This pattern conveys that the speaker is absolutely certain about something, often because it is obvious or logically inevitable. It can be used for predictions, assumptions, or general truths. The nuance is stronger than はずだ or に違いない, carrying a sense of 'it’s natural' or 'it goes without saying.'

It is fairly common in both speech and writing, but be careful: using it about other people’s feelings or actions can sound assertive, so it’s best used when you are genuinely sure or when you want to emphasize an undeniable fact. In casual conversation, you might hear に決まってる (contracted), but standard Japanese uses the full form.

Note that when attaching to な-adjectives, you include だ: 簡単だに決まっている. However, when attaching directly to nouns, do not add だ: 学生に決まっている. Adding だ to nouns is incorrect.