Conventional radiocarbon ages have been corrected for isotope fractionation by normalizing to = -25 PDB or VPDB. CALIB no longer supports the correction for isotope fractionation within the program for the following reason: The δ13C correction depends on whether the original measurement was a 14C/12C ratio (all radiometric and some AMS) or a 14C/13C ratio (some AMS systems). An Excel spreadsheet with the formulas for both types of systems is given here. If you don't know which type of measurement was done it is better to contact the laboratory which supplied the radiocarbon date.
MS Excel spreadsheet for δ