A planar design to non-planar design conversion method includes following steps. At least a diffusion region pattern including a first side and a second side perpendicular to each other is received. A look-up table is queried to obtain a first positive integer according to the first side of the diffusion region pattern and a second positive integer according to the second side of the diffusion region pattern. Then, a plurality of fin patterns is formed. An amount of the fin patterns is equal to the second positive integer. The fin patterns respectively include a first fin length, and the first fin length is a product of the first positive integer and a predetermined value. The forming is performed by at least a computer-aided design (CAD) tool.