For talking about possibility, may and might now mean much the same thing: "she may be at lunch" and "she might be at lunch" are both natural and equally likely. The distinctions that still hold are grammatical, not about odds — might is the past-tense form ("he said he might come"), and it is the one used for things that could have happened but didn't ("I might have drowned"). May is also the usual word for permission: "May I leave?" — "Might I" is standard too, but noticeably more formal.
The traditional rule, and how much of it survives
Usage guides have long taught that may signals a stronger possibility and might a fainter one. Be honest with yourself about this one: almost nobody writes by it, and no reader reliably decodes it. If you write "the meeting may be cancelled", nothing tells your reader you think it more likely than "might be cancelled". Choosing between them on the grounds of probability is a matter of ear, not grammar.
Two distinctions, though, are real and are still observed by careful writers.
1. Might is the past form
When a sentence shifts into the past — especially in reported speech — may becomes might:
- "I may be late," she said. → She said she might be late.
- We thought the ice might hold. (not may hold)
2. Might have, for things that did not happen
This is the distinction worth protecting. Might have describes an outcome that was possible but did not occur. May have describes something that possibly did occur, and we still don't know.
- If the alarm had failed, the fire might have spread. (It didn't spread.)
- The fire may have started in the kitchen. (It started somewhere; we don't know where.)
Write "the passengers may have died" about a crash everyone survived and you have said something misleading. That is why this one matters.
3. May for permission
Asking permission is normally may's job: May I borrow your charger? Might I is perfectly standard for a tentative request — Might I make a suggestion? — but it sounds markedly formal or arch. In granting or withholding permission, though, only may works: Visitors may not use the staff lift.
The test
Ask whether the thing actually happened. If it definitely did not, you need might have. If it may or may not have — you genuinely don't know — may have is correct. Everywhere else, in ordinary sentences about future possibility, pick whichever sounds better in the rhythm of your sentence and move on.
One more use
May turns up in formal wishes and concessions where might cannot follow: May you both be very happy. Whatever the cause may be, the result is the same. These are fixed patterns rather than a rule you need to reason about.
Frequently Asked Questions
Is it "may have been" or "might have been"?
Use "might have been" for something that was possible but didn't happen: "It might have been a disaster." Use "may have been" when it possibly did happen and you don't know: "The email may have been sent to the wrong address."
Should I say "may I" or "can I"?
Both are understood as requests for permission. "May I" is the more formal choice and is safest in writing or with strangers; "can I" is completely normal in speech and has been for well over a century.
Does might mean something is less likely than may?
In theory, yes — that is the traditional teaching. In practice the two are used interchangeably for possibility, so don't rely on the difference to carry meaning. If likelihood matters, say it: "probably", "there's an outside chance".