Can vs. May: What’s the Difference?

These two words cause a lot of confusion in English and leave writers—and speakers especially—unsure about which word to use and when. Does can have a special function that may cannot be used for? Or are they completely interchangeable?

What is the Difference Between Can and May?

Today, I want to highlight the differences between can and may. I will outline the traditional rule regarding can vs. may, provide you with example sentences for each, and offer some advice going forward in your writing.

After reading this post, you shouldn’t ever again wonder which is correct or which is proper, may or can.

When to Use Can

May you pleaseCan is an auxiliary verb (sometimes called a helping verb or a modal verb) and is used to denote physical or mental ability. It derives from the Old English cunnan, which means, “to be able.”

  • Can you do the salsa?
  • Can you play basketball?
  • He can compose entire symphonies in his head.
  • She can run a five-minute mile.

The traditional rule, as you probably learned in grade school, states that can has to do with physical ability or the capacity to do something, not with permission. As such, all of the above sentences could be rewritten by substituting can with able/capable.

  • Are you capable of doing the salsa?
  • Are you able to play basketball?

You probably remember being a young student and asking the question,

  • Can I go to the bathroom?

Your teacher would invariably correct you by saying,

  • I don’t know, can you?

This, of course, was an instructional exercise on the part of the teacher, who was attempting to instill the difference between expressing one’s physical ability and a request for permission: the difference between may I vs. can I.

When to Use May

Can versus may grammar rulesMay is also an auxiliary verb and is used to denote possibility or permission.

  • There may be a storm tomorrow. (Possibility)
  • I may attend the festivities this weekend. (Possibility)
  • You may borrow my jacket. (Permission)
  • If you finish your chores, you may play outside. (Permission)

For a discussion between may vs. might, see our full post on the subject.

Going back to our example from above, what your teacher wanted you to ask was the question,

  • May I go to the bathroom?

This is a request for permission and not a statement of ability.

How to Express the Denial of Permission

Up until this point, we have focused on the person asking the question and not the person answering. If someone asks you for permission, what is the proper way to respond?

If the answer is yes, then no thought is given at all.

  • May I come inside?
  • Yes, you may.

But, if the answer is no, there are a few additional considerations to keep in mind.

A denial of permission is properly phrased formally by saying may not or with cannot or can’t.

  • You may not use my car tonight.
  • You cannot use my car tonight.
  • You can’t use my car tonight.

But, you should not use the contraction mayn’t. Most people are unaware of this contraction to being with, but for those writers/speakers who are, it’s best to avoid it.

Are May vs. Can Interchangeable?

Does it really make a difference whether I use can I vs. may I?

grammar rules for may versus canThe answer is, of course, it depends. Depends on what, you ask? As with most writing and speaking, choosing whether to use can or may depends on your audience.

If you are writing a highly formal letter where politeness and cordiality is a primary concern, you will definitely want to adhere to the traditional rule and use may to ask permission.

Similarly, if you are at a fancy restaurant, it might be more appropriate to as your waiter, May I have some more water? instead of saying, Can I have some more water?

May is the more polite option.

If, however, you are surrounded by peers in an informal setting, there is nothing wrong with asking your friend, Can I have another hot dog? at a cookout.

As with most writing and speaking, you need to know your audience and understand what is best for the situation at hand. When you wish to show courtesy, respect, and politeness, use may. When these are not of concern, using can is fine.

It should also be noted that while you can use can instead of the word may to request permission, you cannot use may instead of the word can to express ability.

  • I may play basketball. (WRONG)
  • I can play basketball. (CORRECT)

To say I may play basketball is a different statement than to say I can play basketball. While the former isn’t incorrect in all of its uses, it is incorrect for the intended meaning in this example. I may play basketball expresses the likelihood or your playing while I can play basketball expressed your ability to play.

Summary

Even if you rarely make the distinction between these words, it’s important to know the difference between may vs. can.

May is used to express possibility or ask permission.

Can is used to express ability and informally used to ask permission.