Determine the height of the toilet bowl by measuring from the top of the bowl surface (just below the lid) to the floor.
Find the total height of the toilet, including the toilet tank, by measuring from the top of the toilet tank lid to the floor.
Measure the width of the base of the toilet. Because the front may be narrower than the back and you want the width of the widest section, measure the back from left to right.
Measure from the wall behind the toilet to one of the bolts that hold down the toilet. The bolts are on the bottom frame of the toilet on either side and may be covered with a plastic cover. This final measurement is the rough-in, which is the most important measurement.