BMI (Body Mass Index) is calculated by dividing your weight in kilograms by your height in metres squared. A BMI of 18.5–24.9 is healthy, 25–29.9 is overweight, and 30+ is obese.

How do I calculate my BMI?

BMI = weight (kg) ÷ height (m)². Example: 75 kg ÷ (1.75 m × 1.75 m) = 24.5, which is healthy weight.

BMI has significant limitations — it doesn't distinguish muscle from fat, doesn't account for fat distribution, and has different risk thresholds for different ethnicities (South Asians develop metabolic risk at lower BMIs). Waist-to-height ratio is a better predictor of cardiovascular risk.

What Is BMI and What Does Your Number Actually Mean?

Q: How does BMI affect life expectancy?

A BMI of 30–35 is associated with a 2–4 year reduction in life expectancy. Severe obesity (BMI 40+) can reduce life expectancy by 8–10 years. However, fitness level significantly modifies these risk estimates.

The BMI Formula

BMI = weight (kg) ÷ height (m)²

Example: A person weighing 75 kg and standing 1.75 m tall has a BMI of 75 ÷ (1.75 × 1.75) = 75 ÷ 3.0625 = 24.5 (healthy weight).

In imperial units: BMI = (weight in pounds × 703) ÷ (height in inches)². The formula was developed by Belgian statistician Adolphe Quetelet in the 1830s — originally as a population-level measure, never intended for individual health diagnosis.

The 4 BMI Categories

Category	BMI Range	Health Implication
Underweight	< 18.5	Increased risk of nutritional deficiency, bone loss, immune weakness
Healthy Weight	18.5–24.9	Lowest all-cause mortality risk
Overweight	25–29.9	Moderately elevated cardiovascular and metabolic risk
Obese	≥ 30	Significantly elevated risk of type 2 diabetes, heart disease, certain cancers

BMI Limitations

It doesn't distinguish muscle from fat. A bodybuilder with 10% body fat and 100 kg of muscle mass may have a BMI of 32 — classified as obese despite having exceptional health. Conversely, a "skinny fat" person with a BMI of 22 but high visceral fat percentage may have elevated metabolic risk.

It doesn't account for fat distribution. Where you carry fat matters enormously. Abdominal (visceral) fat surrounding organs is far more dangerous than subcutaneous fat on hips and thighs. Two people with the same BMI can have very different cardiovascular risk profiles based on fat distribution.

It varies by ethnicity. Research shows that South Asians (including Indians) develop metabolic complications at lower BMIs than European populations. The WHO recommends lower BMI thresholds for Asian populations: overweight at 23+, obese at 27.5+.

It's less useful at older ages. After 65, slightly higher BMI (25–27) is associated with lower mortality — the "obesity paradox" in the elderly.

Better Metrics: Waist-to-Height Ratio

Waist-to-height ratio (WHtR) is a stronger predictor of cardiovascular risk and metabolic syndrome than BMI. The formula: waist circumference ÷ height. A ratio below 0.5 is healthy — meaning your waist should be less than half your height. This measure captures abdominal fat distribution that BMI misses entirely.

BornClock's biological age assessment uses both BMI and waist circumference (via WHO waist risk thresholds) to give a more accurate picture than BMI alone.

How BMI Affects Life Expectancy

A BMI of 30–35 is associated with a 2–4 year reduction in life expectancy. Severe obesity (BMI 40+) can reduce life expectancy by 8–10 years. However, the risks are not linear — moderate obesity (BMI 30–35) in physically active individuals with no other metabolic risk factors carries lower risk than sedentary individuals at a "healthy" BMI. Fitness level modifies BMI risk substantially.