How to Calculate BMI
Body Mass Index (BMI) is the most widely used screening tool for weight categories. It gives you a single number based on your height and weight that indicates whether you fall into the underweight, normal, overweight, or obese range.
The BMI formula
Metric: BMI = weight (kg) / height (m)²
Imperial: BMI = (weight (lbs) / height (in)²) × 703
For example, a person who is 70 kg and 175 cm tall: BMI = 70 / (1.75)² = 70 / 3.0625 = 22.9 (normal weight).
BMI categories
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
How to calculate BMI online
- Enter your weight and height — input your measurements in either metric (kg/cm) or imperial (lbs/inches).
- Click Calculate — the tool computes your BMI instantly.
- Review your category — see which range you fall into and what it means.
What BMI does not tell you
BMI is a useful screening tool, but it has real limitations:
- It does not measure body fat — a bodybuilder and an overweight person can have the same BMI
- It ignores fat distribution — belly fat is more dangerous than fat elsewhere, but BMI does not account for this
- It varies by age and ethnicity — the same BMI can mean different health risks for different populations
- It is a population tool — BMI works well for studying groups, but is less precise for individuals
Tips
- Use BMI as a starting point — it is a quick screening tool, not a complete health assessment. If your BMI is outside the normal range, talk to a doctor rather than self-diagnosing.
- Track trends, not snapshots — a single BMI reading is less useful than tracking changes over time. A rising BMI may signal a need to adjust habits.
- Consider other metrics — waist circumference, waist-to-hip ratio, and body fat percentage give a more complete picture of health than BMI alone.
- Metric or imperial, your choice — the calculator handles both unit systems, so use whichever you are comfortable with.
Frequently Asked Questions
What is a healthy BMI range?
A BMI between 18.5 and 24.9 is considered normal weight. Below 18.5 is underweight, 25.0-29.9 is overweight, and 30.0 or above is classified as obese. These ranges are general guidelines, not definitive health diagnoses.
Is BMI accurate for athletes?
Not always. BMI does not distinguish between muscle and fat. A muscular athlete may have a high BMI but low body fat. For people with above-average muscle mass, other measurements like body fat percentage or waist circumference are more informative.
Can I use both metric and imperial units?
Yes. Most BMI calculators let you switch between metric (kg and cm) and imperial (pounds and inches) with a single click.
Does BMI apply the same way to children?
No. BMI for children and teens is age and sex-specific, using percentile charts rather than fixed categories. A pediatrician can interpret BMI-for-age correctly.