Short answer: 3 inches is the minimum for a toilet drain under plumbing codes, and it's what most homes use for the toilet itself. 4 inches is common too, especially further down the line. Either can be correct.
3-inch
- The code minimum for a toilet.
- Matches the opening on the bottom of a standard toilet.
- Plenty for a home toilet, because modern toilets use far less water per flush than older ones.
4-inch
- Often used for the main building drain and the sewer line, where every fixture in the house adds up.
- Sometimes used right at the toilet, especially in older homes or where a builder ran 4-inch through the bathroom.
- A 4x3 closet flange (the fitting the toilet bolts to) lets a 3-inch toilet outlet sit on a 4-inch pipe. That's normal.
Bigger isn't automatically better for a single toilet. Drain lines are sized for how much they carry and laid at the right slope so waste keeps moving.
The measurement that matters more: rough-in
If you're replacing a toilet, the number that decides which toilet fits is the rough-in: the distance from the finished wall to the center of the drain (the floor bolts are a close guide).
- 12 inches is standard in most homes.
- 10-inch and 14-inch rough-ins exist, mostly in older or unusual layouts.
Measure from the wall (not the baseboard) to the center of the bolts before you buy.
When it's more than a toilet swap
Changing the drain size, moving the toilet, or adding a bathroom means new drain and vent lines, which usually need a permit. See can I do my own plumbing in Arizona? and can a shower and toilet share a drain?
Leaking or rocking toilet?
That's usually the wax ring or the flange, not the drain size. See toilet leaking at the base. OX Plumbing resets and replaces toilets and repairs flanges, 24/7.