Position the edge of a tape measure against the wall located behind the toilet, and mark a distance 18 inches away from the wall on the bathroom floor. This mark represents the distance the center of the toilet bowl must be from the wall.
Measure 18 inches from the wall or object closest to one side of the toilet. Mark this measurement on the bathroom floor.
Connect the two marks with a line. Use a straightedge to ensure that the line is straight. This line represents the available locations where you can install the toilet with the correct setback. Remember that the line provides the location of the center of the bowl, not the front of the toilet.