Understanding BMI and its limits
Body mass index, or BMI, is one of the most widely used health numbers and one of the most misunderstood. It is a useful starting point, but only a starting point.
How BMI is calculated
BMI = weight in kilograms ÷ height in metres squared. For someone 70 kg and 1.75 m tall, 1.75 × 1.75 = 3.0625, and 70 ÷ 3.0625 = 22.9. In pounds and inches, multiply weight by 703 and divide by height squared.
The adult categories
| BMI | Category (WHO, adults) |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy range |
| 25 to 29.9 | Overweight |
| 30 and above | Obesity range |
Why BMI is popular
It is quick, it needs only a scale and a tape measure, and across large populations it relates reasonably well to health outcomes. That makes it a handy screening tool for doctors and researchers.
What BMI cannot tell you
- Muscle versus fat. Muscle is dense. A strength athlete can have a BMI in the overweight range with very little body fat.
- Where weight is carried. Fat around the waist is linked to higher health risk than the same amount elsewhere. Waist measurement adds useful information.
- Age. Older adults may lose muscle and height, which changes the picture.
- Ethnicity. Some health bodies suggest lower cut-offs for people of Asian descent because health risks can appear at lower BMI.
- Children and teens. They are assessed on growth charts, not these adult bands. BMI is also not designed for pregnancy.
Using BMI sensibly
Treat the number as one clue among many. Blood pressure, cholesterol, blood sugar, fitness, sleep, diet and family history all matter as much or more. A healthcare professional can put these together in a way a calculator cannot.
A healthy weight range
Our calculator also shows the range of weights that would give a BMI of 18.5 to 24.9 at your height. It is a general guide, not a target, and being outside it is not automatically a problem.
Calculate your BMI with the BMI calculator, keeping the limits in this guide in mind.