Provided are a method for performing wireless communication by a first device and an apparatus supporting same. The method may comprise: transmitting, to a second device through a physical sidelink control channel (PSCCH), a first sidelink control information (SCI) including information related to a physical sidelink feedback channel (PSFCH) overhead and a beta offset; and transmitting, to the second device through a physical sidelink shared channel (PSSCH) related to the PSCCH, a second SCI, wherein a symbol length related to the PSSCH is obtained based on the information related to the PSFCH overhead, and wherein a number of resource elements (REs) to which the second SCI is mapped is obtained based on the symbol length related to the PSSCH and the beta offset.