How is BMI calculated?

BMI is calculated as weight in kilograms divided by height in meters squared: BMI = kg/m². In imperial units: BMI = (weight in pounds × 703) / (height in inches)². For example, a 70 kg person who is 1.75 m tall has a BMI of 22.9 kg/m².

What are the BMI categories?

WHO BMI categories are: Underweight < 18.5, Normal 18.5–24.9, Overweight 25.0–29.9, Obesity Class I 30.0–34.9, Obesity Class II 35.0–39.9, and Obesity Class III ≥ 40.0 kg/m². These thresholds may differ for Asian populations (overweight ≥ 23, obese ≥ 27.5).

What are the limitations of BMI?

BMI does not distinguish between lean mass and fat mass, so it may misclassify muscular individuals as overweight. It does not account for fat distribution (central vs. peripheral obesity), age, sex, or ethnicity-specific risk profiles. Waist circumference and waist-to-hip ratio may better predict metabolic risk.

When should BMI be measured?

BMI should be calculated at every adult primary care visit as part of routine screening. The USPSTF recommends screening all adults for obesity and offering intensive behavioral interventions for patients with BMI ≥ 30. For children, BMI percentile for age and sex is used instead.

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BMI Calculator

Calculate body mass index (BMI) and view WHO weight status categories.

Inputs

Weight (kg)

Height (cm) Tip: 175 cm = 1.75 m

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Enter weight and height to calculate BMI.

Underweight< 18.5

Normal18.5–24.9

Overweight25.0–29.9

Obesity (Class I)30.0–34.9

Obesity (Class II)35.0–39.9

Obesity (Class III)≥ 40.0

Disclaimer: For educational purposes only. Not a substitute for clinical judgment.

When to use

BMI is a screening tool for weight status in adults. It does not directly measure adiposity and may misclassify muscular individuals, older adults, and some ethnic groups.

Clinical pearls

Consider waist circumference or other cardiometabolic risk markers when BMI is borderline.
Use age- and sex-specific percentiles for children/adolescents (not adult BMI cutoffs).

References

World Health Organization. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894, 2000.

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About This Tool

What Is BMI?

Body Mass Index (BMI) is a simple anthropometric measure calculated as weight in kilograms divided by height in meters squared (kg/m²). First described by Adolphe Quetelet in the 19th century, BMI has become the most widely used screening tool for classifying weight status in adults. The World Health Organization (WHO) defines standard BMI categories: underweight (<18.5), normal weight (18.5–24.9), overweight (25.0–29.9), and obesity (≥30.0), with obesity further subdivided into Class I (30–34.9), Class II (35–39.9), and Class III (≥40).

When to Use BMI Clinically

BMI is appropriate as an initial screening tool in primary care, preventive health visits, and population-level epidemiological studies. It is most useful for identifying patients who may benefit from further cardiometabolic risk assessment, including waist circumference measurement, lipid panels, fasting glucose, and blood pressure monitoring. BMI is incorporated into numerous clinical guidelines, including those from the ACC/AHA, WHO, and NICE, for initiating conversations about weight management and assessing eligibility for bariatric surgery (typically BMI ≥40, or ≥35 with comorbidities).

Interpreting BMI Results

While BMI correlates with body fat percentage at the population level, it does not directly measure adiposity. It can overestimate body fat in muscular individuals (e.g., athletes) and underestimate it in those with sarcopenia (e.g., elderly patients with preserved fat mass but reduced muscle). Asian populations may carry higher metabolic risk at lower BMI thresholds — the WHO suggests lowered cutoffs (overweight ≥23, obese ≥27.5) for Asian populations.

🔑 Clinical Pearls

BMI alone does not capture metabolic health — an "obese" BMI with normal metabolic markers (metabolically healthy obesity) carries different risk than the same BMI with metabolic syndrome.
Waist circumference (>102 cm men, >88 cm women) adds significant prognostic value beyond BMI for cardiovascular and diabetes risk.
For children and adolescents, BMI must be interpreted using age- and sex-specific percentile charts (CDC or WHO growth charts), not adult cutoffs.
BMI is not validated for pregnancy — do not use for weight classification in pregnant patients.

Key References

World Health Organization. Obesity: Preventing and Managing the Global Epidemic. WHO Technical Report Series 894, 2000.
NHLBI Expert Panel. Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults. Obes Res. 1998;6(Suppl 2):51S–209S.
WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy. Lancet. 2004;363(9403):157–163.

Formula last verified: February 2026