Steel profiles, such as angle sections in beams and girders, with slightly tapered cross section sides are not uncommon in older structures. Further, the fracture toughness of older and inhomogeneous steels varies in general across the thickness of a sample. The thickness of a machined, standard parallel-sided specimen of a tapered sample is in practice seldom more than 60-70 % of the full sample thickness and such a specimen captures in general inferior core material only. In view of this conundrum, a modified three point bend specimen with partly tapered sides has been designed so as to accommodate tapered samples. The project includes calibration, testing and evaluation of the modified specimen. The part of the project, which is reported here, is part analytical and part numerical and aimed at calculation of the stress intensity factor for various crack lengths, taper and specimen cross section proportions.