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Health basics guide

BMI: what the number can and cannot tell you

Body mass index, or BMI, is a screening measure based on height and weight. It can be a convenient way to describe weight relative to height in broad populations, but it is not a diagnosis and it does not capture every factor that matters to an individual’s health.

3 min read · Published September 16, 2026

For general information only. This guide is not medical advice and does not diagnose or treat any condition.

What the calculation measures

BMI compares weight with height. In U.S. customary units, the formula divides weight in pounds by height in inches squared, then multiplies by 703. The same relationship can be calculated from kilograms and meters in metric units.

The result is a single ratio. It does not directly measure body fat, fitness, diet, blood pressure, medical history, or how weight is distributed in the body.

Why context matters

People with the same BMI can have different body composition, health histories, activity levels, and needs. For example, muscle mass, pregnancy, age, and some health conditions can make a simple height-and-weight ratio less informative for an individual.

A calculator can help you record a number or discuss it with a clinician, but it cannot determine whether you have a condition or tell you what treatment, diet, or exercise plan is right for you.

Use a result as a prompt for a better question

If a BMI result concerns you, focus on the next useful question rather than trying to interpret it alone. A healthcare professional can consider your overall health, medications, symptoms, family history, and any appropriate measurements or tests.

For day-to-day tracking, use consistent measurements and avoid treating small changes as a complete picture of health.

Try it step by step

  1. 1Choose the measurement system you use most consistently.
  2. 2Enter your current height and weight.
  3. 3Record the result only as a screening value, not a diagnosis.
  4. 4Discuss questions about weight or health with a qualified healthcare professional.

Keep in mind

  • BMI is not designed to replace professional medical assessment.
  • Do not use a single result to self-diagnose or change prescribed treatment.
  • Urgent symptoms or immediate health concerns need timely professional care.