BMI is a popular weight loss term thrown around with little explanation. If you are confused about what it means and how to calculate it here is a brief by Althea Shah Fitness Expert & VP Marketing Gold's Gym India to help you out. What is BMI? BMI stands for Body Mass Index and is a useful measurement for most people over 18. It is a measurement derived from your weight and height that estimates your body composition. It tells you whether you are underweight overweight or an ideal weight for your height. Knowing your BMI is important as a