BMI, or body mass index, is a statistical measurement of a person’s body weight that is based on both weight and height. It is used to determine what a person’s ideal weight is. In comparison, BMR refers to a person’s Basal Metabolic Rate, or the number of calories a person would burn in a day even if he or she did nothing but sit or lie in one place.
Your BMI is calculated only by your weight and height (for a child, his or her age and sex are also factors). Your BMR is determined by your weight, height, sex, and age. The first can help you determine your immediate health needs (where are you in the range of “health”?) and the second can help you determine what changes you may need to make your diet in order to be where you want to be.
A normal BMI will be between 18.5 and 24.9. You are considered underweight if your BMI is less than that. Anyone with a BMI under 16.5 is considered severely underweight and it is an unofficial criterion for anorexia. The BMI category from 25 to 29.9 is overweight. You are considered obese if your BMI is over 30, and are morbidly obese it is over 40.