What Is BMI & How Is It Calculated?

What Is Body Mass Index (BMI)?

BMI is a simple numerical value derived from your height and weight. It was developed by Belgian mathematician Adolphe Quetelet in the 19th century as a quick population-level screening tool for body fatness. It is not a direct measure of body fat percentage.

The BMI Formula

BMI Categories (WHO Standard)

• Below 18.5 — Underweight
• 18.5 – 24.9 — Normal weight ✅
• 25.0 – 29.9 — Overweight
• 30.0 – 34.9 — Obese Class I
• 35.0 – 39.9 — Obese Class II
• 40.0+ — Obese Class III (Severe)

Worked Example

Person: 70 kg, 175 cm tall

• Height in metres = 1.75 | Height² = 3.0625
• BMI = 70 ÷ 3.0625 = 22.86 → Normal weight

Limitations of BMI

BMI is a useful population screening tool but has several well-known limitations:

• Muscle mass: Athletes can have a high BMI despite very low body fat.
• Age and sex: Older adults naturally carry more fat at the same BMI. Women typically have higher body fat at a given BMI than men.
• Ethnicity: Some ethnic groups face higher health risks at lower BMI values. Asian populations use different cut-offs (23 for overweight, 27.5 for obese).
• Fat distribution: Waist circumference is often a better predictor of cardiovascular risk than BMI alone.

Better Metrics to Use Alongside BMI

• Waist-to-Height Ratio: Waist < 50% of height is considered healthy.
• Waist Circumference: <94 cm men / <80 cm women (WHO guidelines).
• Body Fat %: DEXA scan, hydrostatic weighing, or bioelectrical impedance provide more accurate body composition.