What is BMI and How is it Calculated?
Body Mass Index is a simple ratio of weight to height squared. The formula is:
BMI = Weight (kg) ÷ Height² (m²)
Example: A person who weighs 70kg and is 1.75m tall: BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9
For imperial units: BMI = (Weight in lbs × 703) ÷ Height² in inches
Example: 154 lbs, 5 feet 9 inches (69 inches): BMI = (154 × 703) ÷ (69 × 69) = 108,262 ÷ 4,761 = 22.7
The formula was created by Belgian mathematician Adolphe Quetelet in the 1830s. It was not originally designed as a medical tool — it was a way to study populations statistically. Ancel Keys popularized it for individual health screening in the 1970s, calling it the "Body Mass Index." Its simplicity (requiring only height and weight) made it widely adopted in medicine.
BMI Categories: What the Numbers Mean
The World Health Organization (WHO) defines four main BMI categories for adults:
Underweight: BMI below 18.5 A BMI below 18.5 may indicate malnutrition, eating disorders, or other health conditions. Underweight individuals have increased risk of bone loss, immune deficiency, and fertility problems.
Normal weight: BMI 18.5 to 24.9 This range is associated with the lowest risk of weight-related health problems in population studies.
Overweight: BMI 25.0 to 29.9 Statistically associated with increased risk of type 2 diabetes, hypertension and cardiovascular disease, though many people in this range have no health issues.
Obese Class I: BMI 30.0 to 34.9 Obese Class II: BMI 35.0 to 39.9 Obese Class III (Severe): BMI 40.0 and above
Obesity is associated with significantly increased risk of type 2 diabetes, heart disease, sleep apnea, certain cancers, and reduced life expectancy.
BMI for Indians and South Asians: Different Thresholds
An important consideration often overlooked: standard WHO BMI thresholds were derived primarily from studies of European populations. Research has consistently shown that South Asians (including Indians) have higher body fat percentage at lower BMI values compared to Europeans.
The International Obesity Task Force and many Indian medical organizations recommend lower BMI thresholds for South Asians:
Underweight: below 18.5 (same) Normal weight: 18.5 to 22.9 Overweight: 23.0 to 24.9 Obese: 25.0 and above
This means an Indian with a BMI of 24, which is "normal" by WHO standards, would be classified as "overweight" by South Asian guidelines.
Why the difference? South Asians tend to carry more visceral fat (fat around internal organs) at lower BMI values. Visceral fat is more metabolically active and more strongly associated with type 2 diabetes, metabolic syndrome and cardiovascular disease than subcutaneous fat (fat under the skin).
India has one of the world's highest rates of type 2 diabetes, in part because standard BMI thresholds fail to identify many Indians at risk.
The Limitations of BMI
BMI is a useful screening tool but has well-documented limitations that every individual should understand:
Does not measure body fat directly — BMI measures weight relative to height. It cannot distinguish between fat mass and lean mass (muscle, bone, water).
Overestimates health risk in muscular individuals — athletes and people with high muscle mass often have elevated BMI despite very low body fat. A professional bodybuilder with very little body fat might have a BMI over 30, technically classified as obese.
Underestimates health risk in sedentary individuals — a person with low muscle mass but high body fat percentage might have a normal BMI despite poor metabolic health. This is sometimes called "normal weight obesity" or "skinny fat."
Does not account for fat distribution — where fat is stored matters. Abdominal (visceral) fat is far more metabolically harmful than fat stored in the hips and thighs. BMI cannot distinguish between these.
Varies by age — body fat percentage naturally increases with age while muscle mass decreases. An elderly person with a BMI of 23 may have much more body fat than a young adult with the same BMI.
Varies by sex — women naturally have higher body fat percentage than men at the same BMI.
Better Measures to Use Alongside BMI
Health professionals increasingly use BMI alongside additional measurements for a more complete picture:
Waist circumference — measures abdominal fat directly. Health risks increase when waist circumference exceeds 90cm (35.5 inches) for Indian men and 80cm (31.5 inches) for Indian women. Waist measurement is a better predictor of metabolic syndrome than BMI.
Waist-to-height ratio — waist circumference divided by height. A healthy ratio is below 0.5, meaning your waist should be less than half your height. This measure is both simple and strongly predictive of cardiometabolic risk.
Body fat percentage — measured by DEXA scan, hydrostatic weighing, or consumer-grade bioelectrical impedance scales. Healthy ranges: 10-20% for men, 18-28% for women (values increase naturally with age).
Waist-to-hip ratio — compares waist size to hip size. A ratio above 0.90 for men and 0.85 for women indicates central obesity associated with cardiovascular risk.
Blood tests — fasting blood glucose, HbA1c, lipid profile and blood pressure provide direct metabolic health data that BMI cannot.
The most useful approach: use BMI as a quick initial screening tool, but discuss additional measurements and tests with your doctor for a complete picture of your health status.