Installing toilets in new construction requires precise measurements so the drain pipe lines up with the toilet's trap. The drain pipe is installed long before the toilet, using the rough-in measurement that matches the rough-in dimension for the toilet. When these measurements are incorrect, the toilet's tank may end up sticking into the finished wall or positioned several inches away from the wall.
Check the building plans to locate the general spot where the toilet will be installed.
Calculate the distances from neighboring fixtures and walls to the center of the toilet. Use called-out dimensions on the plans, or measure the distances on the plans and convert them using the plan's scale to arrive at the real world measurements.
Mark a line on the sub-floor perpendicular to the wall that will be behind the toilet, and that represents the center of the toilet relative to the objects on its sides.
Measure 12 inches (or 10 inches) outward from the edge of the bottom plate of the wall behind the toilet along the line drawn in Step 3.
Add the thickness of the finished wall to the measurement. If the wall will be 1/2-inch drywall, add 1/2 inch to 12 inches to arrive at 12-1/2 inches. If the finished wall will be 5/8-inch drywall, and the rough-in for the toilet is 10 inches, add 5/8 inch to 10 inches to arrive at 10-5/8 inches.
Mark this location on the line drawn in Step 3. The toilet drain pipe will be centered on this mark.