Many have suggested that 3" pipe is better than 4" for floating the solids away.
At some point, there is a need for 4" based on how many toilets are connected.
Once the waste line leaves the home, you will be looking at 4"
Terry Love

: On another forum, there has been some discussion about whether 3" or 4" waste pipe will work better , the issue being less flow from toilets to "float" the waste down the line. Someone even suggested 2 1/2 ".
: I'm just curious, and you probably can answer this better that the other folks who have been posting there.

: Thanks,
: Jim