This paper describes a program that evaluates tensile and flexural behaviour of steel fibre reinforced concrete. It simulates a random distribution of fibres in a sample volume and, analysing an assigned number of failure surfaces, it determines a characteristic curve that describes the variation of tensile strength in function of crack width. This curve measures the toughness of the composite and can be used to estimate flexural strength and ductility of beams and plates. The procedure is based on theoretical formulas relative to straigth fibres and hooked end fibres. It is also possible to make reference to experimental pullout curves to characterize the mechanical behaviour of the fibre.