Health & Fitness · Glossary
What is BMI (Body Mass Index)?
Quick definition
A numerical value calculated from weight and height that classifies adults as underweight, normal, overweight, or obese.
Full explanation
BMI (Body Mass Index) is a simple screening metric: weight in kg divided by height in meters squared (kg/m²). It was developed in the 1830s by Adolphe Quetelet and is the standard screening tool used by doctors worldwide. Categories (WHO): Underweight <18.5, Normal 18.5–24.9, Overweight 25–29.9, Obese ≥30. BMI is fast, free, and reproducible — but it has major limitations: it does not distinguish muscle from fat, so athletes are often classified as "overweight" or "obese." It also varies by ethnicity (Asian populations have higher risk at lower BMIs). Use BMI as a starting point, not a final diagnosis.
Related calculators
Calculators that use or explain BMI.
Related terms
More from Health & Fitness
BMR (Basal Metabolic Rate)
The number of calories your body burns at complete rest, performing basic functions like breathing and circulation.
TDEE (Total Daily Energy Expenditure)
The total calories your body burns in a day, including all activity. The basis for diet planning.
Macronutrients
The three main nutrient groups the body needs in large quantities: protein, carbohydrates, and fat. The "macros."
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Last reviewed: June 15, 2026 • Category: Health & Fitness