What is a BMI calculator?
A BMI calculator is a quick tool that estimates your Body Mass Index from two numbers: your height and your weight. BMI is a widely used screening measure that places adults into broad weight ranges - underweight, healthy weight, overweight, and obesity. It does not measure body fat directly and it does not diagnose any condition. Think of it as a useful starting point rather than a final answer about your health.
How to calculate BMI
BMI uses a simple formula. In metric units, you divide your weight in kilograms by your height in metres squared. In US units, you divide your weight in pounds by your height in inches squared and multiply by 703.
- Metric: BMI = weight (kg) ÷ [height (m) × height (m)]
- US units: BMI = [weight (lb) ÷ (height (in) × height (in))] × 703
For example, an adult who is 1.73 m tall and weighs 72 kg has a BMI of about 24.1, which is in the healthy weight range. The calculator above does this arithmetic for you and rounds the result to one decimal place.
Adult BMI categories
These categories apply to adults aged 20 and older. They are the same for men and women.
| Category | BMI range |
|---|---|
| Underweight | Below 18.5 |
| Healthy Weight | 18.5 - 24.9 |
| Overweight | 25.0 - 29.9 |
| Obesity | 30.0 and above |
| Class 1 Obesity | 30.0 - 34.9 |
| Class 2 Obesity | 35.0 - 39.9 |
| Class 3 Obesity | 40.0 and above |
What your result means
Your category gives you a general sense of where your weight sits relative to your height. A result such as "Your BMI is 24.2, which falls in the Healthy Weight range for adults 20 and older" is a screening signal, not a diagnosis. Two people with the same BMI can have very different body compositions and health profiles, so the number is most useful as one input in a bigger picture.
Limitations of BMI
BMI is popular because it is simple and free, but that simplicity is also its limit. It cannot tell the difference between muscle and fat, so very muscular people may get a high BMI despite low body fat. It does not show where fat is stored, which matters for health. It can read differently for some older adults who have lost muscle. And it was developed mainly from data on certain populations, so it may fit some groups better than others. For these reasons, treat BMI as a screening tool and talk with a healthcare provider about your personal situation.