Translation guide
Expresses that something is impossible to imagine or believe. In Japanese, this is often conveyed through phrases meaning 'unthinkable', 'can't imagine', or 'beyond belief', rather than a single direct equivalent.
The speaker finds something so surprising or extreme that they cannot imagine it happening or being true.
Literally 'cannot think/consider'. The most common and natural way to say something is inconceivable or unthinkable.
The speaker finds an idea or action so wrong that it should not even be considered.
Literally 'must not exist/happen'. Used for things that are morally or logically impermissible.
考えられない is the most direct equivalent of 'inconceivable' and focuses on the inability to think or reason about something. 信じられない is closer to 'unbelievable' and expresses shock or disbelief. ありえない is casual and emphatic, meaning 'no way' or 'impossible'.
It's inconceivable that he would quit.
It's inconceivable that he would do such a thing.
I never imagined it would turn out like this.
Literally 'cannot imagine'. Emphasizes the inability to form a mental image of the situation.
The vastness of the universe is inconceivable.
Literally 'cannot believe'. Often used when something is so shocking or unexpected that it's hard to accept as true. Closer to 'unbelievable' but overlaps with 'inconceivable'.
That story is simply inconceivable.
Literally 'cannot exist/be possible'. A strong, colloquial way to say something is impossible or out of the question. Often used in casual speech.
That's inconceivable!
Literally 'unexpected, beyond what one could think'. Conveys that something was completely unforeseen or unimaginable. More literary.
It was an inconceivable event.
Such injustice is inconceivable.
Literally 'cannot be forgiven/allowed'. Emphasizes that something is unacceptable.
Human rights violations are inconceivable.
彼が辞めるなんて信じられない。
I can't believe he's quitting.
There's no way he's quitting.
There is no single Japanese word that perfectly matches 'inconceivable' in all contexts. Using a dictionary form like 想像を絶する (souzou wo zessuru) is very strong and literary, and may sound unnatural in everyday speech. Stick to the phrases above for natural communication.