Body Mass Index (BMI) is a standard measure of the ratio of weight to height, calculated as BMI = weight (kg) / height² (m²). Introduced by A. Quetelet in 1832 and popularized by A. Keys in 1972 as a convenient obesity screening tool.
WHO classification for adults: < 18.5 — underweight; 18.5-24.9 — normal; 25.0-29.9 — overweight; 30.0-34.9 — obesity class I; 35.0-39.9 — class II; ≥ 40.0 — class III (morbid). Thresholds are lower for Asian populations (overweight from 23, obesity from 27.5).
BMI is a simple and widely applicable screening tool, but it does not reflect fat distribution or muscle mass. For more precise assessment, use waist circumference (≥ 94 cm in men, ≥ 80 cm in women — abdominal obesity per IDF criteria), waist-to-hip ratio, or bioelectrical impedance analysis. Current international obesity guidance uses BMI as a mandatory initial screening measure, to be supplemented with waist circumference.
When to use
Initial visit — a standard part of anthropometric assessment.
Screening for cardiovascular and metabolic risk.
Risk stratification in patients with diabetes, hypertension, CKD, reproductive disorders, or joint pain.
Monitoring progress during weight-loss programs.
Parameters in detail
Weight
Body weight in kilograms, with clothing, without shoes.
Height
Standing height in meters, measured with a stadiometer.
Clinical example
Case
A 48-year-old woman, height 165 cm, weight 88 kg, waist circumference 96 cm. Reports dyspnea climbing 2 flights of stairs; type 2 diabetes diagnosed 2 years ago.
Calculation
BMI = 88 / (1.65)² = 88 / 2.72 = 32.3 kg/m².
Interpretation and management
Obesity class I plus abdominal obesity (waist > 80 cm). Combined with diabetes and dyspnea, this indicates high cardiometabolic risk. Approach: lifestyle modification (caloric deficit of 500-750 kcal/day, walking ≥ 150 min/week, resistance training 2x/week), discuss pharmacotherapy for obesity (semaglutide/liraglutide for BMI ≥ 30 with diabetes, orlistat), and consider bariatric surgery for BMI ≥ 35 with diabetes. A target weight loss of 5-10% meaningfully improves glycemic control and blood pressure.
Limitations and cautions
Does not distinguish fat mass from muscle mass — BMI is often overestimated in trained athletes without excess adiposity.
Does not account for fat distribution — abdominal obesity carries higher risk than peripheral obesity at the same BMI.
In older adults, sarcopenia can mask visceral obesity (normal BMI with an unfavorable metabolic profile).
Racial and ethnic differences exist — risk emerges at lower BMI thresholds in Asian populations.
Frequently asked questions
At what BMI should pharmacotherapy for obesity be started?
BMI ≥ 30, or ≥ 27 in the presence of comorbidities (diabetes, hypertension, dyslipidemia, OSA). First-line agents include liraglutide, semaglutide (where available), and orlistat; sibutramine may be used with caution where approved.
When is bariatric surgery indicated?
BMI ≥ 40, or ≥ 35 with severe comorbidity (diabetes, OSA, resistant hypertension). International bariatric surgery guidelines (e.g., ASMBS/IFSO) define detailed indications.
Can a person have a normal BMI but an unhealthy metabolic profile?
Yes, this is termed metabolically obese normal weight (MONW). It presents as abdominal obesity, dyslipidemia, and insulin resistance despite a normal BMI. It is more common in older adults and in patients with sarcopenia.
Is BMI suitable for children?
In children, BMI percentile curves (z-scores) are used rather than absolute thresholds. Obesity is defined as above the 97th percentile, overweight as above the 85th percentile.
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Aleksandr A. Aulov
Physician, internal medicine, Sechenov School — independent medical education platform.
Calculators are compiled from the original publications and current clinical guidelines. Interpretation thresholds follow the source study unless stated otherwise.
This calculator is intended for healthcare professionals. Do not use it for self-diagnosis or self-treatment. Management decisions are made by the treating clinician based on the full clinical picture of the individual patient.
Last clinical review: April 28, 2026 ·
Author: Aleksandr A. Aulov,
MD, internal medicine ·
Primary source: WHO. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894, 2000