BMI, or body mass index, is based on a person's height and weight, with a healthy BMI score being between 20 and 25. Calculate BMI by multiplying weight times 703, and then dividing this by height squared withhelp from a certified personal trainer in this free video on calculating body mass index. Expert: Tanya Batts Bio: Tanya Batts has been a certified personal fitness trainer for more than 11 years. She specializes in Pilates, yoga, combat cardio, aerobics,...