What BMI is
Body Mass Index (BMI) is a simple mathematical formula, created in the 19th century by the Belgian mathematician Adolphe Quetelet, that relates a person's weight to their height.
The World Health Organization (WHO) uses BMI as an international standard for quickly assessing whether an adult falls within a weight range considered healthy for their height, or whether they are underweight, overweight or living with obesity.
How to calculate BMI
The BMI calculation is straightforward. You take your weight (in kilograms) and divide it by your height (in meters) squared.
The official formula is:
• BMI = weight (kg) ÷ (height (m) × height (m))
For example: someone who weighs 70 kg and is 1.75 m tall would calculate 70 ÷ (1.75 × 1.75) = 70 ÷ 3.06 = 22.8. That person would fall in the "Normal weight" range.
WHO classification table
The World Health Organization classifies adult BMI (men and women aged 20 and over) into the following ranges:
• Below 18.5: Underweight • Between 18.5 and 24.9: Normal weight • Between 25.0 and 29.9: Overweight • 30.0 or above: Obesity
The limits of BMI — why it doesn't tell the whole story
While BMI is a useful epidemiological tool for assessing large populations, it has significant limitations when used on its own to assess an individual's health:
1. It does not tell muscle from fat: muscle is denser than fat. Athletes and very muscular people often get a BMI that labels them "overweight" or "obese" even with very low body fat and excellent cardiovascular health. 2. It does not assess fat distribution: visceral fat (stored in the abdomen, around the organs) is far more dangerous for metabolic health than subcutaneous fat (stored on the hips and thighs). Two people with the same BMI can have completely different cardiovascular risk profiles. 3. It does not account for age or ethnicity: the standard tables are not ideal for older adults (who naturally lose muscle and bone mass) or for pregnant women. Some populations, such as people of Asian descent, face metabolic risk at lower BMI values than the standard cut-off.
Frequently asked questions
- Does BMI work for children?
- BMI for children and teenagers (up to 19) uses the same formula, but the classification is different. Because they are still growing, the result must be compared against WHO-specific charts (percentiles), split by age and sex. This calculator was built for adults.
- Is BMI different for men and women?
- No. For the World Health Organization's official adult classification, the table and cut-off points are the same for men and women.
- My BMI says overweight — what now?
- BMI is only an indicator. A BMI in the overweight range does not automatically mean you are unwell. Look at other health markers, such as body fat percentage, waist circumference and blood work. Consult a professional for a full assessment.
- Which matters more: BMI or body fat percentage?
- Body fat percentage is a far more accurate health indicator, because it reveals your body's actual composition (lean mass vs fat mass). BMI is used because it is much easier and cheaper to calculate than accurate body composition tests such as a DEXA scan or bioimpedance.
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