Translation guide
Expresses a very high degree of certainty that something will happen or is true. In Japanese, this is often conveyed through adverbs, sentence-ending particles, or conditional patterns rather than a single fixed phrase.
To say that something is very likely to happen or be true, in a neutral or slightly formal tone.
A direct and clear way to say 'almost certainly'. Suitable for both spoken and written Japanese.
彼はほぼ確実に来るでしょう。
He will almost certainly come.
Very common in conversation; literally 'first of all, without mistake'. Conveys strong confidence.
この計画はまず間違いなく成功する。
This plan will almost certainly succeed.
Literally '8 or 9 times out of 10'. A common idiomatic expression for high probability.
十中八九、雨が降るだろう。
It will almost certainly rain.
Similar to まず間違いなく but slightly more formal or written.
その情報はほぼ間違いなく正しい。
That information is almost certainly correct.
To express strong belief about a future event in casual conversation.
Attach to the plain form of a verb/adjective. Means 'must be' or 'no doubt'. Very common in spoken Japanese.
彼はもう着いているに違いない。
He must have arrived already. (He has almost certainly arrived.)
Indicates expectation based on reasoning. 'It should be that...' Often translates to 'almost certainly' in context.
彼なら知っているはずだ。
He should know. (He almost certainly knows.)
Adverb meaning 'surely' or 'certainly'. Often used with 〜に違いない or 〜はずだ for emphasis.
きっと大丈夫だよ。
I'm sure it'll be fine. (It will almost certainly be okay.)
To say something is almost certainly true based on observable evidence or logical deduction.
Colloquial pattern meaning 'it's obvious that' or 'it's bound to be'. Conveys high certainty based on common sense.
そんなこと、誰も信じないに決まっている。
Of course nobody would believe that. (It's almost certain nobody will believe that.)
Formal/literary equivalent of 〜に違いない. Used in writing or formal speech.
この文書は本物に相違ない。
This document is almost certainly genuine.
To express that something is bound to happen, with a nuance of inevitability.
Used with verbs to mean 'can't help but do'. Implies an almost certain reaction.
その話を聞いて、笑わずにはいられなかった。
Hearing that story, I couldn't help but laugh. (I almost certainly laughed.)
Means 'it's natural that...' or 'it's only to be expected'. Conveys that something is almost certain given the circumstances.
彼が怒って当然だ。
It's only natural that he's angry. (He's almost certainly angry.)
There is no single Japanese word that directly corresponds to 'almost certainly' in all contexts. Translating it word-for-word (e.g., ほとんど確かに) sounds unnatural. Instead, choose an expression based on the level of formality and the type of certainty (logical deduction, expectation, inevitability).
に違いない is a subjective, intuitive conviction. はずだ is a logical expectation based on information. に決まっている is a colloquial, emphatic assertion of obviousness. Choose based on the nuance you want to convey.