This paper presents a novel neutron-induced soft-error-rate (SER) estimation tool with a particle transport code: PHITS. The proposed tool can calculate the SER according to various data patterns and the layout of the memory cells in an SRAM. As layouts, two kinds of an NMOS-PMOS-NMOS 6T and an inside-out PMOS-NMOS-PMOS versions are considered. The proposed tool distinguishes a single-event-upset (SEU) SER, a horizontal multiple-cell-upset (MCU) SER, and a vertical MCU SER using an extracting function. The horizontal MCU SER in the inside-out version of the PMOS-NMOS-PMOS 6T SRAM cell layout was expected to be 26-41% less than that of the general NMOS-PMOS-NMOS 6T cell layout.