How do I calculate BMI?
Body Mass Index (BMI) is calculated as a simple ratio of your weight to your height squared. It is the most widely used screening tool for identifying potential weight problems in adults.
Why use BMI?
While not a direct measure of body fat, BMI correlates moderately with more direct measures of body fat (like skinfold thickness measurements or underwater weighing) and strongly with various metabolic disease outcomes.
Normal BMI & Categories
The following standard weight status categories are used by the CDC and WHO for adults aged 20 and over:
| Category | BMI Range |
|---|---|
| Underweight | < 18.49 |
| Normal | 18.5 – 24.9 |
| Overweight | 25 – 29.9 |
| Obese Class I | 30 – 34.9 |
| Obese Class II | 35 – 39.9 |
| Obese Class III | > 40 |
Other Considerations
It is crucial to understand that BMI is a screening metric, not a diagnosis of body fatness or health.
- False Positives (Athletes): Individuals with high muscle mass may have a high BMI but low body fat. For example, a bodybuilder may be classified as "Obese" despite excellent metabolic health.
- False Negatives (Seniors): Older adults often lose muscle mass (sarcopenia). They may have a "Normal" BMI but actually possess excess body fat and low muscle mass.
Frequently Asked Questions
Does a 'Normal' BMI depend on age?
Example Calculation: I am a man, 180cm tall, weighing 75kg. What is my BMI?
Let's walk through the math step-by-step:
- Convert height to meters: 180cm = 1.8m.
- Square the height: 1.8 × 1.8 = 3.24.
- Divide weight by height squared: 75 / 3.24 = 23.15.
Result: Normal Weight.
Why is BMI different for men vs. women?
Is BMI accurate for bodybuilders?
Tools & Data Verified by the EverydayCalculators Medical Research Team.
Last updated: December 2026.