What Is BMI and How Do You Calculate It?
2026-06-03 · 5 min read
BMI is the world's most common weight screening number. Here is the formula, the categories, and what BMI does — and does not — tell you.
Body Mass Index (BMI) is a single number that relates your weight to your height. Health services worldwide use it as a quick screening tool to flag whether someone may be underweight or carrying excess weight.
The formula
In metric units, BMI = weight (kg) ÷ height (m)². In imperial units, BMI = 703 × weight (lb) ÷ height (in)². The 703 factor converts the imperial result onto the same scale as metric.
The WHO categories
- Under 18.5 — underweight
- 18.5 to 25 — normal weight
- 25 to 30 — overweight
- 30 and above — obese
What BMI does not measure
BMI ignores body composition. Muscle is denser than fat, so athletes and very muscular people often read as "overweight" despite low body fat. It also does not account for fat distribution, age, or sex. Treat BMI as a starting point, not a diagnosis.
For a fuller picture, professionals combine BMI with waist measurement, body-fat percentage, and overall health markers.
Calculate yours
The BMI Calculator accepts both metric and imperial input and shows your category against the WHO ranges instantly — nothing is uploaded.