The container behind your toilet is called the toilet tank. This tank fills with water that will enter your bathroom during a flush. Toilet cisterns vary in size. You will have to measure a tank when you are determining if it can fit into a space or when you are trying to replace it.
American Standard 3195A101.020 Champion PRO Right Height Elongated Toilet Bowl, White
Instructions
1 Measure the locations of the three holes in the bottom of the tank, referring to the other. These are important measures because they will help determine if a new tank will fit in the toilet if you are replacing the old tank.
2 Measure the length, width and depth of the toilet tank with your measuring tape.
3 Measure the distance from the hole in the handle to the nearest edge of the toilet. If you are going to replace the tank, but have a handle that you want to use, you want to make sure it fits and looks appropriate. Some handles are larger than others and it will be too long for the new toilet. Although a handle can work mechanically in the new toilet, it may seem strange because of its length in relation to the size of the toilet bowl.