How do you measure BMI?

Simply a formula. formula. Google: "BMI formula". E.g. For US typical units BMI = 703 x Wt (pounds)/Ht (inches). Of course this means BMI does not compensate for type of body build & is commonly very misleading. It was developed by a mathematician, adopted by the insurance/advance-monetary-time-payment industry as a crude indicator of body build for the masses &, in turn, adopted by the medical disease industry.