The Russell periodontal index is calculated by:

The Periodontal Index (PI) was developed by Russell A.L. in 1956, over a trial period of ten years. The Russell's rule states that "When in doubt assign the lesser score. The Periodontal Index score (PI score) per individual is obtained by adding all of the individual scores and dividing by the number of teeth present or examined.  PI score per person = Sum of individual scores/Number of teeth present. Ref: Soben peter, 5th edition  pg no:326