The Body Mass Index (BMI) is used internationally by doctors and other health professionals to assess if someone’s weight is putting their health at risk.

A chart may be used, or it can be calculated as follows: your weight (in kg) divided by your height (in metres) squared.  For example, the BMI of a 72kg (11 stone 5lb) woman who is 1.64m (5ft 5in) tall is: 72 divided by (1.64x1.64) = 26.8. 

If you have worked out your BMI, read about your number (or category on the BMI chart).  

• Less than 18.5 or ‘underweight’ – means you may need to gain weight for your health’s sake. Talk to your doctor if you have any concerns, or if you feel frightened about gaining weight. 
• 18.5- 24.9 or ‘healthy weight’ – means you are a healthy weight so aim to stay in this range.
• 25-29.9 or ‘overweight’ – means aim to lose some weight for your health's sake, or at least prevent further weight gain.
• 30-35 or ‘obese/fat’ - means your health is at risk. Losing weight will improve your health. 
• More than 35 or ‘very obese/very fat’ – means you should visit your doctor for a health check, as you may need extra help to manage your weight and health. This is especially important before taking up any new exercise.

Please note that BMI is not as accurate for athletes and very muscled people (muscle weighs more than fat). It is also not accurate for women who are pregnant or breastfeeding, or people who are frail. Please note that pregnant women should not be trying to lose weight.