How to Use “～だろう” (darou) and “～だろうと思う” (darou to omou) for Guessing in Japanese

Introduction

Have you ever wanted to say things like:

• “It will probably rain tomorrow.”
• “I think the economy will get better.”
• “He will surely be happy.”

In Japanese, when you want to express a guess, prediction, or probability, you can use:

～だろう / ～だろうと思おもう = probably… / I think it will…

Let’s learn how to use these natural Japanese expressions to share your predictions and thoughts.

1. Meaning: What does ～だろう mean?

～だろう (darou) has the exact same meaning as ～でしょう (deshou). It is used to express:

• a guess or presumption,
• a prediction about the future,
• probability.

Common English translations:

• probably…
• I guess…
• I think…

2. Formation

Basic structure

To use this grammar, simply attach だろう to the plain form of a verb, adjective, or noun.

Verb / い-adjective

Plain form + だろう

• 増ふえる (to increase) → 増えるだろう
• 良よい (good) → 良いだろう

な-adjective / Noun

Dictionary form (without だ) + だろう

• 元気げんき (energetic) → 元気だろう (NOT 元気なだろう, 元気だだろう)
• 雨あめ (rain) → 雨だろう (NOT 雨だだろう)

Examples by word type

How to guess about the past

You can also use だろう to guess about something that already happened. Simply change the verb or adjective into the past tense (Ta-form) before adding だろう.

• 昨日きのうは雨あめだっただろう。(It probably rained yesterday.)
• そのテストは難むずかしかっただろう。(That test was probably difficult.)

Make it Sound Natural: Use たぶん and きっと

Native speakers often pair ～だろう with specific adverbs to show exactly how confident they are in their guess.

• たぶん (Probably) + だろう: Standard guess (~70% sure).
  • たぶん、明日あしたは雨あめだろう。(It will probably rain tomorrow.)
• きっと (Surely / Definitely) + だろう: Strong confidence (~90% sure).
  • 彼かれはきっと喜よろこぶだろう。(I’m sure he will be happy.)

3. Spoken vs. Written Japanese (だろう vs だろうと思う)

It is important to know when to use ～だろう and when to use ～だろうと思おもう.

• ～だろう： Written Language (書かき言葉ことば)
  Used in news articles, essays, and reports. It sounds objective and declarative.
• ～だろうと思おもう： Spoken Language (話はなし言葉ことば)
  Used in daily conversations. Adding と思う softens the sentence and emphasizes that it is your personal opinion or guess.

Compare:

• これからは外国人がいこくじんの旅行者りょこうしゃが増ふえるだろう。
  Foreign tourists will probably increase. (Written)
• 外国人がいこくじんの旅行者りょこうしゃが増ふえれば、経済けいざいがもっと良よくなるだろうと思おもいます。
  If foreign tourists increase, I think the economy will get better. (Spoken)

4. Watch Your Intonation! (Rising vs. Falling)

When using ～だろう in casual conversation, your intonation changes the entire meaning!

• Falling Intonation (だろう ⬇): Expresses a guess or prediction (Probably…).
  • 明日あしたは雨あめだろう ⬇ (It will probably rain tomorrow.)
• Rising Intonation (だろう？ ⬆): Seeking agreement or confirming a fact (…, right? / Didn’t I tell you?).
  • 言いっただろう？ ⬆ (I told you so, didn’t I?)
  • これ、おいしいだろう？ ⬆ (This is delicious, right?)

5. Example Sentences

Written style (～だろう)

弟おとうとの誕生日たんじょうびにゲームを買かってあげるつもりだ。きっと喜よろこぶだろう。
I plan to buy my younger brother a video game for his birthday. I’m sure he’ll love it.

次つぎのワールドカップはフランスが優勝ゆうしょうするだろう。
France will probably win the next World Cup.

娘むすめの成績せいせきが悪わるいのは、勉強時間べんきょうじかんが足たりないからだろう。
My daughter’s poor grades are probably because she doesn’t study enough.

Spoken style (～だろうと思う)

この曲きょくは人気にんきが出でるだろうと思おもいます。
I think this song will probably become popular.

姉あねが幼おさななじみと結婚けっこんしました。二人ふたりはきっと幸しあわせになるだろうと思おもいます。
My older sister got married to her childhood friend. I’m sure they’ll be happy together.

田中たなかさんなら、この仕事しごとを手伝てつだってくれるだろうと思おもいます。
I think Tanaka-san would be willing to help with this task.

6. Related grammar (comparison)

～だろう vs ～でしょう

Both expressions mean “probably” or “I suppose”, and in many situations they are interchangeable.

However, でしょう is more polite, while だろう is the plain form.

Examples:

• 明日あしたは雨あめだろう。
  It will probably rain tomorrow.
• 明日あしたは雨あめでしょう。
  It will probably rain tomorrow. (more polite)

～だろう vs ～かもしれない

Both express uncertainty, but the degree of confidence is different.

Compare:

• 明日あしたは雨あめだろう。
  It will probably rain tomorrow.
• 明日あしたは雨あめかもしれない。
  It might rain tomorrow.

The speaker is more confident when using ～だろう.

～だろう vs ～そうだ

These grammar patterns are both used for predictions, but the basis of the prediction is different.

～そうだ

• Based on something the speaker can directly observe.
• Often based on appearance or immediate evidence.

～だろう

• Based on reasoning, knowledge, experience, or general judgment.

Example:

• 空そらが暗くらい。雨あめが降ふりそうだ。（≒雨が降るだろう）
  The sky is dark. It looks like it’s going to rain.

7. Practice

Translate the following sentences into Japanese using ～だろう or ～だろうと思います.

FAQ

Conclusion

In this article, you learned how to use ～だろう and ～だろうと思う to express guesses, predictions, and probabilities.

• Use ～だろう for writing or objective predictions.
• Use ～だろうと思う in conversations to share your personal guess or opinion.

Mastering this grammar will help you express your thoughts and predictions naturally. Try using it the next time you predict the weather or talk about the future!