～ことにする / ～ことになる: Decide vs. It Was Decided

The difference between ことにする and ことになる comes down to who decided. ことにする marks a decision the speaker made of their own will, while ことになる frames the same outcome as something that simply came about.¹² Both share the same front half, a plain-form verb plus こと plus に. Only the final verb changes, which makes this pair one of the clearest places to see how Japanese encodes agency.

Overview

ことにする and ことになる both wrap a verb clause in こと, a nominalizer that turns "do X" into "the matter of doing X." They then attach に and a final verb.³ The split between them is volitional versus non-volitional: する says the speaker chose, while なる says circumstance settled it.¹²

Both patterns are standard register, used in writing and speech, and are not dialectal. The polite endings ことにします and ことになりました are the default in announcements and self-introductions.¹²

The shared skeleton: [verb] + ことに + する / なる

こと is a 形式名詞 (formal noun, a noun with no concrete meaning of its own). It attaches to a plain-form clause and lets that clause behave like a noun, referring to the act, fact, or matter of doing something.³ Pair it with にする or になる, and it builds the decision and outcome patterns.³

The front half is identical for both: the plain form of the verb (dictionary form or ない form), then こと, then に. Only the final verb differs: する for a volitional decision, or なる for an outcome that comes about.¹²

The に here is the same に that marks a goal or result with する ("make it into X") and with なる ("become X"). So ことにする is literally "make it the case that X," and ことになる is "it becomes the case that X."⁴²

Where this sits in JLPT

The base forms sit at the N4 boundary. The pattern is taught in Genki II, Lesson 23, the standard second-year elementary course, which aligns with N4 sequencing.⁵ References disagree by one level: JLPT Sensei files both ことにする and ことになる at N4, while Bunpro files ことにする at N3.⁶⁷⁸ The social agency-deflection use, and the standing-arrangement form ことになっている, lean N3. Bunpro files ことになっている as high as N2.⁹

Why no single "official" level exists

The JLPT does not publish an official grammar list; the test is specified only by can-do competency summaries.¹⁰¹¹ Any single level for a grammar point is a consensus drawn from learner references, not an official label. Treat ことにする / ことになる as N4 core form with N3-leaning nuance.

Form and conjugation

ことにする: dictionary form and ない form attach

ことにする follows the short (plain) present form of a verb. A negated verb in ない form is also allowed.⁵ The affirmative 行く becomes 行くことにする ("decide to go"), and the negative 行かない becomes 行かないことにする ("decide not to go").⁵⁷

タバコをやめることにした。⁷
"I've decided to quit smoking."

これ以上いじょう言いわないことにします。⁷
"I've decided not to say anything more."

The pattern has a full set of polite and tense variants: ことにする (plain), ことにします (polite), ことにした and ことにしました (past, "have decided"), and ことにしている and ことにしています (standing personal practice).⁷

ことになる: same attach, different verb

ことになる takes the same front half: the plain dictionary form or ない form, plus こと plus に. Only the final verb is なる.¹² The affirmative 転勤する becomes 転勤することになる ("it comes about that I transfer"), and the negative 行かない becomes 行かないことになる ("it comes about that I don't go").⁸²

来月らいげつ、転勤てんきんすることになった。²
"It's been decided that I'll be transferred next month."

来月らいげつ、日本にほんへ行いくことになりました。⁸
"It has been decided that I will go to Japan next month."

The tense forms match ことにする: ことになる (plain), ことになります (polite), ことになった and ことになりました (past, "it has been decided"), and ことになっている and ことになっています (standing rule or arrangement).⁹¹²

The tense layer: した vs している vs なっている

The tense ending changes the meaning more than learners often expect. Most single-page guides stop short here. ことにした is a one-time decision made at a point in time ("I have decided to ...").⁵⁷ ことにしている means "do something as a regular practice": you made up your mind and have stuck to that determination, so it reads as a standing personal policy or habit.⁵² ことになっている is a standing external arrangement or rule currently in force ("it is the rule that ..."). The speaker reports that rule rather than creates it.⁹¹²

The contrast in one line: した is a fresh choice, している is a kept personal rule, and なっている is an externally fixed rule. The first two use する, where the chooser is the speaker. The third uses なる, where the rule stands independent of the speaker.⁵⁹²

Form	Meaning	Who is the agent
ことにした	a fresh one-time decision	the speaker
ことにしている	a kept personal policy or habit	the speaker
ことになっている	a standing rule or arrangement in force	someone or something else

毎週まいしゅう、金曜日きんようびはジムに行いくことにしている。²
"I make it a rule to go to the gym every Friday."

