ことになる

1. 1It has been decided or arranged, by others or by circumstances, that something will happen.Ex. 1
2. 2Something turns out to be, or amounts to, a certain result or conclusion.

Neutral in register and common in announcements, workplace reports, and news of plans or cancellations, often in the past form ことになりました. Speakers sometimes use it even for their own decisions to sound modest or less assertive. Unlike ことにする, which presents a decision as someone's deliberate choice, ことになる presents the outcome as arranged by others or by circumstances.

• ことにするN4「ことにする」 highlights the subject's own active decision, while 「ことになる」 refers to a decision formed by others or by circumstances, with an objective, softened tone; both can describe one's own decision, but the latter sounds more humble. 「ことにする」 also has the sense of "treating something as the case".
• ことになっているN3「ことになっている」 refers to rules, customs, or arrangements that are already decided and still in effect, emphasizing the current state, while 「ことになる」 emphasizes the change of "being decided". Use 「ことになりました」 to report a new decision, and only 「ことになる」 can express the calculation sense.
• ようになるN4「ようになる」 describes a gradual change in ability, habit, or state and often attaches to the potential form, whereas 「ことになる」 covers formed decisions, arrangements, or calculated results; the two cannot be interchanged — to say "I've learned to ride a bike," use 「乗れるようになった」.
• わけだN3「わけだ」 does not express decisions or arrangements; it is close to 「ことになる」 only in the calculation sense of "it turns out that…", where the two are often interchangeable. It also pairs with 「どうりで」 to mean "no wonder", and can follow nouns or adjectives, with 「な／の／である」 required after a noun.
• はずだN4「はずだ」 is the speaker's evidence-based inference ("it should be…"), not a settled arrangement, while 「ことになる」 expresses a decision already made or a calculated result. 「はずだ」 can also attach to adjectives and nouns, and the two are generally not interchangeable.

ことになる presents a decision as arranged by others or by circumstances, whereas ことにする presents it as someone's deliberate choice (the speaker's or another person's, as in 彼は行かないことにした), and ことになっている describes an established rule or standing arrangement. ようになる expresses a gradual change in ability or habit, while わけだ gives a logical explanation or conclusion and はずだ gives an expectation based on reasoning; neither of the last two reports a decision.