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

The decision is yours to make

〜ことにする is how you announce a decision you reach by your own will. 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," which lands as "I've decided to do X." What matters is whose choice it is: with ことにする, the speaker chose it. And because that choice usually happens in a single moment, you'll meet it most often in the past, as ことにした. To decide against something, make the verb negative with the ない-form.

Snapping it onto the verb

The one rule for attaching ことにする is that it clips onto the plain form — dictionary or ない. Not the て-form, not the ます-stem: it's 行くことにする (✓), never 行きことにする or 行ってことにする. And when you shift to the past, the tense rides on する → した, not on the inner verb, so 行くことにした — say 行ったことにする and you've said something else entirely.

Settling it now vs. already settled

The tense of する tracks when you decided. Reach for ことにする / ことにします when you're making the call in the moment you speak, and for ことにした when the matter is already closed.

When the decision becomes a habit: ことにしている

Swap する for the ている form and a one-time decision turns into a standing rule you keep — "I make a point of…," "I make it a habit to…" It describes something you resolved once in the past and still hold yourself to today.

The look-alikes: なる, and plain にする

Two neighbours get mixed up with ことにする, and both hinge on that "whose will" question.

The first is ことになる. Where 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:

• 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're happy about it, ことになる frames the outcome as arranged for you. So 留学することにした is "I chose to study abroad," while ことになった hands the choice to circumstances. The shorthand to keep: する = I chose it, なる = it turned out that way.

The second is plain 〜にする, which you use to pick among options, usually a noun: コーヒーにする ("I'll have coffee"), お茶にする. When what you're settling is an action, though, a verb can't take にする raw — you nominalize it with こと first. That's the whole reason ことにする exists: it lets you "choose" an action the way にする lets you choose a noun. So don't reach for ことにする with a bare noun.

The short version

• 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.

Practice it