Maintaining a healthy weight is an important part of being fit. Being underweight or overweight  can take a huge toll on the body and can seriously impact your health now and in the future. Body Mass Index, or BMI, is a screening tool that measures body size. It is a way to determine if you are at a healthy weight for your given height, or if you need to make some adjustments to your lifestyle. BMI can be a tool to help you stay aligned with your fitness goals and can also encourage you establish new goals.