BMI, or Body Mass Index, is a single number worked out from your height and weight. It's used as a quick, general screening figure — the sort of thing a doctor might glance at before a fuller look. It's genuinely useful as a rough guide, and also widely over-read, so it's worth understanding both what it is and what it isn't.
How BMI is calculated
BMI divides your weight by the square of your height. In metric, that's your weight in kilograms divided by your height in metres squared. In imperial, it's your weight in pounds times 703, divided by your height in inches squared — the 703 simply rescales the imperial units onto the same scale, so both give the same result. For example, 70 kg and 1.75 m gives 70 ÷ (1.75 × 1.75) = about 22.9.
What the categories mean
The standard adult categories are: under 18.5 is underweight, 18.5 to 24.9 is the normal range, 25 to 29.9 is overweight, and 30 and above is obese. These are broad population-level bands, not a verdict on any individual.
What BMI doesn't measure
This is the important part. BMI only knows your height and weight — it can't tell muscle from fat. That's why it misclassifies people at both ends: a muscular, very fit person can score as “overweight” while carrying little fat, and someone with low muscle can sit in the “normal” range while carrying more fat than is healthy. It also ignores where fat is carried, and doesn't account for age, sex or ethnicity, which all matter. For a fuller picture, measures like body-fat percentage and waist-to-height ratio add what BMI leaves out.
- Open the BMI Calculator and enter your height and weight.
- Choose metric or imperial — the result is the same either way.
- Read your figure and category as a starting point, not a diagnosis.
This is general information, not medical advice, and BMI is a screening figure rather than a health assessment. For anything about your own health, a doctor who can weigh the whole picture is the right person to ask. Your height and weight are used only in your browser and never stored.