What is BMI and How is it Calculated?

The Formula

Body Mass Index is calculated from two measurements you almost certainly already know:

BMI = weight (kg) ÷ height² (m²)

Example: a person who is 1.70 m tall and weighs 70 kg has a BMI of 70 ÷ (1.70 × 1.70) = 24.2.

In imperial units the formula adjusts slightly: BMI = (weight in lbs ÷ height in inches²) × 703. The result is identical — it is the same formula with unit conversion baked in.

Where Did BMI Come From?

BMI was invented by Belgian mathematician Adolphe Quetelet between 1830 and 1850. He was studying statistical averages across populations — not individual health. His "Quetelet Index" was never designed as a clinical tool. It was adopted by the medical community in the 1970s when physiologist Ancel Keys tested seven candidate indices against direct body fat measurements in 7,500 men and found Quetelet's formula performed best. Keys renamed it Body Mass Index.

WHO BMI Categories for Adults

The World Health Organization defines four categories for adults aged 18 and over:

These thresholds were established in WHO's 1995 consultation and formalised in the 2000 Technical Report Series 894. They are based on population-level epidemiological data linking BMI to mortality and chronic disease risk in predominantly European-origin populations.

What BMI Actually Measures

BMI is a measure of body size relative to height — not a direct measure of body fat. It cannot tell the difference between fat tissue, muscle, bone, or water. A professional rugby player may have a BMI above 28 while carrying very little fat. Conversely, an older adult with low muscle mass may have a "normal" BMI while carrying a high proportion of body fat.

Despite this, BMI correlates reasonably well with body fat at the group level in population studies, which is why it remains a useful screening tool.

BMI Thresholds Are Not Universal

The WHO standard was derived from studies of European-origin populations. Subsequent research found that people of South Asian and East Asian origin develop metabolic complications at lower BMI values. The WHO now publishes separate cut-off points for Asian populations — our BMI calculator lets you switch between all three standards.

What BMI Cannot Tell You

• Where body fat is located (visceral fat around organs is more harmful than subcutaneous fat)
• Muscle mass versus fat mass
• Bone density
• Cardiovascular fitness or metabolic health
• Blood pressure, blood sugar, or cholesterol

Use BMI as a first screening step. Combine it with waist circumference, blood markers, and clinical assessment for a complete picture.

Calculate your BMI →