Main drain line

lmanwarren

I made my bathroom bigger and will be relocating the bath and toilet and sink.I bought a new 1.6 gpf toilet and wondering is 3 inch big enough for the main line that runs to the septic.I made the hole for the flange 5 inch being I planned on 4 inch pvc and the flange is 5 inch outside.I was told by a friend 3 inch is plenty big enough.Now if I bought a flange for 3 inch would it be big enough to go out past my 5 inch hole on the floor surronding it so that I can screw it to the floor or am I stuck using 4 inch now.Now the original cast iron pipe looks to be 4 inch inside and using a fernco fitting would it be fine to go from say 3 inch pvc to the cast or does it have to be the same size for the fitting to seal ok.

Bonus

The flange for the 3" will fit fine, 3" is good to use and you can adapt it no problem. There's ferncos that are made for that. I'd say you're good to go!