ことにする

Phrase (Basic ㊦ 204)

A volitional decision to do something is made.

Equivalent: Decide to

私は会社を辞めることにした・しました。
I decided to quit my company.
私は毎日三十分ぐらい運動をすることにしている・います。
I made it a rule to exercise for about 30 minutes every day.
今年の夏は北海道を旅行することにしました。
I've decided to make a trip to Hokkaido this summer.
京都までバスで行くことにしましょう。
Let's (lit. decide to) go as far as Kyoto by bus.
毎日漢字を十覚えることにしました。
I've decided to memorize ten kanji every day.
私は肉をあまり食べないことにしている。
I make it a rule not to eat very much meat.

Formation

(i) Vinformal nonpast ことに{しる/した}  
  話すことに{しる/した} Someone decides/has decided to talk
  食べることに{しる/した} Someone decides/has decided to eat

Notes

1. If one decides not to do something, the verb before ことにする should be negated as in:

2. ことにする is a more complex version of Nounにする 'decide on Noun', 'make it Noun'. (⇨ にする) The complexity is due to a noun phrase nominalized by こと. (⇨ こと2) An example of Nounにする is given below:

【Related Expressions】

I. ことにする indicates someone's volitional decision, whereas ことになる indicates a non-volitional decision. Therefore, if you perceive a given decision to be your own decision, you should use ことにする; on the other hand, if you don't perceive a given decision to be your own, you should use ことになる instead. That is why ことにする and ことになる are very awkward in [1a] and [1b], respectively.

[1]

II. ことにする and ことに決める 'determine to do something' are virtually identical in meaning. The difference is that the former is an idiom and, therefore, frequently used in colloquial speech, while the latter is appropriate when the speaker is talking about a relatively important decision in a rather decisive manner. Also, ことにする can be used to mean 'I hereby decide to ~' but ことに決める cannot- Thus, [1] below cannot be rephrased by ことに決める.

[1]