What Is BMI? Understanding Your Body Mass Index

Updated June 2026 - 4 min read

BMI - Body Mass Index - is a simple number calculated from your height and weight. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and has been used by physicians and public health researchers ever since as a quick screening tool. It is not a diagnosis, but it can flag potential health risks worth discussing with a doctor.

How Is BMI Calculated?

The formula differs depending on which measurement system you use:

• Imperial: BMI = 703 x weight (lbs) divided by height (in)²
• Metric: BMI = weight (kg) divided by height (m)²

For example, a person who is 5'9" (69 inches) and weighs 160 lbs has a BMI of: 703 x 160 divided by (69 x 69) = 23.6

What Do the Categories Mean?

Limitations of BMI

BMI is a useful starting point, but it has real limitations:

• It does not measure body fat directly. A muscular athlete can have an "overweight" BMI despite low body fat.
• It does not account for age. Older adults naturally carry more fat relative to lean mass.
• It does not reflect fat distribution. Belly fat is riskier than fat stored in hips or thighs, but BMI cannot distinguish the two.
• It may be less accurate across ethnicities. People of Asian descent face elevated health risks at lower BMI thresholds.

Why BMI Still Matters

Despite its limitations, BMI remains widely used because it is free, fast, and reasonably correlated with health outcomes at the population level. Studies consistently show that people with a BMI above 30 have higher rates of type 2 diabetes, cardiovascular disease, certain cancers, and sleep apnea. If your BMI falls outside the normal range, it is a prompt to dig deeper - not a verdict.