The 2" line is the vent, the portion that is enlarged to 3" or in some jurisdictions 4" is considered a frost proof fitting on the end of the vent line.

For the purpose of computing aggregate total cross sectional area the UPC lists the areas as:

1-1/4" = 1.23sq.in
1-1/2 = 1.77sq.in
2" = 3.14 sq.in
3" = 7.07 sq.in.
4" = 12.57sq.in.

You have two 2" bathroom vents which would yield a total combined cross sectional area of 6.28sq.in which is still 0.79sq.in less than the cross sectional area of a 3" main drain, however you should also have a vent for the kitchen which is most likely an 1-1/2" vent line. An 1-1/2" vent line has a cross sectional area of 1.77sq.in. When you add the cross sectional area of the kitchen vent to the two bathroom vents you now get a total combined aggregate of 8.05sq.in, which is greater than the 7.07 cross sectional area of the 3" main drain.

Thanks, I was afraid I was only able to use the 2" in the calculation. The issue is my main drain to the septic tank is 4", so I would need two 3" vents or four 2" vents. My kitchen sink will have to be vented via an AAV, there is no way to route a vent due to log walls behind it. I will have to vent my washing machine though and it will probably end up going through the roof by itself but it will only be 1 1/2" which won't really help me in my totals. So it looks like I'll go with two 3"s and a 1 1/2".

Sometimes running 3" vents can prove to be a bit challenging.

You laundry standpipe drain is already 2" so I would begin by extending a 2" vent on the standpipe. You could then install a 3" vent on one bathroom and a 2" vent on the other and it would yield a combined total of 13.35sq.in which is greater than the 12.57sq.in in the 4" main drain.

On the other hand I would strongly consider another approach. Unlike the IRC which permits and unlimited number of AAV's, the UPC only permits a maximum of one AAV per structure with the expressed written consent of the AHJ. In some jurisdictions they will only approve an AAV for venting a sink on an island and in others they simply refuse to allow any AAV's.

If your house is built over a basement or crawlspace where you can access the lines under the floor I would strongly consider installing an island loop vent for your kitchen sink then you could run the kitchen, laundry standpipe and both bathrooms with 2". That would yield 12.56sq.in of vents and even though your 4" main drain is actually 12.57sq.in, in most cases the inspector would sooner give you the 0.01sq.in than approve an AAV.

If you need an illustration on how to configure an island loop vent let me know and I will post it.

Yes, I would like to see your illustration on an island loop vent. There are pictures with a description on how to run it in a book I have, but how I understand it is you'd tie in this vent line under the kitchen floor into an existing vertical vent line in the basement which I do not have. Maybe your illustration/description will make more sense in my case. And even though this vent does not exit the roof, it still counts towards the combined cross section total?

Ok, a 2" vent off of the laundry standpipe makes sense and gives me another option, though I'd still have to bump it up to 3" before going through the roof. However, this is new construction and only running two 3" vents may not be too large of a problem. I would like to avoid having too many pipes protruding through the roof if possible as well. What if I turn the 2" laundry vent horizontal at some point (above flood rim height of course) and tie into the 3" vent coming from the main floor bathroom? Or would I then run into the concern of too long of horizontal offset if that would be longer than 3'? Does that 3' rule apply in normal houses with attics where you are combining one horizontal vent into a vertical one before exiting the roof?

Thanks again in advance. Jeff

Lazypup, any chance of posting that illustration? I was doing more research and it looks like a laundry standpipe, though needing a 2" drain, can use a 1 1/2" vent. Therefore if I know I'm going to have too long of a horizontal offset, I could bump it up to 2" and not have to worry about going to 3" which might be hard getting it manuevered where I need. Your thoughts?