The Traditional Distinction
- Can: expresses ability — what someone is capable of doing.
- May: expresses permission — what someone is allowed to do; also expresses possibility.
Under this rule:
- She can speak three languages. (she has the ability)
- You may leave early today. (you have permission)
- He may be at the office. (it is possible)
Can: Ability and Informal Permission
Can is a modal verb. Its primary, undisputed use is to express physical or mental ability — what someone or something is capable of doing:
- The software can process up to ten thousand records per second.
- She can finish the report by noon.
- The system can handle multiple concurrent users.
Can also expresses permission in informal and conversational contexts. In everyday spoken English, Can I go? is how most people ask for permission. Style guides that insist on may for permission in casual speech are applying a formal register to a context where it does not naturally belong.
May: Permission and Possibility
May expresses two things: formal permission and possibility.
May for permission
- You may submit the application by email. (formal authorization)
- Attendees may bring one guest.
- Employees may use the facility on weekends.
This use of may is standard in formal writing, policies, and official communications where granting or denying permission is the explicit purpose.
May for possibility
- The results may vary depending on the conditions.
- This approach may not be suitable for all users.
- She may be able to join the call later.
The possibility sense of may is not shared with can in the same way. The results can vary suggests the results have the capability to vary; the results may vary suggests they possibly will.
When the Distinction Matters
In formal writing — official documents, policies, legal texts, academic papers, and professional communications — maintaining the can/may distinction is worthwhile. Using may for permission signals that the granting of permission is the point, not merely confirming capability. In regulatory language especially, the difference between what someone can do (is physically capable of) and what they may do (is permitted to) is sometimes legally significant.
Compare:
- Users can access this data. (they are technically capable of it)
- Users may access this data. (they are authorized to do so)
In a terms-of-service document, the second sentence says something different from the first.
When the Distinction Does Not Matter
In conversational and informal writing, the distinction between can and may for permission has largely collapsed. Most speakers use can to ask and grant permission without any ambiguity. Insisting on may in informal contexts sounds stilted. The correction "you mean 'may'" is most appropriate in formal or instructional settings where maintaining the register matters.
Might: Can vs. May vs. Might
Might is the past tense of may but functions in the present as a weaker form of possibility. Where may suggests a reasonably likely possibility, might suggests something more tentative or less probable:
- She may be at her desk. (fairly possible)
- She might be at her desk. (less certain, more tentative)
- The update may cause issues for some users. (this is a real possibility)
- The update might cause issues for some users. (possible but less expected)
In formal writing, using might when you want to express genuine uncertainty and may when the possibility is more substantial is a useful distinction to maintain.
Cannot vs. May Not
The distinction matters in the negative forms. Cannot means "is not able to." May not means "is not permitted to" or "it is possible that it will not."
- Users cannot access files without authentication. (technical inability)
- Users may not share passwords. (prohibition)
- The results may not be available until Monday. (possibility — they might not be)
In formal policy writing, the difference between "users cannot do X" (they lack the capability) and "users may not do X" (they are prohibited) should be maintained carefully.
Frequently Asked Questions
Is it wrong to use "can" for permission in formal writing?
In formal writing — particularly legal, regulatory, and official policy documents — using may for permission is standard. Using can for permission in those contexts risks ambiguity between capability and authorization. In less formal professional writing, can for permission is generally acceptable.
Which is correct: "Can I help you?" or "May I help you?"
Both are used. May I help you? is more formal and traditional. Can I help you? is more casual and widely used in everyday service contexts. Neither is incorrect; register determines which fits the situation.
Do style guides weigh in on this?
Yes. AP Style, The Chicago Manual of Style, and most academic style guides acknowledge that the traditional ability/permission distinction is increasingly blurred in informal English, but recommend using may for permission in formal writing where precision matters. Garner's Modern English Usage notes that the distinction is worth preserving in careful writing.