Because obesity can lead to so many health problems, it's important to help and support people who are obese or overweight to lose weight. Healthcare professionals use a measure called body mass index (BMI) to help decide if you are overweight or obese.
Your BMI is your weight in kilograms (kg) divided by the square of your height in metres (m2). If you want to work out your BMI, you can use an online BMI calculator.
What BMI means for adults
Between 18.5 and 24.9 – healthy weight.
Between 25 and 29.9 – overweight.
Between 30 and 39.9 – obese.
BMI of 40 or more – severely obese.
In adults, BMI is used as an estimate of body fat. Different people are affected by obesity at different BMIs. For example, people who are very muscular or some older people may not be at high risk of health problems due to their weight even if their BMI is over 25. But, people from black, Asian family origin or minority ethnic groups may be at increased risk even if their BMI is 25 or under (see Other NICE guidance for details of our guidance on BMI and waist circumference in black, Asian and minority ethnic groups). This means you might be advised to lose weight or keep your weight steady even if your BMI looks like it is OK or a little high. If this is the case your healthcare professional should explain why.
For children and young people, a different system is used. The healthcare professional should check their BMI against special charts that take account of their age and sex to decide if they are overweight or obese. You can also use an online BMI calculator, but if you do remember it must be one that says it will work for children as adult BMI calculators will give the wrong results. See Other NICE guidance for details of our guidance on obesity in children and young people.