The Prime is a simple modification of the BMI system, which consists in the ratio between the actual BMI with the upper normality limit referred to the BMI scale (currently defined BMI = 24.9). By definition, BMI Prime is also the ratio between body weight and the upper normality limit of body weight, calculated on the BMI of 24.9.
Since it constitutes the relationship between two distinct values of BMI, the Prime is a dimensionless number without associated units.
Individuals with BMI Prime less than 0.74 are underweight, those between 0.74 and 1.00 exhibit optimal weight and subjects with Prime greater than 1.00 are overweight.
BMI Prime is clinically useful because it is possible to quickly determine what percentage of weight deviates from the upper limit of normality.
For example, a person with a BMI of 34 has a BMI Prime of 34 / 24.9 = 1.37, which means that it is 37% above its upper limit.
In Southeast Asia and South Chinese populations, BMI Prime should be calculated using an upper BMI limit of 22.9 instead of 24.99.