The main goal of this study is to obtain an optimum mix design for steel-fibre-reinforced concrete (SFRC) plates that establishes the optimum aspect ratio (length/diameter) and volume fraction of fibres by maximising the plate toughness (as specified by the EFNARC standard) and minimising the mix costs. To achieve this goal, 60 plate specimens with dimensions of 600 x 600 x 100 mm were produced based on different SFRC mixtures and tested. The compressive and splitting tensile strengths and the elastic modulus of the mixtures were also determined. Based on the experimental results, a multi-objective simultaneous optimisation technique was used to determine the optimum SFRC mix design. The variables considered during optimisation were the aspect ratio and volume fraction of the fibres for two different concrete classes. For each concrete class, the optimum mix design was determined.