What it means
に決まっている expresses the speaker’s strong conviction that something is certainly true or will certainly happen. It is often used when the conclusion seems obvious from the situation, evidence, or common sense, but it can also emphasize a personal judgment. The expression commonly follows a plain verb, an i-adjective, a na-adjective stem, or a noun. Its polite counterpart is に決まっています.
Usage note
に決まっている is emphatic and is more common in familiar or assertive speech than in highly formal writing. It presents the conclusion as obvious rather than as a cautious probability, so it can sound forceful when directed at another person.
How to use it
彼が犯人に決まっています。
He must be the culprit.
この方法が安全に決まっています。
This method must be safe.
その映画は面白いに決まっています。
That movie must be interesting.
彼は来るに決まっています。
He is certain to come.
彼が犯人に決まっている。
He must be the culprit.
この方法が安全に決まっている。
This method must be safe.
その映画は面白いに決まっている。
That movie must be interesting.
彼は来るに決まっている。
He is certain to come.