Don't worry about your BMI, especially if you lift. It is an old method used to calculate your proper weight.
The formula used is: weight in kg/(height in m) squared.
General guidelines are that anyone within 20-25 is fine, 25-30 is overweight, over 30 is obese. Many fitness people I know are over 30 and are in fantastic shape. If you are a lifter worry more about your body composition.