The Body Mass Index, also known popularly as your BMI, is calculated by dividing your weight (in kilograms) by your height (in meters). It is designed for men and women over the age of 18 and represents a measurement of body fat based on height and weight.
Underweight = <18.5
Normal weight = 18.5–24.9
Overweight = 25–29.9
Obesity = BMI of 30 or more
You can check out the entire table over at the HHS website.