• だ/です
    Often translated as "probably," "I assume," or "I believe," だろう is used to speculate based on your interpretation of something.

    Table of Contents

    What Is だろう?

    だろう is one way to express speculation based on opinions and perspectives.

    While there are other ways to express speculation—such as かもしれない—the level of certainty varies depending on the expression. だろう is used when the speaker is pretty sure about whatever they are saying. In fact, だろう emphasizes the feeling that the speaker has a reason to make their claim, even though they often don't specify what this is.

    Let's look at an example to make this clear:

    • コウイチは金持ちだろう
    • I believe that Koichi is rich.

    In this sentence, だろう shows that the speaker has a reason to speculate Koichi is rich. Maybe he has a private helicopter.

    It is also important to note that だろう is a derivative of . For this reason, だろう carries an assertive tone when used in conversations, whereas in writing it only indicates a speculative nuance. (You can read more about だ in this article!) When it is used in conversations, other particles such as ね are often added at the end of the sentence.

    • A: どのチームが優勝すると思う？
      B: ジャイアンツだろうね
    • A: Which team do you think will win the tournament?
      B: I assume it will be the Giants.

    B's use of だろう indicates that B has a reason to support his speculation. Maybe the Giants are the strongest team and always win.

    How to Make Sentences with だろう

    Now let's look at how to form sentences with だろう. It can come after nouns, adjectives, and verbs in both their positive and negative forms.

    Noun + だろう

    Positive Negative
    彼は歌手だろう。 彼は歌手じゃないだろう。
    I assume he is a singer. I assume he is not a singer.

    な-adjective + だろう

    Positive Negative
    きれいだろう。 きれいじゃないだろう。
    I assume it is clean. I assume it is not clean.

    い-adjective + だろう

    Positive Negative
    今週は暑いだろう。 今週は暑くないだろう。
    I assume it's going to be hot this week. I assume it's not going to be hot this week.

    Verb + だろう

    Positive Negative
    彼は会社を休むだろう。 彼は会社を休まないだろう。
    I assume he will take a day off. I assume he won't take a day off.

    For nouns and adjectives, you can indicate that your speculation is about the future by adding a word that indicates time, such as 明日, 来週, or 来月. When using a verb, however, you have to change its form. So to talk about a current action or situation, we use the ている form.

    • 彼は会社を休んでいるだろう。
    • I assume he is taking a day off.

    When using だろう to describe events or situations in the past, you have to change the form of the verb into the past tense. The good news is that you never have to change the form of だろう.

    • 彼は歌手だっただろう。
    • I'm pretty sure he was a singer.
    • 彼は会社を休んだだろう。
    • I'm pretty sure he took the day off.

    You can manipulate the level of certainty of the speculation by using adverbs such as 多分, 恐らく, and きっと. 多分 and its formal version 恐らく indicate a lower level of certainty while きっと indicates a higher level of certainty:

    • 明日はきっと寒いだろう
    • It will definitely be cold tomorrow.
    • 明日は多分寒いだろう
    • It will probably be cold tomorrow.

    It is important to know that lower-probability adverbs such as もしかして and もしかしたら, which are roughly translated as "maybe" in English, cannot be used with だろう. These adverbs don't match the certainty level that だろう carries.

    ❌ 彼はもしかしたら歌手だろう

    So far we've discussed cases where だろう is used at the end of a sentence, but it can also be used in the middle of a sentence, before conjunctions such as から (so) and けど (but).

    • 明日雨だろうから、傘が必要だね。
    • It will probably rain tomorrow, so we might need an umbrella.
    • 勉強大変だろうけど、頑張って。
    • Study must be hard for you, but hang in there.

    These sentences are common in both writing and speaking.

    だろう for Seeking Confirmation

    だろう makes what you say sound like a confirmation question when speakers use it to speculate about something that they think the listener knows about. For example, imagine you ask your friend if he ate your pudding while you were in the bathroom. You might tell him:

    • 僕のプリン、食べただろう。
    • You ate my pudding, right?

    While this question is your speculation, in reality it is often understood as seeking confirmation or agreement. For another example, say you were bragging about how cute your girlfriend is and said:

    • 僕の彼女、可愛いだろう。
    • My girlfriend is cute, right?

    Just like the previous example, it is not you, but your listener who knows what their own opinion about your girlfriend is. This is why this kind of question often comes off as seeking confirmation.

    This use of だろう is common in both writing and speaking. However, when it is used in seeking confirmation, だろう is almost always shortened to だろ. Because of the assertive and definitive tone it carries, だろ is often associated with masculine speech.

    だろう for Seeking Information

    Unlike when using だろ to ask a confirmation question, it won't come off as masculine when using だろ to ask information-seeking questions. In such a case, question words such as なん, だれ, どこ, いつ, and なんで are used together:

    • あれ、なんだろ(う)？
    • What do you think that is?
    • コウイチはどこだろ(う)？
    • I wonder where Koichi is.
    • いつかなえは来るだろ(う)？
    • I wonder when Kanae will come.

    Depending on the context and how you say it, you will either sound like you're asking a question or speculating to yourself when using だろう to ask for information. For example, you might say どこだろう to yourself while you are looking for your phone.

    Indeed, you can also use だろう to indicate that you're in the process of thinking:

    • A: 一番好きな動物はなに？
      B: うーん、なんだろ(う)。猫か犬？
    • A: What is your favorite animal?
      B: Well, l don't know… cats or dogs?

    While か can be used together with だろう to explicitly indicate the sentence is a question, it makes the speech more formal and authoritative. If you say コウイチはどこだろうか, it can make you sound like a character in a novel.