The formulas available are Holladay 1 & 2, Hoffer Q and SRK/T. The H2 gives the best results and the others are just for comparison for the surgeon to see how much better. The Holladay 1 Formula is the first formula Dr Holladay developed using 2 variable predictor (AL,K) of the Effective Lens Position (ELP). The Holladay 2 is newer and uses 7 variables (AL, K, ACD, LT, WTW, Age and Ref) to predict the ELP.