判断・推量N2
〜に決まっている
it's certain that... / definitely...
Structure
普通形/N + だ + に決まっている
Explanation
Indicates that something is certain, a very confident speculation. Attached to the plain form of verbs/adjectives or noun + だ.
Examples
そんなに遅くまで遊んでいたら、叱られるに決まっている。
Playing that late, you're bound to get scolded.
彼が来るに決まっている。約束したから。
He's definitely coming—we made a promise.
Note
More assertive and colloquial than に違いない. Not used with past tense; the subject is usually a third person.