What Is BMI and Why It Matters for Your Health

Body Mass Index (BMI) is a widely used screening tool in the United States to help estimate whether an adult’s body weight may be associated with health risks. While BMI does not directly measure body fat or muscle mass, it remains a useful population-level indicator of potential health concerns related to weight.

Body composition matters because both excess body fat and low muscle mass are linked to negative health outcomes. Higher levels of body fat are associated with increased risk for conditions such as heart disease, type 2 diabetes, and certain cancers, while insufficient muscle mass can raise the risk of falls, injury, and functional decline—especially as we age.

How BMI Is Calculated and Classified

BMI Formula

BMI is calculated using the following formula:

BMI = weight (kg) ÷ height (m²)

For reference, many people in the U.S. use online BMI calculators that automatically convert pounds and inches into metric units.

Adult BMI Categories (Ages 20 and Older)

• Underweight: Less than 18.5

• Healthy weight: 18.5 to less than 25

• Overweight: 25 to less than 30

• Obesity: 30 or higher

These BMI classifications are still used by the Centers for Disease Control and Prevention (CDC) and other major health organizations and remain current as of today.

Important Limitations of BMI

BMI Is a Screening Tool, Not a Diagnosis

BMI estimates weight-related health risk but does not directly measure body fat, muscle mass, or fat distribution.

When BMI May Be Misleading

• Athletes or highly active individuals may have a high BMI due to increased muscle mass, not excess fat

• Older adults may have a “normal” BMI but low muscle mass, which can increase health risks

• BMI does not account for where fat is stored in the body, which is an important factor in disease risk

Because of these limitations, many healthcare providers now recommend using BMI alongside other measures, such as waist circumference, physical activity level, and metabolic health markers.

How BMI Is Used to Guide Health Goals

Focus on Health, Not Just a Number

General guidance is to aim for both a healthy weight range and good physical fitness, rather than focusing on BMI alone.

Small Weight Changes Can Make a Big Difference

Research continues to show that even a modest weight loss of 5% can meaningfully improve health markers such as blood pressure, blood sugar, and cholesterol. For example, a person weighing 200 pounds may see health benefits from losing just 10 pounds.

Tracking Progress Over Time

If weight is a concern, tracking BMI, body weight, or waist circumference over time can help set realistic goals and monitor progress. For individuals who are overweight, simply preventing further weight gain is often a meaningful first step toward better health.

The Bottom Line on BMI

BMI remains a helpful, accessible starting point for understanding weight-related health risk—especially at the population level. However, it works best when combined with other indicators such as physical activity, muscle strength, waist circumference, and overall lifestyle habits.

For personalized guidance, it’s always best to discuss BMI results and health goals with a qualified healthcare provider.