How to Determine Toilet Size and Toilet Fitting Sizes

Toilet Sizes  . . .

Most common standard toilets are generally considered to be either “2-inch or 3-inch” by naming convention. This refers to the INSIDE pipe diameter within the toilet and down through the floor drain. There are also some in between and larger size toilets, but these are considered non-standard and require fittings from the original manufacturer because no standard flush valves of any type will fit, whether they are standard flapper or dual flush piston valves.
Since the toilet size refers to the inside pipe diameter, this means the drain hole cut-out inside the tank will naturally be larger. How much larger depends on the initial hole cut-out and the after-cure tolerance of the ceramic or plastic composite during manufacturing.
Toilet Fitting Sizes – Flush Valve Drain Hole Sizing
In most cases for 2-inch toilets, this will result in a drain hole size in the range of 2 3/8″ to 2 3/4″, and so the flange and gasket combinations on our 2″ dual-flush valves will fit and seal this size range effectively. Any drain hole size larger than 2 3/4″ does not allow enough gasket overlap and may result in leaking. It is certainly possible to substitute a larger gasket or base adapter, but that gets beyond the scope of a basic DIY retrofit and is best left up to the advanced installer or plumber.
Similarly for 3-inch toilets, the drain hole should fall in the range of 3 3/16″ to 3 3/4″, and so the flange and gasket combination on our 3″ dual-flush valves will fit and seal this range. Any drain holes size larger than 3 3/4″ does not allow enough gasket overlap and may result in leaking.
Note that in most cases, the toilet size can be determined by measuring the lip of the flapper valve base. If the flapper lip is roughly 1 7/8″ to 2 3/8″ in diameter, that indicates a 2-inch toilet. If it is more in the 3″ range, that is most likely a 3-inch toilet. See our illustration of measuring the flapper valve base lip over on the Products > Help Me Choose tab.
In other cases where the flapper base is an odd design or the valve is not a flapper type, the valve and tank will need to be removed to allow direct measurement of the tank drain hole.
Filler Valve Inlet Hole Sizing
The filler valve inlet hole in the tank is much easier to fit because it is almost always 1″ or slightly larger and fits standard 15/16″ threaded filler valves. Most filler valves, including our models, will fit and seal a hole size in the range of 1″ to 1 3/8″.
Flush Handle or Push Button Actuators Mount Hole Sizing
The flush actuator mount hole can be a little tricky sometimes because there are two commons sizes, 5/8″ and 3/4″ square, which will also vary a little due to the same manufacturing tolerances. Most flush actuators, whether lever handle or push button, will have a removable rubber collar that will accommodate both sizes. All of our kits have this collar arrangement unless otherwise specified. In extreme cases where the hole is a little smaller than 5/8″, the plastic shoulder of the flush mechanism may need to be sanded or filed slightly. Then on the other extreme, where the hole is slightly larger than 3/4″, the rubber collar may need to be shimmed with tape or other suitable material to enlarge it further for a tight fit.
Still other flush actuator mount holes are round or odd-sized, again intended for a specific manufacturer actuator.
Anytime a non-standard smaller hole size is encountered, there is always the option of enlarging it with a ceramic file or grinder. This may sound like a lot of work, but in most cases, the hole only needs to be enlarged slightly and the ceramic or plastic composite usually cuts away fairly quickly and easily. Then upon completion, the resulting fitting hole is now at a standard size and can accept any number of standard fittings on the market.