How to Calculate Your BMI and What the Number Really Means
Body mass index, or BMI, is one of the most widely used numbers in health. Doctors mention it, fitness apps display it, and forms ask for it. Yet most people have never been told what it measures or how it is worked out. This guide explains BMI in plain language, shows you how to calculate it, walks through what each range means, and covers where the number falls short.
What BMI actually measures
BMI is a simple ratio between your weight and your height. The idea, developed in the 1830s by a Belgian mathematician named Adolphe Quetelet, was to describe the build of an average population. It was never meant to diagnose an individual, a point worth keeping in mind.
In practice, BMI gives a single number that roughly places you on a scale from underweight to obese. Because it only needs height and weight, it became the default screening tool worldwide.
The BMI formula
BMI = weight (kg) ÷ height (m)²
Divide your weight in kilograms by your height in metres multiplied by itself.
If you weigh 70 kg and stand 1.75 m tall, square the height (1.75 × 1.75 = 3.0625), then divide: 70 ÷ 3.0625 = 22.9. That sits in the healthy range. Rather than do this by hand, you can use our BMI calculator.
What the ranges mean
- Below 18.5 — underweight
- 18.5 to 24.9 — healthy weight
- 25 to 29.9 — overweight
- 30 and above — obese
These bands are guides, not hard walls. Someone at 25.1 is not suddenly at risk the moment they cross from 24.9.
Where BMI falls short
BMI does not measure body fat directly and cannot tell muscle from fat. A muscular athlete may read as overweight while carrying little fat. An older adult who has lost muscle may read normal while carrying more fat than is healthy. BMI also ignores where fat is stored, and the standard categories fit some ethnic groups better than others.
Bottom line: BMI is a useful starting point, not a verdict. Treat it as one piece of a bigger picture.
Try it yourself
Work out your own BMI in seconds with our free BMI calculator. Read the result as a guide, and bring any real concerns to a qualified professional.