Measure the distance from the rearmost bowl hold-down bolts, the bolts on the bottom of the toilet covered by plastic caps, to the wall. When buying a replacement toilet, this forms the rough-in measurement to use when buying the new toilet.
Measure 12 inches from the rear wall and mark on the floor with a vertical line. For a new toilet, this measurement represents how far to make the center of the toilet flange from the rear wall. The toilet flange or closet flange is a circular fitting that connects the toilet drain at the bottom of the toilet to the sewer drain pipe on the floor.
Measure 15 inches from the nearest side wall and mark this on the floor with a horizontal line. The distance on the sides for a toilet installation requires a minimum of 15 inches from the center of the toilet to any side obstruction.
Mark the point where the horizontal and vertical lines intersect. Locate the center of the toilet flange here, at the exact point where the floor drain connects to the toilet. Allow a minimum of 2-feet from the front wall to the toilet.