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. Use the free BMI Calculator above to try your own numbers instantly.
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.
Planning further? You may also find our Calorie Calculator useful, or explore all calculators.
Further reading: Body mass index (Wikipedia) · Obesity (World Health Organization, Africa)