Walking shoes, a measuring tape, water and fresh food, representing healthy weight and body mass index

Body Mass Index for Adults

Body mass index, usually shortened to BMI, is a simple screening number that compares your weight to your height. Because it is quick to calculate and requires nothing more than a scale and a tape measure, BMI is widely used as a first look at whether an adult's weight may pose a health risk. It is a useful starting point — but it is only a starting point, and understanding what it does and does not measure helps you put the number in context.

This article is for general education only and is not a substitute for professional medical advice. BMI is a screening tool, not a diagnosis. Talk with your healthcare provider about what a healthy weight means for you.

What BMI Measures

BMI is calculated from your weight and height — in metric terms, weight in kilograms divided by the square of height in meters. The result is a single number that sorts adults into broad weight categories. It was designed for populations and screening, which is why it works well for spotting general patterns but less well for judging any one individual.

The Adult BMI Categories

For most adults aged 20 and over, the standard categories are interpreted as follows:

  • Below 18.5 — underweight
  • 18.5 to 24.9 — healthy weight range
  • 25.0 to 29.9 — overweight
  • 30.0 and above — obesity

These cut-offs are the same for adult men and women. They are different for children and teens, whose BMI is interpreted using age- and sex-specific growth charts rather than these fixed numbers.

What the Number Can — and Cannot — Tell You

A higher BMI is associated, across large groups of people, with a greater risk of conditions such as high blood pressure, type 2 diabetes, heart disease and certain other illnesses. That makes it a helpful flag. What BMI cannot do is measure body composition. It does not distinguish muscle from fat, and it does not account for where fat is stored. A very muscular person may have a high BMI without excess fat, while someone with a "normal" BMI may still carry health risks. For that reason, clinicians often look at waist measurement and other factors alongside BMI.

Putting BMI in a Fuller Picture

Think of BMI as one instrument on the dashboard rather than the whole picture of health. Habits matter at least as much as a single number: a varied diet built on the principles in nutrition basics, regular physical activity, adequate sleep, and managing blood pressure and cholesterol all contribute to long-term well-being. If your BMI falls outside the healthy range, modest and sustainable changes — rather than crash dieting — are the approach supported by the evidence on losing weight safely.

Authoritative Resources

For more on body mass index and healthy weight, see the CDC's healthy-weight resources, the National Heart, Lung, and Blood Institute, and MedlinePlus on body weight.

Related reading: Losing Weight Safely, Nutrition Basics, and Understanding High Blood Pressure. Return to the health article index.