January 2025 · 5 min read · Health
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What Is BMI?
Body Mass Index (BMI) is a numerical value derived from a person's weight and height. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and has since been adopted worldwide as a simple screening tool for weight categories — underweight, healthy weight, overweight, and obese.
BMI is not a diagnostic tool, but it can be a useful starting point for a conversation with a healthcare provider.
Formula: Metric Units (kg and cm)
BMI = Weight (kg) ÷ (Height (m) × Height (m))
Example: A person weighs 70 kg and is 1.75 m tall.
Step 1: 1.75 × 1.75 = 3.0625
Step 2: 70 ÷ 3.0625 = BMI 22.9
This falls in the "Healthy weight" range.
Formula: Imperial Units (lbs and inches)
BMI = (Weight (lbs) ÷ (Height (in) × Height (in))) × 703
Example: A person weighs 154 lbs and is 5'9" (69 inches) tall.
Step 1: 69 × 69 = 4,761
Step 2: 154 ÷ 4,761 = 0.03234
Step 3: 0.03234 × 703 = BMI 22.7
BMI Categories (WHO Standard)
| BMI Range | Category |
|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese (Class I) |
| 35.0 – 39.9 | Obese (Class II) |
| 40.0 and above | Severely obese (Class III) |
BMI for Children and Teens
The standard BMI formula applies to adults aged 20 and over. For children and teenagers (ages 2–19), BMI is calculated the same way but interpreted using age- and sex-specific percentile charts, because body fat levels change with age and differ between boys and girls.
Limitations of BMI
BMI is a widely used but imperfect measure. It does not account for:
- Muscle mass: Athletes often have high BMIs due to muscle, not fat. A bodybuilder may register as "obese" despite having very low body fat.
- Fat distribution: Where fat is stored matters. Abdominal fat carries more health risk than fat stored in the hips or thighs — and BMI does not distinguish between them.
- Age and sex: Older adults typically have more body fat than younger adults at the same BMI. Women generally have more body fat than men at the same BMI.
- Ethnicity: Research shows that health risks associated with BMI can vary significantly across ethnic groups.
For a more complete picture of health, doctors often combine BMI with waist circumference, blood pressure, cholesterol levels, and blood sugar measurements.
A Healthier Way to Track Progress
Rather than fixating on a single BMI number, focus on trends over time. A consistent healthy lifestyle — regular physical activity, balanced nutrition, adequate sleep — matters far more than hitting a specific BMI target. If you have concerns about your weight or health, consult a qualified healthcare professional.
💡 Use our
BMI calculator on the home page to instantly calculate your BMI in both metric and imperial units.