The standard rough-in dimension for a toilet drain pipe is 12” off the back wall. The official requirements in the International Plumbing Code state that a toilet cannot be closer than 15 inches from the centerline to any side wall, partition, cabinet, etc... If toilets are side by side without a wall in between, there cannot be less than 30 inches between the centerlines of the two toilets. Finally, there cannot be less than 21 inches of clearance in front of the toilet to any wall or other obstruction.