It isn't often that you have to cut a toilet, and you definitely don't ever want to cut into the bowl or tank for any reason. However, on rare occasions, it may be necessary to cut a small portion off the base or the tank lid to fit the toilet into a corner.
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Porcelain is a material almost identical to ceramic — it's just made from denser clay and fired at higher temperatures — so the methods tile setters use to cut ceramic tiles also work for porcelain. Pro Tool Guide lists six such methods, and you can apply at least two of them to toilets, provided you wear goggles and work carefully.
Use an Angle Grinder
The most common method for cutting porcelain and ceramic tiles is to use a wet saw, which is a small table saw with a diamond cutting wheel and a trough of water through which the blade passes to keep it cool. You can't use a wet saw to cut a toilet, but you can use a 4-inch diamond cutting wheel installed in an angle grinder. Keep the blade manually wet by squirting it with water from a plastic bottle.
Mark the cut line on the toilet using a wax pencil or felt tip marker, then put on goggles and lower the grinding wheel gently onto the mark, keeping steady pressure as it works its way through the porcelain. It's best if you have a helper to squirt water continuously as you cut. If you're working alone, stop frequently and squirt the water yourself. It's important to keep both the cutting wheel and the porcelain wet to prevent cracks.
Cutting Tight Curves
An angle grinder can cut wide curves but not tight ones that you might need to fit the toilet around a water pipe sticking out from the wall. Some tradespeople might try to make narrow curved cuts with a jigsaw and a diamond blade, but the vibrations of the jigsaw make this method risky. A better way is to drill a series of holes with a masonry bit. If the holes are close enough together, you can break the porcelain between them by tapping with a cold chisel and a mallet.
Drilling through porcelain can be challenging because the material is so hard that the tip of the bit walks along the surface. To prevent this, a poster in an online plumbing forum recommends angling the drill about 45 degrees and creating a divot deep enough to prevent the bit from wandering.
It's just as important to keep the drill bit cool as it is to keep a cutting wheel cool, but you don't have to spray water continuously. Just dip the bit periodically in a bowl of water. Once all the drilling is done, put on goggles to protect your eyes before you tap the porcelain to break it, because small shards are bound to go flying.
Nippers for Small Notches
You may need to cut a small portion from a corner of the bowl base or tank lid to get the toilet to fit in a tight corner, and rather than sawing, it may be better to use tile nippers. Use these to break tiny sections of the porcelain away and keep nipping until the notch is the right size. Nippers don't make a clean cut, but that won't matter if the section of the toilet you're cutting is hidden behind it.