荷物にもつは午前中ごぜんちゅうに届とどくことになっている。²
"The package is supposed to arrive in the morning."

Nuance and usage contexts

ことにする: the decision is mine

ことにする expresses what someone has decided of their own will. The speaker is the agent.⁵¹ It describes something the speaker decided for themselves, and it pairs naturally with the first person because the decider is the speaker.¹⁸

少すこし太ふとったから、毎朝まいあさジョギングをすることにした。²
"Since I've gained a little weight, I've decided to jog every morning."

来月らいげつ、仕事しごとを辞やめることにした。¹
"I've decided to quit my job next month."

ことになる: it came about

ことになる expresses something decided or determined by factors outside the speaker's own resolve: a rule, an organization, a group, or circumstance.¹² The speaker is not the agent. The focus is on the outcome that came about.¹⁸

The mechanism is the verb itself. する marks someone performing an action (transitive), while なる marks the result of an action (intransitive). That is why ことになる typically presents the matter as decided by someone or something other than the speaker.²¹³

会議かいぎは来週らいしゅうの月曜日げつようびに行おこなわれることになった。⁸
"It's been decided that the meeting will be held next Monday."

来月らいげつ、結婚けっこんすることになりました。²
"It's been decided that I'm getting married next month."

Using ことになる to deflect agency (the polite move)

Even when the speaker did make the decision, ことになりました frames it as something that came about. This softens the announcement and sounds more humble and less self-assertive.² In meaning, the construction sits close to the passive: it presents the speaker as having no visible control over the event. That lets a speaker deliver a significant personal announcement modestly.⁸²

This is the article's key insight. It is well attested across learner references and pedagogy, rather than being a formal academic finding. Classic face-saving uses include 結婚することになりました ("we're getting married") and 仕事をやめることになりました ("I'll be leaving my job"). In these cases, the ことにしました version would sound blunter and more self-centered because it foregrounds the speaker's own will.²

このたび、結婚けっこんすることになりました。²
"I am pleased to share that I am getting married."

私わたしが会かいの代表だいひょうを務つとめることになりました。⁸
"It has fallen to me to serve as the group's representative."

Pick ことになりました for personal announcements

For self-referential news such as marriage or a job change, ことになりました is socially preferred because it deflects agency and reads as humble. The ことにしました version is grammatically fine but foregrounds your own will and can sound blunt or immodest.²

ことになっている: the standing rule

ことになっている points to a standing arrangement, rule, custom, schedule, or plan currently in force ("it has been established that" or "it is the rule that"). It does not imply that the speaker decided it.⁹¹² It often describes something expected or scheduled that has not yet happened. The arrangement is held in a state of having become so.⁹¹²

This is distinct from a one-time ことになった. なった reports the moment a single outcome was settled, while なっている reports the ongoing in-force state of a rule.⁹¹²

9時くじに集合しゅうごうすることになっている。¹²
"We're supposed to meet at nine o'clock."

この学校がっこうでは、制服せいふくを着きることになっている。⁹
"At this school, you're required to wear a uniform."

Good to know

Why なる feels humbler than する

する is the transitive, controlling member of a する/なる pair. なる is the intransitive, agentless member: する implies something is being controlled or acted upon, while なる implies something happens on its own.¹³ Transitive (他動詞) verbs foreground who did the action. Intransitive (自動詞) verbs foreground the change itself, leaving the doer unimportant.¹³

That is the mechanism behind the politeness use. By choosing intransitive なる, the speaker drops the visible agent. As a result, ことになる reads as agentless and therefore more modest than the agent-foregrounding ことにする.²¹³

The trap of translating ことになる as a passive

Learners often reach for the passive (される) to render "it was decided." This produces an unnatural or wrong form. ことになる already carries the "it was decided" or "it came about" sense and is grammatically active, so the idiomatic choice is a plain verb plus ことになる.

来月らいげつ、転勤てんきんすることになった。²
"It's been decided that I'll be transferred next month."

