Should I use "Can I", "Could I", or "May I"?

A student sent me a question about how to ask for permission in English:

I have a question about when to use "Can I...?", "Could I...?", or "May I..."to ask someone for permission. For example, If I(a student) asked you (the teacher) to do something, should I use "Could I...?" instead of "Can I...?"

This is a good question. It's confusing that English has all of these different ways of asking for permission. Here are some guidelines for using each expression:

Can I..?

"Can I...?" is the most casual way to ask for permission. It's common for talking to friends, coworkers, and family members:

Can I see it?

Can I get something to drink?

In traditional English grammar, "Can I...?" was not used for asking permission. That's changed in the last 50 years, though. These days, it's the most common of the three expressions.

May I...?

"May I...?" is the most formal way to ask for permission in English. Formal language is useful for talking to strangers and when there's a large power gap between you and the person you're talking to.

You can ask a stranger for a small favor like this:

May I borrow your pen for a second? 

Some teachers in elementary, junior high, and high school require their students to ask for permission using "May I...?"

Student: Can I go to the bathroom?

Teacher: "May I...?"

Student: May I go to the bathroom?

Teacher: Yes, you may.

Could I...?

"Could I...?" is a good way to ask for permission when you need to ask for something that's a "bigger" request. In other words, you don't feel as comfortable asking for it. For example, you might ask your sister:

Could I borrow your other car when I'm in town?

"Could I...?" is not as formal as "May I...?" but it's better for big requests.

The big picture

The differences between "Can I...," "Could I...," and "May I...?" are very small. It's not a big deal if you mix them up. So learn the differences if you can, but when you need to ask for permission, just choose the expression you think is best and ask confidently!