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
What’s a healthy BMI?
Roughly 18.5–24.9. Below that is underweight; 25–29.9 is overweight; 30+ is obese.
Does BMI work for everyone?
It’s a population-level measure. Muscle mass, age and ethnicity all change how it should be read.
Is BMI the same as body fat percentage?
No. BMI only compares weight to height, so it cannot tell fat from muscle. Two people with the same BMI can have very different body compositions — a body fat test or waist measurement gives a more direct picture.
Does BMI apply to children?
Not this version. Children and teens need age- and sex-specific BMI percentile charts because their expected body composition changes as they grow, unlike the fixed adult bands used here.
Why do some healthy athletes get a high BMI?
Muscle is denser than fat, so a heavily muscled person can weigh enough to score “overweight” or “obese” on BMI despite having low body fat. This is BMI’s best-known blind spot.
Is waist size a better health indicator than BMI?
Waist circumference or waist-to-height ratio can flag risk that BMI misses, since they capture where fat is stored. Central belly fat carries more health risk than fat elsewhere, even at the same BMI.
How often should I recalculate my BMI?
There’s no fixed schedule — it’s only useful as a rough checkpoint, such as every few months or after a noticeable weight change, rather than something to track daily.
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.