ことになる

ことになる says that something comes about without the speaker choosing it: a decision made by others, or where events lead. It follows a verb in the dictionary form or the ない-form. In the past, ことになった, it announces a decision.

Even a decision the speaker took part in is often announced this way. It sounds modest and keeps the speaker's own will in the background.

An announcement of something already settled takes the past: ×来月転勤することになる → 来月転勤することになった.

ことになる or ことにする

ことにする is a decision the speaker makes. ことになる is one that was made for them, or is presented that way.

Where things lead

In the non-past, ことになる says what will follow from a situation: "you'll end up…". After a た-form, it says what an act amounts to.

Usage notes

These go beyond a decision or a result.

ことになっている

With 〜ている, the decision is in force: a rule, a custom or a schedule. It is a grammar point of its own.

ということになる

After a noun, or to sum up a line of reasoning, ということになる means "that means", "that makes it".