Wax rings come in two diameters, 3 inches and 4 inches, because — as you might expect — those are the two standard sizes for toilet waste openings. Besides diameter, thickness is also an important parameter when it comes to wax rings. Again, there are two possibilities: regular and extra thick. The wax ring forms the seal between the toilet and the waste pipe, so to determine the wax ring size, you have to remove the toilet.
Determining the Wax Ring Diameter
You determine the wax ring diameter by turning over your toilet and measuring the diameter of the waste opening. If the toilet has been leaking from the bottom, which is a good sign the existing wax ring has failed, you'll have to pull the toilet. This means unbolting it from the floor and lifting it off the flange. Be sure to turn off the water, empty the tank and unhook the water supply before you do this. A good pair of tongue-and-groove pliers is all you need to do this, assuming no problems.
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Once you've got the toilet off, you'll want to empty the bowl to prevent water spilling all over the bathroom floor. You can do this with a sponge or an old turkey baster, which you've now recommissioned as a bathroom tool. Once the toilet is on its side, clear off remnants of the old wax ring sufficiently to allow you to measure the outlet diameter with a tape measure.
Regular or Extra Thick?
To determine the thickness of the wax ring you need, look at the distance between the top of the flange, which is the fitting to which you attach the toilet, and the surface of the floor. If it's at the same level, or no more than 1/4 inch below it, a regular wax ring, which is between 3/4 and 1 inch thick, is all you need. If the flange is recessed deeper than that, you need an extra-thick wax ring, which is about an inch thicker than a regular one. If the flange is deeply recessed, and the old wax ring was a regular one, that's probably why the toilet was leaking onto the floor.
In some cases, the flange is recessed too deeply for even an extra-thick wax ring to make a seal. This often happens when you install new flooring in the bathroom that is thicker than the existing flooring. The solution is to install a flange extender, which is a plastic ring that bolts on top of the existing flange and raises the height by as much as 3/4 inches.
If the flange is within 1/4 inch of floor level, do not use an extra-thick wax ring. You won't be able to compress it sufficiently to prevent the toilet from rocking.
Installing a New Wax Ring
The most difficult and unpleasant part of installing a new wax ring is removing all remnants of the old one. The standard method is to scrape it off bit-by-bit with a putty knife. Be sure to wear gloves and deposit all the old wax on a newspaper or paper towels for disposal.
Some plumbers like to fit the new wax ring around the toilet waste opening and push on it to make it stick, while others prefer setting the wax ring on the flange. Either way works, and when you reset the toilet, it's a good idea to sit on it facing the tank and rock back and forth to compress it before you bolt the toilet down.