or (for imperial units)

For instance, if your weight is 91 Kg and height is 1,77 meters, it makes:

BMI = 91 / (1,77*1,77) = 29 or calculate your BMI here

According to the table below, it indicates an overweight.

Table source: National Heart, Lung, and Blood Institute

For adults, weight status is based on the absolute BMI level. For children and adolescents, weight status is determined by the comparison of the individual's BMI with age- and gender-specific percentile values.
BMI is more accurate at approximating body fat than is measuring body weight alone. However, BMI has some limitations. BMI overestimates body fat in people who are very muscular and underestimates body fat in people who have lost muscle mass. The relationship between BMI and body fat varies somewhat with age, gender, and ethnicity. In addition, for adults, BMI is a better predictor of a population's disease risk than an individual's risk of chronic disease.
As another surrogate measure, waist circumference can approximate abdominal fat but should be measured very carefully. Fat located in the abdominal region is associated with a greater health risk than peripheral fat.