The wrong move is to insert the passive 転勤される before ことになった; the plain 転勤する is correct.⁸²

Do not confuse it with the よう set

ことにする and ことになる act on a one-time decision or a settled outcome. By contrast, ようにする and ようになる act on an ongoing effort or a gradual change of state or ability. To say "gradually become able to speak Japanese," use 日本語が話せるようになる, not 日本語が話せることにする. Within the standing-practice forms, the same split holds: ことにしている stresses the rule itself, while ようにしている stresses one's effort to follow it.²

Negative ことにする vs the separate ことはない pattern

A negated decision puts ない before こと: 行かないことにする means "I've decided not to go."⁷ Do not read this as "no need to." That sense belongs to the separate N3 pattern こと(は)ない, meaning "there is no need to." For example, 心配することはない means "there's no need to worry," and the は is often dropped in speech.⁷¹⁴

する = "I'll make it so," なる = "it becomes so"

Anchor the contrast in the two verbs' core meanings: する means "do or make," and なる means "become." That maps directly onto the volitional versus non-volitional split. With that anchor, a learner can derive the nuance instead of memorizing it case by case.¹¹³

See also

• ～ことができる: How to Say "Can Do" in Japanese
• ～ことがある: How to Say "I Have Done X Before" in Japanese
• Suru-Verbs (する-Verbs): How する Turns Nouns Into Verbs
• The Polite Volitional ～ましょう: How to Say "Let's" in Japanese
• The Plain Volitional Form ～よう / ～おう: How to Say "Let's" and "I Will" in Japanese

References

Footnotes

1. Oka, Mayumi; Tsutsui, Michio; Kondo, Junko; et al. Tobira: Gateway to Advanced Japanese (Learning Through Content and Multimedia). Kurosio Publishers / Tokyo: 9640.jp. Grammar index entries for ことにする and ことになる. https://www.9640.jp/nihongo/en/detail/ ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹

2. Maggie Sensei. "ことになる/する ( = koto ni naru/suru ) & ようになる/する ( = you ni naru/suru )." https://maggiesensei.com/2017/08/22/%E3%81%93%E3%81%A8%E3%81%AB%E3%81%AA%E3%82%8B%E3%81%99%E3%82%8B-koto-ni-naru-%E3%82%88%E3%81%86%E3%81%AB%E3%81%AA%E3%82%8B%E3%81%99%E3%82%8B-you-ni-narusuru/ (limitation) ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹ ↩¹² ↩¹³ ↩¹⁴ ↩¹⁵ ↩¹⁶ ↩¹⁷ ↩¹⁸ ↩¹⁹ ↩²⁰ ↩²¹ ↩²² ↩²³ ↩²⁴ ↩²⁵

3. Tofugu. "こと (事) for Abstract Things." https://www.tofugu.com/japanese-grammar/koto/ (limitation) ↩ ↩² ↩³

4. Makino, Seiichi; Tsutsui, Michio. A Dictionary of Basic Japanese Grammar. The Japan Times. Entries organized by romanized headword; "koto ni suru" and "koto ni naru" appear under the K headwords. ↩

5. Banno, Eri; Ikeda, Yoko; Ohno, Yutaka; Shinagawa, Chikako; Tokashiki, Kyoko. Genki II: An Integrated Course in Elementary Japanese (2nd ed.). The Japan Times. Grammar point ～ことにする, Lesson 23. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷

6. Bunpro. "ことにする (JLPT N3)." https://bunpro.jp/grammar_points/ことにする (limitation) ↩

7. JLPT Sensei. "N4 Grammar: ことにする (koto ni suru) Meaning." https://jlptsensei.com/learn-japanese-grammar/ことにする-koto-ni-suru-meaning/ (limitation) ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸

8. JLPT Sensei. "N4 Grammar: ことになる (koto ni naru) Meaning." https://jlptsensei.com/learn-japanese-grammar/ことになる-koto-ni-naru-meaning/ (limitation) ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹

9. Bunpro. "ことになっている (JLPT N2)." https://bunpro.jp/grammar_points/ことになっている (limitation) ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸

10. The Japan Foundation and Japan Educational Exchanges and Services. "N1-N5: Summary of Linguistic Competence Required for Each Level." Japanese-Language Proficiency Test official site. https://www.jlpt.jp/e/about/levelsummary.html ↩

11. Japanese-Language Proficiency Test official site. "FAQ / About the test" (the JLPT does not publish official kanji, vocabulary, or grammar lists). https://www.jlpt.jp/e/ ↩

12. JLPT Sensei. "N3 Grammar: ことになっている (koto ni natteiru) Meaning." https://jlptsensei.com/learn-japanese-grammar/ことになっている-koto-ni-natteiru-meaning/ (limitation) ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

13. Tofugu. "Japanese Transitive and Intransitive Verbs." https://www.tofugu.com/japanese-grammar/transitivity/ (limitation) ↩ ↩² ↩³ ↩⁴ ↩⁵

14. JLPT Sensei. "N3 Grammar: ことはない (koto wa nai)." https://jlptsensei.com/learn-japanese-grammar/ことはない-koto-wa-nai/ (limitation) ↩