The BMI is a mathematical calculation invented by the MetLife corp. in the 1950's (or sometime around that time) for determining life insurance premiums based on longevity.
Using data from 100 years or so, their actuarials came up with a remarkable highly correlated predictor using height and weight as the inputs that predict lifespan.
The formula is your weight in pounds multiplied by 703, divided by the square of your height in inches. (The reason for the 703 multiplier is that the formula is really your weight in kilos divided by the square of your height in meters).