Understanding BMI: What It Tells You and What It Doesn't
Body Mass Index (BMI) is a simple number that compares your weight with your height. Doctors and health organisations use it as a quick first check of whether a person's weight may be affecting their health. It's easy to calculate and useful, but it has limits that are worth understanding.
The BMI formula
For example, a person who is 1.70 m tall and weighs 68 kg has a BMI of 68 ÷ (1.70 × 1.70) = 23.5. In imperial units the formula is BMI = 703 × weight (lb) ÷ height (in)². The calculator converts units for you.
Adult BMI categories (WHO)
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25 – 29.9 | Overweight |
| 30 – 34.9 | Obesity class 1 |
| 35 – 39.9 | Obesity class 2 |
| 40 and above | Obesity class 3 |
Some health bodies use lower cut-off points for people of South Asian, Chinese and some other Asian backgrounds, because health risks can start at a lower BMI. Ask your doctor which range applies to you.
Limitations of BMI
- Muscle vs fat: athletes and very muscular people can have a high BMI without excess fat.
- Body shape: BMI doesn't show where fat is stored. Fat around the waist carries more health risk; waist measurement adds useful information.
- Age and sex: body composition changes with age and differs between men and women.
- Children and teens: under 20s need age- and sex-specific BMI charts, so this calculator isn't suitable for them.
- Pregnancy: BMI isn't meaningful during pregnancy.
What to do with your result
Treat BMI as one piece of information, not a diagnosis. If your result is outside the healthy range, or you have concerns about your weight, a doctor can look at the full picture, including waist size, blood pressure, blood tests and lifestyle. For an estimate of your daily energy needs, see our calorie calculator.