Last updated: July 2026
What Is BMI and How Is It Calculated?
Body Mass Index (BMI) is a screening measurement that estimates whether your weight is in a healthy range relative to your height. It's calculated two ways depending on your unit system:
- Imperial: BMI = (weight in lbs × 703) / (height in inches)²
- Metric: BMI = weight in kg / (height in meters)²
For example, someone who is 5'9" (69 inches) and weighs 160 lbs has a BMI of (160 × 703) / 69² = 23.6, which falls in the normal weight range.
BMI Categories
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obesity |
These ranges are the standard adult categories used by the CDC and World Health Organization. They apply to adults aged 20 and older — children and teens are assessed using age- and sex-specific percentile charts instead.
US Obesity Statistics
According to the CDC's most recent NHANES data (August 2021–August 2023), 40.3% of US adults have obesity (BMI of 30 or higher) — 39.2% of men and 41.3% of women. Obesity prevalence is highest among adults aged 40–59 (46.4%) compared with younger and older age groups. This rate has remained relatively stable over the past decade after a period of earlier increases.
Limitations of BMI
BMI is a useful, low-cost screening tool, but it doesn't measure body fat directly and has known limitations. It doesn't distinguish between muscle and fat, so it can overestimate body fat in muscular or athletic individuals and underestimate it in older adults who've lost muscle mass. Body fat at a given BMI can also vary by sex, age, and race or ethnicity — for example, research suggests health risks may begin at a lower BMI for some Asian populations compared with other groups. For these reasons, BMI is best used as a general screening indicator rather than a standalone diagnosis, and a healthcare provider can offer a fuller picture using additional measurements.