Home›Health›BMI

BMI

Body Mass Index for adults

BMI (Body Mass Index) is a simple ratio of weight to height squared, intended as a quick population-level screening metric. It does not measure body fat directly and is not appropriate for athletes, the elderly, children, or anyone with significant muscle mass.

Used responsibly, BMI gives a rough indicator that may prompt further conversation with a healthcare provider. Used carelessly, it can be misleading.

BMI categories (adults)

  • Underweight: BMI < 18.5
  • Normal weight: 18.5 ≤ BMI < 25
  • Overweight: 25 ≤ BMI < 30
  • Obese: BMI ≥ 30

These thresholds come from the World Health Organization. Some Asian-population guidelines use slightly lower cutoffs.

The formula

  • Metric: BMI = weight (kg) / height (m)²
  • Imperial: BMI = (weight (lb) / height (in)²) × 703

Worked example

An adult who is 175 cm (1.75 m) tall and weighs 75 kg:

  • BMI = 75 / (1.75 × 1.75) = 75 / 3.0625 ≈ 24.5
  • That falls into the "normal weight" category.

When BMI is useful

As a quick conversation starter or as part of a larger health checkup. Public health authorities use it to track population trends. It is cheap, requires no equipment, and is broadly correlated with body fat in non-athletic populations.

When BMI is misleading

  • Athletes often have BMI in the "overweight" or "obese" range due to muscle mass, despite very low body fat.
  • Elderly individuals may lose muscle and gain fat without BMI changing.
  • Children use age- and sex-specific percentile charts, not adult BMI cutoffs.
  • Different ethnic backgrounds have different fat-percentage-vs-BMI relationships.

For a more direct picture, body fat percentage (via the Navy method calculator on this site, DEXA scan, or other measurement) is more informative.

Frequently asked questions

Is BMI a measure of body fat?

No. It is a height-weight ratio that correlates with body fat in the average non-athletic adult. For a direct body fat estimate, use the Body Fat Percentage calculator.

My BMI says I am overweight but I am muscular. What does that mean?

BMI does not distinguish muscle from fat. If you are athletic, BMI is the wrong tool — body fat percentage or waist-to-hip ratio is more meaningful.

Should I aim for the middle of the normal range?

BMI is a screening tool, not a target. A healthcare provider can interpret BMI in context with other measurements (waist circumference, body composition, family history).

Related tools