The height requirement for toilets under the Americans with Disabilities Act is between 17 to 19 inches, as stated by the ADA Accessibility Guideline in section 4.16.3. If the toilet is for a private home instead of a public restroom, then the person's disability needs to be taken into account before choosing the height of the toilet. The 17-inch height can be better for some wheelchair users, while someone who has a different disability may do better with the 19-inch height.
In the appendix notes of the ADA Accessibility guidelines, it states that 18 inches is normally the best height to go with if installing the toilet into a public area. Seats and pads can be added to adjust the height and comfort of toilets installed in this situation. When measuring to install a toilet under these guidelines, make sure to measure from the floor to the top of the toilet seat or you will not have the proper measurement. Also, when installing toilets that will be used by children with disabilities, the measurements are different and range between 11 and 17 inches. The Americans with Disabilities Act Accessibility Guidelines appendix gives all of the age ranges and the various heights that go with those ages.