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It was dropped in preference to body fat calculation, as a more accurate measure, since BMI penalizes those with more muscles, and rewards weight loss, regardless of fat loss or muscle loss.
Just enter your height and weight and hit calculate. There is a table which indicates what the results mean. As Tamarian said, BMI doesn't take into account muscle mass, so you have to take the result with a grain of salt. I found it helpful as a starting out point, and will re-evaluate when I get closer to my goal weight.