Another useful measure for evaluating the level of obesity is BMI (Body Mass Index). This value is calculated by dividing weight, expressed in kilograms, by height squared, expressed in meters. This indicates approximately how much body mass is made up of fat, while the remainder (lean mass) is for the most part comprised of muscle tissue and bone. So, a person weighing 77kg and 1.75m high has a BMI equal to 77 / 1.75 squared, which equals 77 / 3.0625, which works out as 25.14. A BMI over 25 is overweight; a BMI over 30 indicates obesity. This varies according to age, as shown in Figure 6, while Figure 7 shows the boundaries between normal weight, overweight and obesity during the first years of life.