BMI (Body Mass Index) is calculated by dividing weight in kilograms by the square of height in meters: BMI = kg / m². The formula was developed by Belgian mathematician Adolphe Quetelet in the 1830s — not by a medical doctor, and not originally intended as a health metric. It became widely adopted by health organizations in the 1970s as a simple, cost-free population screening tool. Understanding what it does and doesn't measure is essential for interpreting your result correctly.
The WHO BMI Categories
- Below 18.5 — Underweight: may indicate nutritional deficiency or underlying condition
- 18.5 to 24.9 — Normal weight: associated with lower risk of weight-related health issues
- 25.0 to 29.9 — Overweight: elevated risk of cardiovascular disease, type 2 diabetes
- 30.0 and above — Obese: substantially elevated risk of multiple chronic conditions
How to Calculate Your BMI
In metric units: divide your weight in kilograms by the square of your height in meters. Example: 70 kg at 1.75 m = 70 / (1.75 × 1.75) = 70 / 3.0625 = 22.9. In imperial units: divide weight in pounds by the square of height in inches, then multiply by 703. Example: 154 lbs at 5'9" (69 inches) = (154 / (69 × 69)) × 703 = 22.9. Our BMI calculator handles both unit systems automatically.
The Limitations of BMI
BMI's main limitation is that it cannot distinguish between muscle and fat. Professional athletes frequently test as "overweight" or "obese" by BMI despite having very low body fat percentages, because muscle is denser than fat. A 90 kg bodybuilder and a 90 kg sedentary person with the same height have the same BMI but very different health profiles. BMI also does not account for fat distribution — visceral fat (around the abdomen) carries higher cardiovascular risk than subcutaneous fat regardless of total weight.
BMI and Ethnicity
Research has shown that the standard WHO BMI cutoffs may not apply equally across all ethnic groups. Some studies suggest that people of South Asian, East Asian, and African descent show increased cardiometabolic risk at lower BMI values than the standard cutoffs suggest. The WHO has proposed alternative cutoffs for Asian populations (overweight at 23, obese at 27.5) though these are not universally adopted.
What to Do With Your BMI Result
A BMI result is a starting point, not a diagnosis. Use it as one data point alongside waist circumference (a direct measure of abdominal fat), blood pressure, cholesterol levels, and blood glucose. A BMI in the normal range does not guarantee good health; a BMI in the overweight range does not mean a medical problem exists. For personalized health assessment, consult a qualified healthcare professional.