Fracture toughness is a measurement of fracture resistance and is a crucial parameter in designing and manufacturing structural engineering components, including the components of ships and offshore structures. However, accurate measurement of fracture toughness requires a fatigue pre-cracked specimen which can be challenging to prepare in extremely brittle materials. Moreover, specimen thickness is another requirement which can be challenging to achieve as well, particularly in very tough materials. Therefore, a physics-based closed-form analytical expression is proposed to determine fracture toughness of isotropic materials simply by utilizing a uniaxial tensile test specimen. The expression naturally introduces a length scale parameter, consistent with non-local applications, such as peridynamics, as well. Fracture toughness of various metallic materials, including both brittle and ductile, are predicted and compared to the experimental results in the literature. Predicted fracture toughness values are in good agreement with the experimentally measured ones.