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'Could' expresses reasonable possibility or polite permission; 'might' expresses more remote or uncertain possibility.
Side-by-side comparison
Use "could" to ask for permission in a polite but fairly direct way. It works well when you expect a positive answer with reasonable confidence.
Could you pass me the salt, please? I cannot reach it from here.
Could I borrow your charger for a minute? My phone is almost dead.
Could you speak a little more slowly, please? I am still learning English.
"Might" sounds more hesitant and uncertain than "could". Use it when you are unsure whether you will be allowed, or when you want to sound especially considerate.
It might rain later, so you might want to bring an umbrella just in case.
You might want to call ahead before going, the restaurant closes early on Mondays.
We might be late if there is heavy traffic on the motorway.
Common mistake
"Might I use your office?" sounds archaic in modern English. For formal permission use "Could I" or "May I". And "might not" and "mustn't" are not the same: "She might not attend" = she may not attend (uncertainty). "She mustn't attend" = she must not attend (prohibition).
Trick to remember it
- •Possibility scale: WILL > SHOULD > MAY > COULD > MIGHT (most to least probable).
- •For permission: CAN (informal) > COULD (formal) > MAY (very formal) > MIGHT (archaic).
- •"You might want to..." = the softest possible suggestion.
Quick rule
- •"Could" = reasonably probable possibility + formal permission.
- •"Might" = more remote or uncertain possibility.
- •"Might I...?" = archaic; use "Could I...?" instead.
Did you know?
"Could" and "might" are both past forms of modals: "could" is the past of "can" and "might" is the past of "may". But in modern English, both are used primarily in the present for possibility or permission. This evolution of past forms acquiring present meaning (or greater politeness) is one of the most fascinating phenomena in the English modal system. The only context where "might" clearly denotes future-from-a-past-point is reported speech: "He said she might attend."
Practise what you learned
Exercise 1 · Pick the right one
"_____ I leave a bit early today? I have a dentist appointment."
Exercise 2 · Pick the right one
"_____ I ask who's calling, please?" (formal phone etiquette)
Exercise 3 · Form the word
Related word pairs
"Can" expresses ability or capacity (to be able to do something); "may" expresses formal permission or possibility (something might happen).
'Could you...?' asks if someone is able to do something; 'would you...?' asks if they would be willing to.
"May" expresses higher probability or formal permission; "might" expresses lower probability or is the past tense of "may".
'May well' expresses high probability; 'might well' expresses reasonable possibility with slightly more uncertainty.
Bryan López
Founder of SpeakUP Academy