The word "obesity" means being so overweight that it is dangerous for your health. The most common way to measure obesity is using the body mass index (BMI), which compares your weight to your height. To work out your BMI, square your height in metres. Divide your weight in kilograms by this figure. For example you might be 1.7 metres tall and weigh 80kg . Multiply 1.7 by 1.7 to get 2.89. Divide 80 by 2.89. Your BMI is 27.68. If your BMI is 20-25, you have a healthy weight. If it is 25-30, you are overweight. If it is above 30, you are obese.