に決まっているObviously; bound to be
に決まっている forcefully presents the speaker's conclusion as obvious, settled, or inevitable, leaving little room for another answer.
Treating the answer as settled
に決まっている does more than express high probability. It conveys an attitude like “Of course that is the answer” or “What else could it be?” The speaker may have a clear reason, but emotion often helps explain why this forceful form is chosen.
Typical forceful assertions
These conclusions are presented as too obvious to debate.
そんな無責任な提案は、みんなに反対されるに決まっている。
Such an irresponsible proposal is obviously going to be opposed by everyone.
AnalyzeThe criticism of the proposal gives the conclusion an emotional edge.
駅前の店なんだから、家賃は高いに決まっている。
Since the shop is right by the station, the rent is obviously high.
Analyzeそんな説明では、誰も納得しないに決まっている。
With an explanation like that, obviously no one will be convinced.
AnalyzeIn conversation
この勝負、どっちが勝つと思う？
Who do you think will win this match?
Analyze経験のある美咲に決まってるよ。
Obviously Misaki—the one with experience.
Analyzeに決まってる is the ordinary conversational contraction. Use the full form when you do not want the specifically casual sound.
Choosing among certainty expressions
に決まっている strongly shuts down competing answers and often carries emotion. に違いない expresses a confident belief based on available information without as strongly dismissing alternatives. はずだ presents something as the expected result of known circumstances, while に相違ない is markedly more formal or literary.
足跡から見ると、犯人は裏口から入ったに違いない。
Judging from the footprints, the culprit must have entered through the back door.
AnalyzeThis neighboring form emphasizes a confident conclusion drawn from evidence rather than an emotionally forceful dismissal of other answers.
More examples
あれだけ準備したんだから、発表は成功するに決まっている。
After all that preparation, the presentation is obviously going to succeed.
Analyzeこの値段なら、偽物に決まっている。
At this price, it has to be a fake.
Analyze子ども一人でこんな重い箱を運ぶのは無理に決まっている。
Of course it is impossible for one child to carry such a heavy box.
Analyzeあんな失礼なメールを送ったら、相手は怒るに決まっている。
If you send such a rude email, of course the other person will get angry.
Analyze外は雪なんだから、寒いに決まってるよ。
It is snowing outside, so of course it is cold.
AnalyzeThe contraction gives this sentence a casual conversational tone.
彼女は約束を守る人だから、来るに決まっている。
She is someone who keeps her promises, so of course she will come.
Analyzeあの二人が組めば、試合は楽勝に決まっている。
If those two team up, the match is bound to be an easy win.
Analyze