Fatigue and leak-before-break calculations for a pressure vessel require knowledge of the stress intensity factor at the deepest point of a straight- or curved-fronted (semi-elliptical) surface crack emanating from the bore of an internally pressurized, autofrettaged thick cylinder. A limited number of available solutions is curve fitted. The concept of a tube equivalent plate (TEP), which exhibits crack-constraint characteristics matching those of a thick cylinder, is developed, and the resulting equations are curve fitted. Ratios of the TEP stress intensity factor results are then used to interpolate between certain existing solutions. This provides wide-ranging solutions covering radius ratios from 1.8 to 3.0, autofrettage overstrain from 0 to 100% and crack shapes from straight fronted to semi-circular. The calculation procedure is described using worked examples.