A calculation method for generating a layout pattern in a photomask includes at least the following steps. A two-dimensional design layout including several geometric patterns distributed in a plane is provided to a computer system. The computer system is used to mark portions of the geometric patterns and generate at least one marked geometric pattern and at least one non-marked geometric pattern. The marked geometric pattern is then simulated and corrected by the computer system so as to generate a 3-D design layout. Through the simulation and correction, the marked geometric pattern and the non-marked geometric pattern are arranged alternately along an axis orthogonal to the plane. The 3-D design layout is outputted to a mask-making system afterwards.