Body mass index (BMI) is a measure of body fat based on height and weight that applies to adult men and women. Based on this, it classifies whether a person is underweight, normal, overweight or obese.
If you're trying to lose, maintain, or gain weight, it's a good idea to start by calculating your Basal Metabolic Rate (BMR). Your basal metabolism is the amount of energy your body uses when it is completely at rest - it's simply the energy needed to operate your organs and keep you alive each day without taking any physical activity in to account. Your BMR is affected by a large number of variables - gender, age, height and weight are the most important, but your body fat percentage, diet, and exercise habits also play a role.