BMI is the most widely used weight-screening tool in the world — and one of the most misunderstood. This guide covers the formula, all 6 BMI categories, how to find your healthy weight range, and where BMI falls short.
What is BMI?
BMI (Body Mass Index) is a number calculated from your height and weight. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and adopted by the World Health Organisation as a population-level screening tool for weight-related health risk.
It is a screening tool, not a diagnostic one. A high BMI flags potential risk — it does not diagnose anything.
The BMI formula
Metric: BMI = weight(kg) ÷ height²(m)
Example: 70 kg, 175 cm → 70 ÷ (1.75²) = 70 ÷ 3.0625 = 22.9
Imperial: BMI = (weight(lbs) × 703) ÷ height²(inches)
Example: 154 lbs, 5′9″ (69 in) → (154 × 703) ÷ (69²) = 108262 ÷ 4761 = 22.7
BMI categories (WHO standard)
| Category | BMI range |
|---|---|
| Severe underweight | < 16.0 |
| Underweight | 16.0 – 18.4 |
| Normal weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obese Class I | 30.0 – 34.9 |
| Obese Class II | 35.0 – 39.9 |
| Obese Class III | ≥ 40.0 |
How to find your healthy weight range
The healthy weight range is the weight that gives you a BMI between 18.5 and 24.9 for your specific height.
Formula (metric):
Min healthy weight = 18.5 × height²(m)
Max healthy weight = 24.9 × height²(m)
Example for 175 cm:
Min = 18.5 × 3.0625 = 56.7 kg
Max = 24.9 × 3.0625 = 76.3 kg
Where BMI falls short
- Muscle mass: A bodybuilder with very low body fat can have a BMI in the “obese” range. BMI does not distinguish between fat and muscle.
- Fat distribution: Abdominal fat carries more cardiovascular risk than fat elsewhere. BMI does not account for where fat is stored.
- Age and sex: Older adults tend to carry more fat at the same BMI as younger adults. Women typically carry more fat than men at the same BMI.
- Ethnicity: Some studies suggest lower BMI thresholds are more appropriate for Asian populations.
For a fuller picture, combine BMI with waist circumference, body fat percentage, and a healthcare professional's assessment.
Calculate your BMI instantly in metric or imperial — with healthy weight range:
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