What Is BMI?
Body Mass Index (BMI) is a simple calculation that uses your height and weight to estimate whether you're underweight, normal weight, overweight, or obese. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s.
The BMI Formula
BMI is calculated differently depending on your unit system:
// Metric (kg, meters)
BMI = weight / (height * height)
// Imperial (lbs, inches)
BMI = (weight * 703) / (height * height)
// Example: 70kg, 1.75m
BMI = 70 / (1.75 * 1.75) = 22.86BMI Categories (WHO Classification)
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 - 24.9 | Normal weight |
| 25.0 - 29.9 | Overweight |
| 30.0 - 34.9 | Obesity Class I |
| 35.0 - 39.9 | Obesity Class II |
| 40.0+ | Obesity Class III |
Limitations of BMI
BMI is a useful screening tool, but it has significant limitations:
- Muscle mass: Athletes with high muscle mass may have a high BMI despite being very fit. A bodybuilder at 100kg and 1.80m has a BMI of 30.9 ("obese") despite having low body fat
- Age: BMI doesn't account for age-related changes in body composition. Older adults tend to have more fat and less muscle
- Sex: Women typically have more body fat than men at the same BMI
- Ethnicity: Health risks associated with a given BMI can vary by ethnic group
- Distribution: BMI doesn't tell you where fat is stored. Visceral fat (around organs) is more dangerous than subcutaneous fat
Better Alternatives
- Waist-to-height ratio: Your waist should be less than half your height
- Body fat percentage: More accurate but harder to measure
- Waist circumference: Risk increases above 94cm (men) or 80cm (women)
Calculate Your BMI Now
While BMI has limitations, it remains a useful first-step screening tool. Use the PureTools BMI Calculator to get your BMI instantly. Just enter your height and weight, and get your result with the WHO category classification.