On a one-piece toilet, you'll find the TANK NUMBER molded into the back wall of the tank. Just carefully remove the tank cover and look for the four digit number. The number is sometimes preceded by the letter K and is usually the second row in the tank wall.

On a two-piece toilet, you will need the BOWL NUMBER as well, found on the underside of the bowl tail.

If you are unable to locate the model number of your toilet, check with your plumbing contractor or builder for the number.