May and might both express possibility, and for everyday guesses they are often interchangeable — it may rain and it might rain mean much the same thing. The differences that genuinely matter are two: use might for hypothetical or imagined situations, and use might have (not may have) for a past event that did not actually happen. The traditional idea that may means 'more likely' and might means 'less likely' is a real tendency, but a weak one — not a rule worth agonising over.
The short version
- Possibility (present or future): both work — It may snow tonight / It might snow tonight. 'Might' sounds a shade more tentative.
- Hypothetical or unreal 'if': use 'might' — If I had the time, I might learn Italian (not 'may').
- Past that didn't happen: use 'might have' — You might have been hurt! (but you weren't).
- Past that's simply unknown: either works — She may have or might have already left.
- Reported speech: 'may' usually shifts to 'might' — He said he might be late.
Do 'may' and 'might' really differ in likelihood?
Plenty of style guides teach that 'may' signals a stronger possibility and 'might' a more remote one. There is a grain of truth in it: We may finish today can sound a touch more confident than We might finish today. But the effect is subtle, and most fluent speakers treat the two as interchangeable when talking about possibility. Don't lean on the distinction to carry important meaning — if you need to show how likely something is, say so directly with words like 'probably' or 'there's a slim chance'. And note that both 'may' and 'might' can point to the present or the future, so 'might' is not somehow weaker or more childish English:
- ✓ The road may be icy in the morning.
- ✓ I might call you after work. (future possibility — perfectly standard, nothing to do with the past)
Might as the past tense of may
Historically, 'might' is the past tense of 'may', and that job survives mainly in reported (indirect) speech, where a present-tense 'may' shifts back to 'might':
- Direct: 'I may be late.' → Reported: She said she might be late. ✓
- Direct: 'We may need help.' → Reported: They told us they might need help. ✓
Modern usage also lets you keep 'may' when the possibility is still open (She said she may be late), but 'might' is the traditional, always-safe choice. Here 'might' isn't really describing a past event — it's just a change of form.
Might for hypothetical and unreal situations
This is where the two words part company cleanly. For imagined or contrary-to-fact situations — the kind introduced by second- and third-conditional 'if' — use 'might', never 'may':
- ✓ If I won the lottery, I might buy a vineyard.
- ✗ If I won the lottery, I may buy a vineyard.
- ✓ She might have said yes if you'd actually asked.
- ✗ She may have said yes if you'd actually asked.
Keep 'may' for real, open conditions, where the outcome is still genuinely on the table:
- ✓ If you hurry, you may still catch the six o'clock train. (a real, live possibility)
'May have' vs 'might have': the one rule that really matters
Both 'may have' and 'might have' can express uncertainty about the past. When you honestly don't know what happened, they're interchangeable:
- ✓ I can't find my keys — I may have left them at work.
- ✓ I can't find my keys — I might have left them at work.
The difference bites when the event did not happen. For a past possibility that never came true — a near miss, a lucky escape, a what-if — you need 'might have'. 'May have' is wrong there, because it implies the outcome is still open, that perhaps it did occur:
- ✓ If the lifeguard hadn't spotted him, he might have drowned. (but he didn't)
- ✗ If the lifeguard hadn't spotted him, he may have drowned.
- ✓ That was reckless — you might have been killed! (you weren't)
- ✗ That was reckless — you may have been killed!
Set the two meanings side by side and you can see why:
- He may have drowned. = It's possible he drowned; we don't yet know his fate.
- He might have drowned. = In the counterfactual sense, it didn't happen — something saved him.
Because 'may have' can't clearly signal 'and it didn't', careful writers default to 'might have' whenever the thing described never actually occurred. This is the single most useful may/might distinction in English — and the one newspapers most often get wrong.
Tricky cases and exceptions
- 'May I' vs 'might I' (permission): both ask permission; Might I make a suggestion? is simply more tentative and formal, and sounds distinctly British to American ears. May I leave? is the neutral standard.
- 'May not' is ambiguous: You may not leave can mean 'you're not allowed to' (prohibition) or 'possibly you won't'. Context decides. Might not only ever means the possibility (you might not leave in time).
- 'May well' / 'might well': both strengthen a possibility — It may well be true means 'quite probably true'.
- Fixed expressions keep 'may': blessings and concessions don't swap — May you both be very happy; come what may; be that as it may.
- 'Might could', 'might should': these double modals are dialectal (chiefly the American South) and nonstandard in formal writing.
A quick memory trick
'Might' is the past tense of 'may', so give it every job that sits one step away from plain reality: the past, the hypothetical, and the thing that didn't happen. Distance in form mirrors distance in fact. For the past specifically, remember: might have = it might have, but it didn't; may have = maybe it actually did.
US vs UK: does it matter?
Barely. The rules above hold on both sides of the Atlantic — the 'might have' counterfactual, the reported-speech shift, and the fading likelihood distinction are all shared, and the 'may have' slip in near-miss sentences turns up in American and British journalism alike. The only mild regional flavour is that 'Might I…?' for permission, and heavily hedged 'might', sound more characteristically British and formal, while American English leans a little harder on 'may' and 'can'. Anyone who tells you 'Americans always say may, Britons always say might' is overselling a difference that isn't really there.
