Fellow Texan here. "Y'all" is plural, but certainly not formal.

I hasten to add that "y'all" is clearly not used exclusively by Texans.

Y'all is singular. "All y'all" is plural.

If the group being spoken to is exceptionally large, or it's clearly a group made of smaller groups (a group of groups), in some places "Alls y'all" is used. It's fascinating to note it when you hear it.

edit also, if there's a possessive it gets pretty crazy, for example a tour guide, assembling several groups for a tour might say "Let's get together for alls y'all's tour".

Native of Georgia (US) here. We use "all y'all" as a possessive. I've never heard "alls y'all's" here, but have in the Southwest and thought it was linguistically slightly excessive. :-)

Oklahoman here. We say "y'all" as plural. We also say "you all."

Y'all is most certainly plural where I'm from (Southeast US).

It confused me when I asked a native Texan something about what I should do next and she responded with "y'all could [foo]". But there's only one of me? I suppose that technically I am the 'all' of the 'you' that is being addressed?

In some regional vernaculars, "y'all" is singular and "all y'all" is plural.

Interesting. Here in North Carolina, "y'all" is generally plural, whereas "all y'all" is also plural but more emphatic, as in "I really do mean every last one of you", or sometimes simply to indicate a stronger emotional content, especially when angry.

South Carolinian here that spent a lot of time in Raleigh & Charlotte and is now living in Austin. "y'all" can be both singular and plural, depending on the context. "All y'all" applies to larger groups (sometimes including people that aren't present), as well as with additional emphasis.

"All y'all come by for supper" meaning you and your close family. And maybe even some other people as well, if there's room at the table for them.

Sounds like a slippery slope.

And now we're back in the "use the plural when talking to a single person to be formal" that started this whole thing.

Native Texan here. I've never really heard it being used as a form of "you" and that just seems silly to me. "Y'all" is definitely plural.

I'm gonna upvote that just for you being a Texan and all.

Though, to be honest, I think that the point of the op is formal, and not plural, 2-nd person address.

Unfortunately, y'all gets used as a singular pronoun as well.

Similarly in Australia we're happy with 'youse'.

I wonder if a high percentage of illiteracy in each place coincides with a high amount of alternative language forms being used?

47% of Australians are functionally illiterate - http://www.abc.net.au/local/videos/2012/09/07/3585457.htm

Illiteracy in Texas (still ranked #1 in US?) - http://www.literacytexas.org/literacy_in_texas/facts_statist...

I thought in Texas it was all y'all.

Not so much. That's farther east, generally speaking.

It seems that where all y'all is used, you have the re-emergence of the distinction between the formal (y'all) and plural (all y'all). Which is pretty fascinating.