BMI (Body Mass Index) is a simple ratio of weight to height that was invented in the early 1800s by Belgian mathematician Adolphe Quetelet. It's designed to quickly assess whether a person's weight falls in a healthy range relative to their height.
The result places you in one of four categories: Underweight, Normal (Healthy Weight), Overweight, or Obese. It's the most widely used screening tool for categorizing body weight — used by doctors, insurance companies, and public health researchers worldwide.
These ranges apply to adults aged 20 and older. For children and teenagers, BMI is age and sex-specific, with separate percentile charts.
You can calculate it manually using metric or imperial measurements:
Person: 75 kg, 175 cm (1.75 m)
BMI = 75 ÷ (1.75 × 1.75) = 75 ÷ 3.0625 = 24.5
BMI = 24.5 → Normal weight
Person: 165 lb, 5'9" (69 inches)
BMI = (165 × 703) ÷ (69 × 69) = 116,000 ÷ 4,761 = 24.4
BMI = 24.4 → Normal weight
Person: 210 lb, 5'10" (70 inches)
BMI = (210 × 703) ÷ (70 × 70) = 147,630 ÷ 4,900 = 30.1
BMI = 30.1 → Obese (Class 1)
It uses weight as a proxy for fat, but muscle weighs more than fat. This is why athletes often get classified as "overweight" or even "obese" despite having very low body fat percentages. A 200-pound bodybuilder at 5'10" has a BMI of 28.7 — "overweight" — despite having almost no excess body fat.
| Measurement | What It Measures | Better For |
|---|---|---|
| Waist Circumference | Abdominal fat | Visceral fat risk assessment |
| Waist-to-Height Ratio | Fat distribution | More accurate than BMI alone |
| Body Fat Percentage | Actual fat mass | Direct measurement (requires calipers/DEXA) |
| Waist-to-Hip Ratio | Hip-to-waist fat ratio | Heart disease risk estimation |
💡 Practical rule: Measure your waist at navel level. For men, aim for under 40 inches (102 cm). For women, under 35 inches (88 cm). This is a better health indicator than BMI alone.
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