What is the difference between に違いない and きっと～でしょう?

Question

Do に違いない and きっと～でしょう have the same meaning and usage? I learned that に違いない is based on feeling of the speaker, but I couldn't find what きっと～でしょう is based on. So I'm wondering if きっと～でしょう is based on logic or feeling just like に違いない?

Answer 1

I see two differences. Basically きっと is even more subjective.

• きっと is typically used to describe a desirable outcome. きっと失敗するだろう is grammatical but this can sound you are hoping someone will fail. 失敗するに違いない is neutral in this regard.
• きっと tends to be used when you say something based on your subjective/personal expectation rather than an objective estimation. Saying きっとうまくいくよ ("It's gonna be fine") to encourage someone is usually safe even when the objective chance of success is 50%. Saying うまくいくに違いない in a situation like this can sound irresponsible.

Answer 2

Both describe almost same degree of high certainty.

~に違いない implies it is highly likely to be something in an assertive, conclusive sense.

きっと～でしょう also implies it is highly likely to be something generally/objectively/ speaking.

Both are kind of a subjective assumption. ~に違いない sounds more definitive than きっと～でしょう.

For example, if weather forecaster says it will be a heavy rain tomorrow due to typhoon approaching, 明日は大雨に間違いなくなるでしょう。 sounds more natural than 明日は大雨にきっとなるでしょう。 .