What Is BMI and How Is It Calculated?

Body Mass Index — better known as BMI — is a simple numeric measure of a person's body weight relative to their height. It was originally developed by Belgian statistician Adolphe Quetelet in the 1830s and has since become the standard screening tool used by doctors, insurers, and public-health organizations worldwide.

How BMI Is Calculated

The formula is straightforward:

BMI = weight (kg) ÷ height (m)²

If you measure in pounds and inches, multiply by 703:

BMI = (weight (lbs) ÷ height (in)²) × 703

For example, a person who weighs 70 kg and stands 1.75 m tall has a BMI of 70 ÷ 3.0625 = 22.9.

WHO BMI Categories

The World Health Organization defines four primary ranges:

• Underweight: below 18.5
• Normal weight: 18.5 – 24.9
• Overweight: 25.0 – 29.9
• Obese: 30.0 and above

Why BMI Matters

Population studies consistently show that BMI values in the overweight and obese ranges correlate with higher risks of type-2 diabetes, cardiovascular disease, and certain cancers. That makes it a useful first-pass screening tool in clinical settings.

Where BMI Falls Short

BMI cannot distinguish between muscle mass and fat mass. A muscular athlete and a sedentary person of the same height and weight will have identical BMI scores despite very different body compositions. That's why healthcare professionals often pair BMI with measurements like body fat percentage or waist-to-hip ratio for a more complete picture.

Next Steps

Ready to check your own BMI? Use our free BMI Calculator for an instant result with a full interpretation guide. If you want to go deeper, follow up with our Body Fat Calculator or Ideal Weight Calculator to build a more complete health profile.