The formula
How it works
Body Mass Index estimates body fat from height and weight. It’s a quick screening tool, not a diagnosis — athletes and older adults can sit outside the “healthy” band for reasons unrelated to fat.
FAQ
Roughly 18.5–24.9. Below that is underweight; 25–29.9 is overweight; 30+ is obese.
It’s a population-level measure. Muscle mass, age and ethnicity all change how it should be read.
What BMI measures
Body Mass Index compares your weight to your height to produce a single number that estimates whether you carry a healthy amount of weight for your size. It is calculated as weight in kilograms divided by height in metres squared, and it is used worldwide as a fast, free screening tool .
How to read your result
A BMI below 18.5 is considered underweight, 18.5 to 24.9 is a healthy weight, 25 to 29.9 is overweight, and 30 or above is classed as obese. These bands are population averages rather than a diagnosis: very muscular people, older adults and different ethnic groups can sit outside the healthy range for reasons unrelated to body fat.