What Is BMI?
Body Mass Index (BMI) relates weight to height using:
BMI = weight (kg) / height (m)²
In imperial units: BMI = 703 × weight (lb) / height (in)².
It was designed as a population screening tool — quick and cheap — not a direct measure of body fat or health for any one person.
WHO Weight Categories (Adults)
| BMI | Category (general) |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal range |
| 25.0 – 29.9 | Overweight |
| 30+ | Obesity (classes I–III) |
Asian populations sometimes use lower cutoffs for risk — guidelines evolve; a clinician interprets your numbers.
When BMI Misleading
Muscular athletes can register "overweight" with low body fat. Older adults may have normal BMI but low muscle (sarcopenia). Children use percentile charts, not adult cutoffs.
BMI never replaces waist circumference, blood work, or clinical judgment.
Healthy Weight Range
From BMI 18.5–25 (for a given height), you can solve for weight range:
Min weight = 18.5 × height(m)²
Max weight = 25 × height(m)²
Use as a rough target band — not a moral or aesthetic prescription.
How to Use This BMI Calculator
Enter height and weight in your preferred units. See BMI value, category, and healthy range where shown. Discuss results with a healthcare provider if you're changing diet or exercise.