BMI Prime, a modification of the BMI system, is the ratio of actual BMI to upper limit optimal BMI (currently defined at 25 kg/m 2 ), ., the actual BMI expressed as a proportion of upper limit optimal. The ratio of actual body weight to body weight for upper limit optimal BMI (25 kg/m 2 ) is equal to BMI Prime. BMI Prime is a dimensionless number independent of units. Individuals with BMI Prime less than are underweight; those with between and have optimal weight; and those at or greater are overweight. BMI Prime is useful clinically because it shows by what ratio (. ) or percentage (. 136%, or 36% above) a person deviates from the maximum optimal BMI.