"Can I" vs "May I" in restaurant setting when ordering

A while back, while we were getting fast food, my friend commented on my usage of "can" versus "may" when asking to take my order. I said:

Can I have a .......

and my friend argued you're supposed to say

May I have a .......

Although I had never thought much about it, and at first assumed my friend was correct, I later started thinking more on it and have come to doubt this to be true. "May I" implies that what you're asking for is already known to be present and available. "Can I" implies more unsurety and is therefore technically the correct answer since there's always uncertainty that a product may or may not be physically available for taking or purchasing.

At first I didn't really care, but the debate has been drawn out now and has turned into a friendly bet of sorts. I just want to know: when in a food environment, which is technically more correct. I know this stuff can be one way or the other sometimes, but I have to know what the experts believe.

Solution 1:

I believe 'can' is more appropriate in a restaurant.

Firstly it is quite possible that you cannot have something that is on the menu because it is no longer available. Asking if you 'can' have the swordfish is valid because the answer may be no.

Secondly using 'may' implies you are asking for permission which I don't think is appropriate in a restaurant. If a waiter told you "you can get a steak tartare, the question is may you", I imagine that you'd be somewhat suprised, possibly outraged. It is called 'ordering' for a reason.

Using may doesn't sound terribly wrong to me but I do think it is largely sham courtesy.

If you are asking for something that isn't the purpose of the staff to fulfill then sure.

"May I go in and see if my friend is already here?"

Great. Wholly appropriate.

"May I have a double cheeseburger and chips"

Seems overly polite and probably sham courtesy.

Solution 2:

In the dilemma "may" vs. "can" and which form is preferable, it depends on how old the speaker is, where they live and which dialect of English they speak.

There is an age-old debate that can in requests, is asking if something is "possible", e.g.

A: Can I have a glass of water?
B: Yes, you can (=it is possible).

Whereas may, some argue, is asking permission and is used in polite, slightly hesitant, requests.

A: May I have a glass of water?
B: Yes, (you may) I'll pour you a glass.

Older, and perhaps more educated, British speakers will probably prefer using "may", and perhaps most Americans too, regardless of their level of education. In my experience, younger British speakers will nearly always ask "Can I...?"

The modal could is also used in formal, rather polite, requests

Could I have a glass of water, please?

There is a nice summary by Cambridge Dictionary titled Requests: Asking for something And in this six-year-old question posted on EL&U: Difference between "can" and "may"

EDIT (June 5 2018)

No waiter that I know of, would think to reply a customer's order with "Yes, you can/may have a glass of water" that could sound arrogant. A normal response would be “Yes, certainly”. The whole point of can/could/may/might/'d like (etc.) in requests is the acknowledgment that the server or counter attendant is the person who brings and sometimes even prepares the food or drink in person.

The server is doing their job but they're doing it (and many do) to the best of their ability. When a customer uses polite language, it is not sham courtesy but a sign of respect.

A server or waiting staff takes on a very important role in a restaurant which is to always be attentive and accommodating to the customers. Wikipedia