May, might, and could can all express possibility.
That does not mean they follow a fixed scale such as may = more likely, might = less likely, and could = least likely.
Sometimes the three are very close in meaning. In other situations, the choice changes the tone or the kind of possibility being expressed.
May vs Might vs Could: The Main Difference
All three modals can describe something that is possible but not certain.
A useful starting point is:
| Modal | Common possibility use |
|---|---|
| may | uncertain possibility; can sound slightly more formal |
| might | uncertain possibility; often very close to may |
| could | a possible explanation, outcome, or development |
These are tendencies, not fixed probability levels.
The best choice depends on the context and the kind of possibility you want to express.
When May, Might, and Could Mean Nearly the Same Thing
Sometimes all three fit naturally in the same situation.
Compare:
The revised schedule may cause delays.
The revised schedule might cause delays.
The revised schedule could cause delays.
All three sentences say that delays are possible.
The difference is subtle.
May can sound slightly more formal.
Might can sometimes sound a little more tentative.
Could presents delay as one possible outcome.
But none of these sentences gives an exact level of probability.
Another example:
The missing file may be in the archive.
The missing file might be in the archive.
The missing file could be in the archive.
Each sentence presents the archive as a possible location.
In a context like this, the practical difference may be very small.
May vs Might for Possibility
May and might are especially close when they express uncertainty.
May and Might Are Often Similar, Not Fixed Levels of Certainty
Compare:
The committee may postpone the vote.
The committee might postpone the vote.
Both mean that postponement is possible.
You may sometimes hear that might always means a smaller chance than may.
That is too rigid.
In some contexts, might can sound more tentative or remote, but English does not maintain a fixed percentage difference between the two.
For example:
The client may request another revision.
The client might request another revision.
Without more context, both simply express uncertainty.
The speaker is not necessarily assigning two different levels of probability.
Why May Can Sound More Formal
May often appears naturally in formal or careful wording.
There may be further changes to the policy.
Additional checks may be required.
Might would also be grammatical:
There might be further changes to the policy.
But may can sound slightly more formal or neutral in this type of writing.
That is a difference in register, not proof that the event is more likely.
Could for Possibility
Could often presents something as one possible explanation or outcome among several.
For example:
The slowdown could be caused by a network problem.
The network problem is one possible explanation.
Another example:
A second supplier could reduce the delivery time.
Using another supplier is one possible way the situation could develop.
This does not mean could is automatically less certain than may or might.
Its usefulness often comes from presenting an available possibility rather than assigning it a particular probability.
May, Might, and Could for Present and Future Possibility
The same modals can refer to the present or future. The surrounding sentence usually tells you the time.
Present Possibility
Nora may still be in the laboratory.
The finance team might already have the revised figures.
The unusual noise could be coming from the ventilation system.
Each sentence describes something that may be true now.
The speaker does not know for certain.
Future Possibility
The contractor may finish ahead of schedule.
The negotiations might continue into next week.
A stronger currency could reduce import costs.
Here, the possibilities concern future events or results.
The modal itself does not create a fixed level of certainty. Context does much of that work.
Negative Possibility: May Not and Might Not
May not and might not can both express the possibility that something will not happen or is not true.
The shipment may not arrive today.
The client might not approve the revised design.
Rachel may not be available this afternoon.
The repairs might not be finished by Friday.
In these sentences, the speaker is uncertain.
The negative outcome is possible.
Why Couldn’t Is Different
Couldn’t does not normally work as a simple negative equivalent of may or might in this use.
Compare:
The supplier might not deliver on time.
This means late delivery is possible.
Now compare:
The supplier couldn’t deliver on time.
This normally means the supplier was unable to deliver on time.
Another example:
Daniel may not attend the meeting.
means that perhaps Daniel will not attend.
But:
Daniel couldn’t attend the meeting.
normally means that something prevented him from attending.
Couldn’t can also express that something is impossible:
That couldn’t be the correct file. The dates do not match.
So when you mean perhaps not, may not or might not is usually the relevant pattern.
May Have, Might Have, and Could Have for Past Possibility
To describe an uncertain event in the past, English can use:
may / might / could + have + past participle
For example:
The technician may have changed the settings.
Rachel might have taken an earlier flight.
The error could have started during the migration.
In each sentence, the speaker is considering a possible explanation for something in the past.
When They Describe an Unknown Past
Compare:
The package may have been delivered to the neighboring office.
The package might have been delivered to the neighboring office.
Both suggest that this is a possible explanation.
May have and might have are often very close in meaning.
Could have can also make a guess about the past:
The courier could have left the package at the side entrance.
In the right context, this means that leaving it there is one possible explanation for where the package went.
Again, there is no reliable percentage scale separating the three forms.
Could Have Can Mean Something Else
Could have also has another important meaning.
Compare:
Leo could have taken the earlier train. We still do not know which train he used.
Here, could have presents one possible past explanation.
Now compare:
Leo could have taken the earlier train, but he left home too late.
Here, we know he did not take it.
The sentence means that the opportunity existed, but it was not used.
Another example:
We could have avoided the delay if we had checked the address first.
Avoiding the delay was possible, but it did not happen.
Context therefore matters greatly with could have.
Can Is Different From May, Might, and Could
Can can also express possibility, but it often describes something that is generally possible rather than uncertainty about one particular situation.
Compare:
Network outages can disrupt remote meetings.
This describes a general possibility.
Now compare:
The network may be down right now.
The network might be down right now.
The network could be down right now.
These describe uncertainty about one specific situation.
Another contrast:
High temperatures can damage sensitive equipment.
describes something that can happen generally.
This device may be overheating.
describes uncertainty about what is happening to one device now.
You can learn the detailed distinction between these modals in can vs may.
A Simple Way to Choose Between May, Might, and Could
Do not choose by memorizing percentages.
Instead, ask what you want the sentence to do.
You simply want to say something is uncertain.
May or might will often work.
You are presenting one possible explanation or outcome.
Could often fits naturally.
You want slightly more formal wording.
May may suit the context.
You want a somewhat more tentative tone.
Might can sometimes create that effect.
You are guessing about the past.
Use may have, might have, or could have, depending on the context.
You mean something was possible in the past but did not happen.
Could have can express that meaning.
The important question is not which modal has the highest percentage of certainty. It is what kind of possibility you want to express.
May, might, and could overlap in many situations, and sometimes the difference is small. Context, tone, and meaning are more useful guides than a fixed certainty scale.
The First English Grade Editorial creates simple English grammar, vocabulary, and language guides for students, beginners, and everyday learners.