～に決まっている

～に決まっている follows a noun, adjective, or plain-form verb and expresses the speaker's strong conviction. It comes from 決まる (きまる, to be decided), so it literally says “it is already decided that ...”. In English, it's usually expressed as “definitely”, “obviously”, or “bound to”.

It is subjective and emphatic: the speaker offers no proof, but treats the conclusion as beyond question. It is common in casual speech, often as a sharp rebuttal.

1. Strong conviction about a fact

Use ～に決まっている to declare flatly what you are sure is true.

2. Confident prediction

After a plain-form verb, it predicts an outcome the speaker considers inevitable.

Compare this with ～はずだ: はずだ draws a calm conclusion from reasons or evidence, while ～に決まっている is a forceful, emotional assertion that leaves no room for doubt.

How to form it

Noun / adjective / plain-form verb + に決まっている

• うそ = lie → うそに決まっている = it's definitely a lie
• 勝つ（かつ）= to win → 勝つに決まっている（かつにきまっている）= is bound to win
• 無理（むり）= impossible → 無理に決まっている（むりにきまっている）= it's obviously impossible

Key idea

～に決まっている literally means “it has already been decided”: the speaker presents their judgment as settled fact, leaving no room for doubt, even if they have not provided any proof.