Op.Dr.Gökhan ATEŞ — Antalya Obezite CerrahisiOp.Dr.Gökhan ATEŞ
Patient Education

BMI and How Obesity Is Defined

Body Mass Index (BMI) is the most widely used measure for defining obesity. However, BMI is not a diagnosis by itself; it is interpreted together with body composition, coexisting conditions, and metabolic findings.

How BMI Is Calculated

BMI = weight (kg) divided by the square of height (m²). For example, a person who is 90 kg and 1.70 m tall has a BMI of about 90 / (1.70 × 1.70) ≈ 31.1.

WHO Classification

Underweight: below 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

Obesity class III (severe obesity): 40 and above

Limitations of BMI

BMI does not separate muscle from fat. Highly trained athletes can have a high BMI without excess fat; older adults may lose muscle and show a misleadingly normal BMI. Waist circumference, body fat percentage, and metabolic parameters are therefore evaluated together.

When to Consult a Physician

People with a BMI of 30 and above, or with extra weight alongside conditions such as type 2 diabetes, hypertension, sleep apnea, or heart disease, may benefit from a bariatric and metabolic surgery assessment.

References

  • World Health Organization — Obesity and overweight fact sheet
  • IFSO — Indications for metabolic and bariatric surgery, 2022 update
Medical notice: The information on this page is for general patient education only and does not replace medical diagnosis, treatment, or a surgical decision. The appropriate method for you can only be determined after a physician's assessment and necessary tests.

Last updated: 28.06.2026 · Medical content: Op.Dr.Gökhan ATEŞ

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