BMI Calculator

BMI categories

The World Health Organization defines five adult BMI classifications:

• Below 18.5: Underweight. Associated with nutritional deficiency and increased risk of certain conditions including osteoporosis and anemia.
• 18.5–24.9: Normal weight. The range associated with the lowest all-cause mortality risk in most large epidemiological studies.
• 25–29.9: Overweight. Modestly elevated risk of cardiovascular disease and type 2 diabetes in population-level data.
• 30–34.9: Obesity class I. Substantially elevated metabolic risk.
• 35–39.9: Obesity class II. High risk; often qualifies for weight-loss intervention.
• 40 and above: Obesity class III (severe obesity). Very high risk of metabolic, cardiovascular, and musculoskeletal complications.

How BMI is calculated

BMI is your weight divided by the square of your height. In metric units:

BMI = weight (kg) ÷ height (m)²

In imperial units the formula adjusts for unit conversion:

BMI = weight (lb) × 703 ÷ height (in)²

A 70 kg person who is 1.75 m tall: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9. That falls in the normal weight range. The healthy weight range shown is the reverse: the weight that would give a BMI of exactly 18.5 and exactly 24.9 at your height.

Limitations of BMI

BMI was developed in the 1830s by mathematician Adolphe Quetelet as a statistical tool for describing populations, not for diagnosing individuals. Several limitations are well documented:

• Muscle vs. fat: BMI measures weight, not composition. A trained athlete with low body fat and high muscle mass can register as overweight or obese. Conversely, a sedentary person at a "normal" BMI may carry high visceral fat.
• Age: Older adults tend to have more body fat at the same BMI than younger adults. The thresholds were derived from studies skewed toward working-age populations.
• Ethnicity: The WHO cutoffs were calibrated on studies dominated by non-Hispanic white populations. Studies of South Asian, East Asian, and some other populations suggest metabolic risk rises at lower BMIs. Some guidelines use adjusted cutoffs for these groups.
• Sex: Women typically carry more body fat than men at the same BMI; standard cutoffs don't account for this.

Despite its limitations, BMI remains in wide clinical use because it is quick, free, and correlates reasonably well with metabolic risk at the population level. For individual assessment, clinicians pair it with waist circumference, lipid panels, blood pressure, and blood glucose.

Frequently asked questions

What is a healthy BMI?

The WHO defines 18.5–24.9 as normal weight. The range associated with the lowest all-cause mortality in most studies is roughly 20–25. Your individual risk profile depends on many factors beyond BMI alone.

Is BMI accurate for athletes?

No; athletes with high muscle mass routinely show "overweight" or "obese" BMI values despite low body fat. For anyone who trains seriously, body fat percentage (via DEXA, bod pod, or skinfold calipers) is a much more informative measure.

Does BMI apply to children?

Standard adult cutoffs don't apply to children. Pediatric BMI is interpreted using age- and sex-specific percentile charts. This calculator is for adults 18 and over.

What units does this calculator use?

Both metric (kg and cm) and imperial (lb, feet, and inches). It defaults to the system that matches your browser's locale and remembers your choice for future visits.

BMI vs. other body composition measures

BMI is a starting point, not a complete picture. Clinicians often pair it with: