What Is BMI and How to Calculate It
Last updated 2026-09-11
Body Mass Index (BMI) is a simple ratio of weight to height used as a quick screening tool for weight categories — it's not a diagnosis, but a starting point.
Metric formula
BMI = weight in kilograms ÷ (height in meters)². A 70kg person who is 1.75m tall: 70 ÷ (1.75 × 1.75) ≈ 22.9.
Imperial formula
BMI = 703 × weight in pounds ÷ (height in inches)².
Read the category
Under 18.5 is underweight, 18.5–24.9 is normal, 25–29.9 is overweight, and 30+ is obese, per the standard WHO ranges.
Example
A person 1.75m tall weighing 70kg has a BMI of about 22.9, in the normal range.
Important Considerations
- BMI doesn't distinguish muscle mass from fat mass — a muscular athlete can score "overweight" despite low body fat.
- BMI ranges were developed primarily from adult population studies and are less reliable for children, the elderly, and pregnant people.
- It's a population-level screening tool, not an individual health diagnosis — use it alongside other measures, not in isolation.
Frequently Asked Questions
- Is BMI accurate for athletes?
- Not always — since BMI can't distinguish muscle from fat, muscular individuals often score higher than their actual body fat percentage would suggest.
- What should I do if my BMI is outside the normal range?
- Treat it as a starting point for a conversation with a healthcare professional, not a standalone diagnosis.