～ことにする
decide to
～ことにする expresses a decision that the speaker makes by their own will. It is formed with the dictionary form (or ない-form) of a verb plus ことにする, literally “make it the thing that...”.
In English it maps to “I’ve decided to...” or “I’m going to make a point of...”.
1. Announcing a decision
Use ～ことにする (or past ～ことにした) when you choose to do something and want to present it as your own decision.
毎朝走ることにする。
“I’ve decided to run every morning.”
- 毎朝（まいあさ）= every morning
- 走る（はしる）= to run
- ことにする = decide to (announces the speaker’s choice)
2. A decision kept as a habit
With ～ことにしている, the decision is maintained over time: “I make it a rule to...”. The ない-form before ことにする expresses deciding not to do something.
夜はコーヒーを飲まないことにしている。
“I make it a rule not to drink coffee at night.”
- 夜（よる）= night
- は = topic marker
- コーヒー = coffee
- を = object marker
- 飲まない（のまない）= not drink (negative of 飲む, to drink)
- ことにしている = make it a rule (a decision kept up as a habit)
Contrast this with ～ことになる: ことにする means the speaker actively makes the choice, while ことになる means the decision was reached by others or by circumstances (“it has been decided that...”).
How to form it
Verb dictionary form / ない-form + ことにする
- 走る（はしる）= to run → 走ることにする（はしることにする）= decide to run
- 行く（いく）= to go → 行かないことにする（いかないことにする）= decide not to go
- 食べる（たべる）= to eat → 食べることにした（たべることにした）= decided to eat
Key idea
The する in ～ことにする signals an active choice: the speaker makes something the plan. If you chose it yourself, use ことにする; if it was decided for you, use ことになる.
Built on the same stem
Updated .