A device for determining dimensions of a workpiece includes a locating apparatus, a determining apparatus and a main processor. The locating apparatus includes a locating board supporting workpieces and defining at least one detecting aperture therethrough. The determining apparatus includes a detecting module, the detecting module includes at least one laser detector, each laser detector includes a laser emitter and a laser receiver respectively mounted on two opposite sides of the locating board. The main processor is connected to the locating apparatus and the determining apparatus, the laser emitter emits laser beams traveling through the detecting aperture and received by the laser receiver, and the main processor determines the dimension of the workpiece according to the dimension of parts on the laser receiver shielded by the workpiece.