What Is BMI?
Body Mass Index, or BMI, is a calculation that compares your weight with your height. For adults, it is commonly used as a simple screening measure to group BMI values into categories such as underweight, healthy weight, overweight and obesity.
BMI is calculated by dividing weight in kilograms by height in metres squared:
BMI = weight (kg) ÷ height² (m²)
BMI Categories for Adults
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5–24.9 | Healthy weight |
| 25.0–29.9 | Overweight |
| 30.0–34.9 | Obesity — Class 1 |
| 35.0–39.9 | Obesity — Class 2 |
| 40.0 or higher | Obesity — Class 3 |
These ranges are used for adults. BMI assessment for children and teenagers uses age- and sex-specific growth references instead.
How Accurate Is BMI?
BMI is useful because it is simple and can provide a quick weight-for-height screening measure. But it does not directly measure body fat or body composition.
Two people can have the same BMI while having different proportions of muscle and body fat. BMI should therefore be interpreted alongside other information rather than treated as a complete assessment of health.
When Can BMI Be Less Useful?
BMI may be less informative for some individuals, including people with unusually high muscle mass. It also does not account for where body fat is distributed.
Pregnancy, growth during childhood and adolescence, and some health conditions also require different approaches to evaluating weight and health. If you are concerned about your weight or health, discuss your individual situation with a qualified healthcare professional.
BMI vs Calorie Needs
BMI and daily calorie needs answer different questions. BMI compares your weight with your height. Your calorie needs estimate how much energy your body uses each day.
If your goal is to maintain, lose or gain weight, your estimated daily energy needs are usually more useful for setting a calorie target than BMI alone.