How to tell them apart
The core contrast is volition: ことにする marks a decision the speaker or group makes themselves, while ことになる marks a decision made by circumstance, an outside party, or an arrangement already in place. Swapping them sounds odd — saying 来月引っ越すことにした means you chose to move, whereas 来月引っ越すことになった means it was decided for you or things just turned out that way.
- ことになる Use ことになる when a plan or outcome is decided by someone else or by circumstance, not by you.
- ことにする Use ことにする when you or your group actively make a decision to do something.
| Grammar point | Meaning | JLPT level |
|---|---|---|
| ことになる kotoninaru | It has been decided that; it turns out that | N4 |
| ことにする kotonisuru | to make the decision to do something | N4 |
ことになる kotoninaru
It has been decided that; it turns out that
ことになる follows a plain-form verb and expresses that something has been arranged or decided, often by circumstances or others rather than the speaker's own will. It can also state what a situation will amount to, as in counting a visit as a second one. Often paired with でしょう for a settled future outcome.
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来月オーストラリアへ出発することになるでしょう。
I will be leaving for Australia next month.
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私は今日の午後メアリーと会うことになるでしょう。
I'll be seeing Mary this afternoon.
ことにする kotonisuru
to make the decision to do something
ことにする expresses that the speaker has decided on a course of action, often announcing their own choice. It attaches to the plain form of a verb, as in 待つことにする. It carries a casual, personal tone, frequently ending with よ in conversation.
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トムをここで待つことにするよ。
I'll wait for Tom here.
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もし今日店が閉まっていたら、明日また行くことにするよ。
If the shop is closed today, I'll try again tomorrow.
ことになる and ことにする are both JLPT N4 grammar points that Japanese learners often mix up. This page compares them side by side on meanings, usage and example sentences, so you can tell them apart.