Mean?
A BMI below 18.5 falls within the underweight category. In some people, this may simply reflect their natural body type. In others, it may be connected to inadequate nutrition, an underlying health condition, digestive problems, loss of appetite, or unintentional weight loss.
Possible signs of poor nutrition include:
- Tiredness or weakness
- Getting ill frequently
- Taking longer to recover from illness
- Loss of muscle strength
- Unintentional weight loss
A very low BMI may be associated with a greater risk of poor physical performance and illness. Anyone experiencing unexplained weight loss or difficulty eating should speak with a healthcare professional.
