I am putting in a new bathroom which is about 50' from my main drain stack. My question is: do I need to use 3" or 4" pipe for the horizontal drain line? Right now, draining into this will be a clothes washer, tub/shower, vanity and toilet. Eventually, I may drain a kitchen, and/or another bathroom into this line. Adding up fixture units, I only get to 19 and I think that I can put 20 on a horizontal 3" line. But I've also read that a toilet should be discharged downstream of everything else. This will not be the case with my configuration. Which is the other reason I am thinking about upsizing to 4", but am trying to keep costs down. Thanks in advance.

Go with the 4" pipe and slope it at 1/4" per foot. Is this all on one level?

Yes all on one level. The horizontal drain is 16" below the subfloor however.

You are correct about the number of fixture units that you will need. I strongly recommend going with a 4" drain line with 1/4" to the foot slope.
Not to worry just run the branch at 1/4" slope and when you get to the horizontal main 45 down to connect up.
You're thinking of the section that says that a major fixture such as a toilet shall not be discharged past a unvented minor one. As long as your fixtures are vented correctly the toilet can install anywhere. Most bathroom groups are roughed in like this.
Toilet connects to sewer main or the stack vent. Lavatory connects to toilet drain and runs a vent off the top the stubout tee out the roof or revents back into a dry vent in the attic.. The toilet wet vents through the lavatory vent and the tub/shower connects to the lavatory drain and is wet vented by it. This is a normal rough in and is acceptable both by local and state codes and also The Standard Plumbing Code Book in 90 percent of the country. Check your local codes.to make sure you're not in the excluded 10 percent.. The vent off the lavatory may be run out the roof or revented back into a dry vent in the attic or if you're reventing back into a fixtures dry vent you must make your connection at least 6 inches over that fixtures flood rim.
Good luck, Tom

Thanks for your detailed reply Speedball. That answers my questions, but now I have new ones... On ballengerb1's reply I went ahead, bit the bullet and bought 4" pipe that I am going to install at a 1/4" per ft pitch. In the meantime, I found a discussion on Terry Love's site that talks about 3" have better flow or carry for a long distance. What is that about? He also suggests that the only reason to move up to 4" is if you have 4 or more toilets discharging into the line. At this point, I am just curious, as I am going to install the 4" as I mentioned.

Thank you for clarifying that. That makes much more sense.

This is all more or less what I was planning to do. Which part might not be allowed? The wet vent? Because here is what I was thinking to do: run a 2" vent off of the toilet's drain and then connect that into a dry vent in the attic. The part I wasn't so sure about, which I think you sort of answered: can I discharge the sink, AND a washing machine into this vertical 2" vent by way of a 2" horizontal drain? The tub/shower is across the room and will need to be vented and drained differently. The washer may also need a separate revent as it may be too far away from the toilet's vent.

Thanks, Perry

We you 3 on all of our new construction

You'll have a better scouring action with 3" is all.

If properly sized and vented, the water and waste goes down the piping in a vortex, which scours the pipe walls.

The general rule of thumb is that 3" will handle 3 bathrooms just fine -- But adding a fourth means you must upsize the trunk line that the fourth bathroom branch ties into to 4".

Another thing to consider when oversizing waste lines is that you are also drilling bigger holes in framing members.

Good point Growler, that's why I was asking if this was all on one level. Pretty difficult to feed a 4" pipe down interior walls. Scouring action makes sense, had not heard/thought of it that way.

Thanks for explaining that. Fortunately, I won't have to bore through any framing. I can hang it all from the joists in the basement. So, Growler, would you have suggested that I go with the 4" as well?

Forgot to ask you Rendle, got any plans to finish the basement? 4" PVC can be pretty noisey. My wife made be go back an insulate our drains after I finished the basement.

No, probably not, or at least not anytime soon. I can also drop the ceiling if I need to.

Our codes mandate a 4" main and bathroom branch under the slab but allow 3" for a second floor bathroom group and the main stack. If your bathroom group is installed above the slab I would go with 3". My earlier answer was bases on a slab rough in. Good luck, Tom