Understanding BMI: what it tells you and what it does not

Body mass index is your weight in kilograms divided by your height in metres squared. Someone weighing 70 kg who is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.

In pounds and inches, multiply weight by 703 and divide by height squared. The result is the same.

The World Health Organization groups adult BMI as follows: under 18.5 is underweight, 18.5 to 24.9 is a healthy weight, 25 to 29.9 is overweight and 30 or more is obesity.

These ranges are for adults aged 20 and over. Children and teenagers are assessed with age- and sex-specific growth charts instead.

BMI does not distinguish muscle from fat or show where fat is stored. Muscular athletes can have a high BMI while being very fit, and some people with a normal BMI carry unhealthy amounts of body fat.

It can also be less accurate for older adults, pregnant women and people of some ethnic backgrounds, for whom different thresholds are sometimes used.

Treat BMI as a starting point for a conversation, not a diagnosis. Waist measurement, diet, activity, blood tests and family history give a fuller picture.

This guide is general information, not medical advice. Speak to a doctor or other qualified professional about your health.