What is the proper size pipe to use for the drain for a toilet?

Answer 1

3 inches diameter is standard.

This is one case where Bigger is better--so go with the 4" if you have a choice.

Answer 2

assuming you have the necessary permits for installing a drain line where there has never been one before you need to find how many fixture units will be on the line

local codes in your area may be different

a toilet equals 3 fixture units
a tub is 2
a stand up shower with a 2 inch drain is 3
a laundry is 4
a lavatory sink is 1
a kitchen sink is 2
a bidet is 2
your total number of fixture units can not exceed 11 on a 3 inch drain

all fixtures in a bathroom group must be vented
depending on local codes venting may have to be direct vent or an air admittance valve may be OK

you should pick up a copy of your local plumbing code book and any commentary that may be available since state codes vary slightly from state to state

your local health department should be able to tell you where to get a code book

hope that helps

Answer 3

4"is best but older homes used 3"(toilet will fit either).

Answer 4

I'd go with 4" myself.