What is the correct question 何だろう or 何だろうか


How to ask a question with だろう in the correct way? Should I add か in the end?

Like 何だろうか？  
I saw a lot of examples and almost always guys omit か in the end. And just say
何だろう；いつだろう。。。




～だろう forms a question that is mainly addressed to yourself. It's like "I
wonder". If you are clearly requesting an answer from someone else, don't use
it.

Next, か after だろう in a yes/no type question corresponds to English
"whether/if", and it's mandatory. か in a wh-question (i.e., a question
including いつ, だれ, 何, なぜ, etc) is purely optional, but is usually omitted
especially in casual speech.

>   * 彼は来るだろう **か** ？ I wonder **if** he will come.
>   * これは食べられるだろう **か** ？ I wonder **if** I can eat this.
>

>   * これは何だろう？ (I wonder,) What's this?
>   * これは何だろうか？ (I wonder,) What's this?
>   * どうしてだろう？ I wonder why.
>   * どうしてだろうか？ I wonder why.
>

Related:

  * [だろう in question context?](https://japanese.stackexchange.com/q/62238/5010)
  * [Difference between だろうか and ではないだろうか](https://japanese.stackexchange.com/q/34189/5010)
  * [願わくばこのまま穏やかなままとはいかねぇもんだろうか meaning?](https://japanese.stackexchange.com/q/69057/5010)




～だろう forms a question that is mainly addressed to yourself. It's like "I
wonder". If you are clearly requesting an answer from someone else, don't use
it.

Next, か after だろう in a yes/no type question corresponds to English
"whether/if", and it's mandatory. か in a wh-question (i.e., a question
including いつ, だれ, 何, なぜ, etc) is purely optional, but is usually omitted
especially in casual speech.

>   * 彼は来るだろう **か** ？ I wonder **if** he will come.
>   * これは食べられるだろう **か** ？ I wonder **if** I can eat this.
>

>   * これは何だろう？ (I wonder,) What's this?
>   * これは何だろうか？ (I wonder,) What's this?
>   * どうしてだろう？ I wonder why.
>   * どうしてだろうか？ I wonder why.
>

Related:

  * [だろう in question context?](https://japanese.stackexchange.com/q/62238/5010)
  * [Difference between だろうか and ではないだろうか](https://japanese.stackexchange.com/q/34189/5010)
  * [願わくばこのまま穏やかなままとはいかねぇもんだろうか meaning?](https://japanese.stackexchange.com/q/69057/5010)

