What Is BMI and How Is It Calculated

Body Mass Index (BMI) is the most widely used nutritional screening tool in public health. With just two measurements — weight and height — it provides a quick estimate of a person's weight status. But what does it actually measure, and where does it fall short?

History: the Quetelet index

BMI was developed by Belgian mathematician and statistician Adolphe Quetelet between 1830 and 1850 as a population-level measure, not an individual diagnostic tool. It was not until the 1970s that Ancel Keys popularised the term "Body Mass Index" and demonstrated its correlation with body fat in large samples.^[1] The World Health Organization adopted the current categories in 1995.

The BMI formula

• BMI = weight (kg) / height² (m)
• Example: a person weighing 75 kg at 1.75 m → BMI = 75 / (1.75 × 1.75) = 24.5

In the imperial system (pounds and inches): BMI = [weight (lb) / height² (in)] × 703.

WHO categories

BMI	Category	Metabolic risk
< 18.5	Underweight	Elevated (malnutrition, osteoporosis)
18.5 – 24.9	Normal weight	Low
25.0 – 29.9	Overweight	Moderate
30.0 – 34.9	Obese class I	High
35.0 – 39.9	Obese class II	Very high
≥ 40.0	Obese class III (morbid)	Extremely high

Uses in public health

At the population level, BMI is a valuable tool for several reasons:

• Low cost and universality: no special equipment is needed. Any clinician can calculate it in seconds.
• Population-level risk prediction: there is a robust correlation between elevated BMI and cardiovascular disease, type 2 diabetes, and certain cancers in epidemiological studies involving millions of people.^[2]
• Trend monitoring: it allows comparison of obesity prevalence across countries and over time, as the WHO and OECD do in their annual reports.

Limitations of BMI

BMI does not distinguish between muscle mass and fat mass. This is its most important limitation:

• An athlete with substantial muscle mass may have a BMI of 27–28 and be classified as "overweight" despite low body fat.
• A sedentary person with a normal BMI (22–23) may have high body fat — the phenomenon known as skinny fat or normal-weight obesity.^[3]
• It does not account for fat distribution. Visceral (abdominal) fat is metabolically far more dangerous than subcutaneous fat, but BMI cannot distinguish between them.
• The cut-off points were established in predominantly Caucasian populations. For Asian populations, the WHO acknowledges that metabolic risk begins at BMI 23, not 25.^[2]

When is BMI sufficient?

For most adults with a moderate lifestyle, BMI remains a useful first-line assessment. When the result is clearly normal (19–24) or clearly elevated (>30), the correlation with metabolic health is strong enough to guide decisions. However, in athletes, older adults, or patients with chronic disease, BMI should be complemented by waist circumference, body fat percentage, or waist-to-height ratio.

To calculate your BMI and see your WHO health category instantly, use our free BMI calculator.

Scientific references

1. Keys A et al. "Indices of relative weight and obesity." J Chronic Dis. 1972;25(6):329-343.
2. World Health Organization. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894. Geneva: WHO; 2000.
3. Romero-Corral A et al. "Accuracy of body mass index in diagnosing obesity in the adult general population." Int J Obes (Lond). 2008;32(6):959-966.
4. Flegal KM et al. "Prevalence of obesity and trends in body mass index among US adults, 1999–2010." JAMA. 2012;307(5):491-497.