A layout design method may include receiving predetermined values related to first to third normal fin designs extending in a first direction and arranged in parallel in a second direction perpendicular to the first direction, generating dummy fin designs based on the predetermined values, generating mandrel candidate designs based on the first to third normal fin designs and the dummy fin designs, decomposing the mandrel candidate designs to first and second mandrel mask designs, and generating a final mandrel mask design using one of the first and second mandrel mask designs that satisfies a predetermined condition. A first interval distance in the second direction between the first normal fin design and the second normal fin design may be different from a second interval distance in the second direction between the second normal fin design and the third normal fin design.