Linear regression formulae are given for converting 1H and 13C magnetic shielding constants calculated at common ab initio and density functional theory levels of calculation into chemical shifts relative to tetramethylsilane. Accuracies of roughly +/-2.2 ppm (13C) and +/-0.15 ppm (1H) or better are found for the training set for most levels. The highest level calculations do not always give better results than economical standard calculations.