A number of standard methods have been proposed to determine the Mode I plane strain fracture toughness of rock. The semi-circular bend (SCB) specimen has been widely used for fracture toughness determination of geomaterials owing to inherent favourable properties such as its simplicity, minimal requirement of machining and the convenience of testing. However, measurements of Mode I fracture toughness using SCB specimen have shown variability with specimen size. The minimum size requirements suggested by many theories are usually too large for practical applications. This research suggests an approximate, but relatively easily applied method, to satisfy the minimum size requirement. Fracture toughness of Kimachi sandstone was determined using the SCB specimen. By testing specimens of varying size, it was confirmed that the apparent fracture toughness increases with the specimen size. The trend line of the apparent fracture toughness displayed eventual stabilisation giving a value that can be considered as specimen size-corrected fracture toughness.