What Is BMI?
BMI (Body Mass Index) is a number calculated from your height and weight. It’s widely used as a quick screening tool because it’s fast, consistent, and easy to compare across large groups of adults.
How BMI is calculated
BMI is based on the idea that, for many people, body weight tends to scale with the square of height.
Metric formula
BMI = weight (kg) ÷ height² (m²)
Imperial formula
BMI = (weight (lb) × 703) ÷ height² (in²)
What BMI is useful for
BMI is most helpful when you use it as a starting point, especially for tracking changes over time, making quick comparisons in population research, and prompting a closer look when combined with other information.
It’s not designed to describe your exact body fat percentage, fitness, or overall health by itself.
How to interpret a BMI number
A BMI value is typically interpreted using standard adult ranges, but the right interpretation depends on context such as age, sex, muscle mass, and health history.
For the full ranges and an easy breakdown, use the BMI Chart.
Limitations of BMI (important)
BMI does not measure body fat directly. Two people can have the same BMI but very different body composition. BMI can be misleading for some groups, including:
- Very muscular people (BMI may overestimate body fat)
- Older adults (BMI may underestimate fat if muscle mass is low)
- Pregnant people (BMI is not interpreted the same way)
- Children and teens (they use age- and sex-specific percentiles)
BMI works best alongside other measures such as waist circumference, waist-to-height ratio, fitness level, and clinical markers.
What to do next
If you’re using BMI for personal tracking, these are sensible next steps:
- Calculate BMI consistently (same units, similar time of day)
- Track the trend over weeks/months instead of one reading
- Add waist measurement or waist-to-height ratio for extra context
- If you’re concerned about symptoms or risk factors, discuss results with a clinician