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 expresses that something has been decided or arranged by circumstances or another person, or that a situation turns out in a certain way. It follows the plain form of a verb, as in 食べることになる or 行かなかったことになる, and ことになっている often describes an arrangement that remains in effect. The grammatical こと is normally written in hiragana, and the pattern sounds neutral and indirect because it avoids stating who made the decision.
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私たちは６時に食事することになっている。
We are to eat at six.
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私は今日そこへ行くことになっています。
I am to go there today.
ことにする kotonisuru
to make the decision to do something
ことにする expresses a decision made by one's own will to do something or not do something. It follows the dictionary form of a verb for an affirmative decision, as in 勉強することにした, and the ない form for a negative decision, as in 行かないことにした. The tense matters: ことにした reports a past decision, ことにしている describes a continuing habit or personal rule, and ことにしよう proposes making a decision together.
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私は毎朝六時に起きることにしている。
I make it a rule to get up at six in the morning.
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フランス語を勉強することにした。
I've decided to study French.
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.