〜に決まっている
Bound to be 〜に決まっている
Meaning
Says an outcome is obvious — "of course ~", "it is bound to be ~".
Formation
[plain form] + に決まっている · [noun] / [na-adj] + に決まっている (no だ)
Notes
A confident, conversational assertion — the speaker treats the conclusion as too obvious to argue with. It expresses the speaker's conviction, not a fact that has been decided, so it is not the passive of 決める. Too blunt for formal writing.
Examples
こんな時間（じかん）に電話（でんわ）したら、迷惑（めいわく）に決（き）まっている。
Phoning at this hour is bound to be a nuisance.
Breakdown
そんなに食（た）べたら太（ふと）るに決（き）まっています。
Eat that much and of course you will put on weight.
Breakdown
一人（ひとり）で全部（ぜんぶ）やるのは無理（むり）に決（き）まっている。
Doing it all alone is obviously impossible.
Breakdown
新（あたら）しいほうがいいに決（き）まっています。
The new one is obviously better.
Breakdown
彼（かれ）が知（し）っているに決（き）まっているよ。
Of course he knows.
Breakdown
⚠️ Often confused with
〜わけだ
That is why 〜わけだ
Draws a conclusion that follows naturally — "so that's why ~", "no wonder ~", "it works ou...
〜わけではない
It is not that 〜わけではない
A partial denial — "it is not that ~", "that does not mean ~". Softens rather than rejects...
〜わけがない
There is no way 〜わけがない
A flat denial of possibility — "there is no way ~", "it is impossible that ~".
〜とは限らない
Not necessarily 〜とは限らない
"Not necessarily ~", "it is not always the case that ~".
〜ものだ
One should / used to 〜ものだ
Three uses: a general truth, nostalgia for what one used to do, and mild moral advice.
〜っこない
Could not possibly 〜っこない
A blunt, casual denial — "there is no chance ~", "could not possibly ~".
〜に過ぎない
Merely 〜に過ぎない
"Merely ~", "no more than ~".
〜というものだ
That is what ~ is 〜というものだ
"That is what ~ really is" — the speaker's verdict stated as a general truth.
〜にほかならない
Nothing other than 〜にほかならない
"Is nothing other than ~", "is precisely ~".