Chelae crushing forces produced by crabs at a single specific gape size have been explored in numerous studies, but very rarely are multiple gape sizes tested. Here, we investigated the chela strength generated at differing gape sizes of the mangrove mud crab, Scylla olivacea. Crabs were induced to grip metal levers fixed to a modified digital pull-push force gauge to generate a force reading. Crushing forces produced by the proximal and distal regions of chelae were significantly different (P<0.05). The major chela of Scylla olivacea had a mechanical advantage compared to its minor chela. Overall, as gape size widened, the force generated increased until an optimal (i.e., greatest force achieved) gape size was attained. Thereafter, a decline in force generation was recorded. The optimal gape size has important implications for foraging behaviour, including prey selection and handling time.