Body Mass Index (BMI) is one of the most widely used — and widely misunderstood — numbers in everyday health screening. Here’s what it actually measures, and what it doesn’t.
How BMI is calculated
BMI = weight in kilograms ÷ (height in metres)2. A person weighing 65 kg at a height of 1.70 m has a BMI of 65 ÷ (1.70 × 1.70) = 22.5.
The standard adult BMI ranges
- Below 18.5 — commonly described as underweight
- 18.5 to 24.9 — commonly described as a healthy range
- 25 to 29.9 — commonly described as overweight
- 30 and above — commonly described as an obesity range
What BMI does not measure
BMI uses only height and weight — it cannot distinguish muscle from fat, and it says nothing about where fat is distributed, both of which matter for health. A very muscular adult can register a “high” BMI despite low body fat. BMI is also less informative for children, pregnant people and older adults, and it does not account for sex-based or ethnic differences in body composition that some researchers argue should adjust the standard thresholds.
How to use a BMI number sensibly
Treat BMI as a quick screening signal, not a diagnosis. If your number falls outside the “healthy” range, or you have specific health goals, a doctor or dietitian can interpret it alongside other measurements like waist circumference, body fat percentage and your overall health history.
Calculate yours instantly with the free BMI Calculator — enter weight in kilograms and height in centimetres.