The Body Mass Index (or BMI) is a way of seeing if your weight is appropriate for your height. The actual calculation is your weight divided by your height squared. BMI can be divided into several categories and generally the higher your BMI, the greater your risk of a large range of medical problems including Pre-Diabetes and Type 2 Diabetes.
BMI Calculator at the MayoClinic
Or you can you this one from the CDC (Centers for Disease Control and Prevention):