〜ことにする: How to Say You've Decided To Do Something

What it means

〜ことにする is how you announce a decision you make yourself. You take an action (in plain form), turn it into a "thing" with こと, and then "do" that thing with する — literally "I make it the case that I do X." The result is "I've decided to do X."

The important nuance is whose will: ことにする is a choice the speaker makes on their own. Because the decision usually happens at one moment, you'll very often see it in the past: ことにした ("I (have) decided").

How to form it

Attach ことにする to the plain (dictionary or ない) form of a verb. Note that the past tense lands on する → した, not on the main verb.

ことにした vs ことにする — when you decide

Use ことにする / ことにします when you're making the decision right now as you speak, and ことにした when the decision is already settled.

〜ことにしている — a habit or personal rule

Swap する for the ている form and the meaning shifts from a one-time decision to an ongoing rule you keep: "I make a point of…" or "I make it a habit to…" This describes a decision you made in the past and still follow.

ことにする vs ことになる — your will or the circumstances

This is the contrast to lock in. 〜ことにする means you decided. 〜ことになる means the decision came about through circumstances or someone else — your company, a group, the situation — not your own will. Compare:

• 転勤することにした = "I decided to transfer" (my own choice).
• 転勤することになった = "It's been decided that I'll transfer" (the company decided; it just happened to me).

Even when you personally are happy about it, ことになる frames the outcome as something arranged for you rather than something you willed. ことになる has its own guide; for now, just remember: する = I chose it, なる = it turned out that way.

にする for nouns

Don't confuse verb + ことにする with plain 〜にする, which you use to choose among options, usually with a noun: コーヒーにする ("I'll have coffee"). When what you're deciding is an action, you need a verb, and a verb can't take にする directly — you nominalize it with こと first. That's exactly why ことにする exists: it lets you "choose" a whole action the way にする lets you choose a noun.

Common mistakes

1. Using the て-form or ます-stem. It attaches to the plain form: 行くことにする (✓), not 行きことにする or 行ってことにする.
2. Putting past tense on the wrong word. Say 行くことにした, not 行ったことにする — the past goes on した.
3. Mixing up する and なる. 留学することにした = "I decided to study abroad" (my choice). If circumstances decided, that's ことになる.
4. Reaching for ことにする with a noun. To choose a noun, use plain にする (お茶にする). Use ことにする only with a verb.

Quick recap

• plain verb + ことにする = decide to do (your own will).
• ない-form + ことにする = decide not to do.
• ことにした = the decision is already made; ことにします = deciding now.
• ことにしている = a habit or personal rule you keep.
• ことになる (own guide) = decided by circumstances/others, not you.

Your turn

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