What is BMI?
Body Mass Index (BMI) is a simple calculation that compares your height and weight to give an estimate of body fat. While it doesn’t measure fat directly, BMI has long been used as a screening tool to help gauge whether someone may fall into categories like underweight, healthy weight, overweight, or obesity.
Healthcare professionals often use BMI as an initial reference point—not a final diagnosis. A higher or lower BMI can suggest potential risks for weight-related health conditions, such as type 2 diabetes, heart disease, or high blood pressure. It may also determine eligibility for certain medical treatments or prescription weight-management medications.That said, BMI has its limitations. It does not consider muscle mass, body fat distribution, age, sex, or ethnicity—all of which can significantly affect your overall health picture. Someone with a muscular build may technically fall into the “overweight” range even if their health markers are excellent.
Despite these shortcomings, research shows that BMI remains strongly linked to the likelihood of developing certain chronic conditions. When combined with other indicators such as waist size, blood sugar levels, and cholesterol, BMI can serve as a useful starting point in evaluating health.
BMI Calculator
Check your Body Mass Index and category. (Screening tool only.)
The formula for BMI is straightforward:Metric system:BMI = weight (kg) ÷ height² (m²)U.S. system:BMI = [weight (lb) ÷ height² (in²)] × 703Once calculated, your BMI falls into one of these general adult categories (CDC standards):
- Underweight: <18.5
- Healthy weight: 18.5–24.9
- Overweight: 25–29.9
- Obesity, Class I: 30–34.9
- Obesity, Class II: 35–39.9
- Obesity, Class III: 40+
Note: These categories apply to adults aged 20 and older. They are not designed for children, teens, or people who are pregnant.
- Underweight (<18.5): May signal increased risk of malnutrition, weakened bones, or compromised immunity.
- Healthy weight (18.5–24.9): Generally considered to carry the lowest risk for chronic conditions.
- Overweight (25–29.9): Can raise the likelihood of high blood pressure, insulin resistance, and other metabolic issues.
- Obesity Classes I–III (30+): Higher levels of obesity are linked with greater risks of type 2 diabetes, sleep apnea, cardiovascular disease, and certain cancers.
While BMI can highlight possible concerns, it doesn’t reflect:
- Muscle vs. fat: Athletes may appear “overweight” despite low body fat.
- Fat distribution: Visceral fat (around the abdomen) is more harmful than fat stored elsewhere.
- Metabolic health: BMI doesn’t measure cholesterol, blood pressure, or blood sugar.
- Fitness level: A physically active person with a higher BMI may still be healthier than someone with a “normal” BMI who rarely exercises.
- Age, sex, ethnicity, and genetics: All play major roles in body composition and health risk.
Think of BMI as a conversation starter, not a verdict. A “normal” BMI doesn’t guarantee good health, just as being in the overweight or obesity range doesn’t automatically mean you’re unhealthy.
If your number falls outside the “healthy” category, it may be worth looking at your lifestyle, activity levels, and other health markers. In some cases, a higher BMI can qualify you for structured medical support or prescription treatments.
The most valuable step? Talk with a qualified healthcare provider who can interpret your BMI alongside other factors—lab results, family history, nutrition, fitness, and mental health—to help you create a plan that truly fits your body and goals.
