BMI (Body Mass Index) is the most widely used tool in the world for quickly assessing whether your weight falls within a healthy range. It is a simple, inexpensive calculation endorsed by the World Health Organization (WHO) — although, as you will see, it has limitations worth knowing.
What is BMI?
BMI relates your weight to your height through a mathematical formula. It does not measure body fat directly; rather, it estimates whether your body mass is proportional to your stature. It is calculated as follows:
BMI = weight (kg) / height (m)²
For example, for a person who weighs 70 kg and is 1.75 m tall:
70 / (1.75 × 1.75) = 22.9
A BMI of 22.9 falls within the normal weight range.
WHO BMI classification
| BMI | Classification |
|---|---|
| Less than 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40 or above | Obesity class III (severe) |
This table is the most widely used reference for adults of both sexes aged 18 to 65.
How to calculate your BMI step by step
If you prefer to do it by hand, it takes three steps:
- Weigh yourself in kg (in the morning, fasted, for greater accuracy).
- Note your height in metres: if you are 178 cm, use 1.78.
- Divide weight by height squared and compare with the table.
If you want the exact result without doing any maths, use our online BMI calculator: enter your weight and height and instantly get your BMI and its WHO classification.
BMI and health risk
BMI is a screening indicator: it quickly flags whether someone may have an unhealthy weight. A BMI above 25 is associated with a higher risk of:
- Cardiovascular disease
- Type 2 diabetes
- High blood pressure
- Joint and mobility problems
- Sleep apnoea
Conversely, a BMI below 18.5 may indicate malnutrition, vitamin deficiency or eating disorders.
The problems with BMI: when is it unreliable?
BMI has important limitations because it does not distinguish between muscle mass and fat. This produces false positives and false negatives:
- Highly muscular athletes: may have an overweight or obese BMI without carrying excess fat.
- Older adults: tend to lose muscle mass and gain visceral fat, so BMI may underestimate risk.
- Pregnant women: weight changes during gestation and BMI tables do not apply.
- Fat distribution: a person with a normal BMI but a lot of abdominal fat has greater cardiovascular risk than this index captures.
In these cases it is worth complementing BMI with other measurements such as waist circumference, body-fat percentage or the waist-to-height ratio.
BMI and age: older adults
After age 65, some studies suggest that a BMI slightly above “normal” (25–27) may be associated with lower mortality, in contrast to the higher risk in younger adults. For this reason, in older adults BMI should be interpreted alongside actual nutritional and functional status.
Conclusion
BMI is a useful, quick and free tool for getting a first impression of your weight, but it is not a diagnosis. It is the ideal starting point for a conversation with your doctor or dietitian, who will assess your body composition, habits and medical history.
Calculate your BMI now and, if the result surprises you, consult a healthcare professional.