わけだ

1. 1Expresses understanding of why something is so after learning the reason ('no wonder', 'that's why'), often with どうりで or なるほど.Ex. 1
2. 2States a conclusion that follows logically from given facts or figures ('that means', 'so it comes to').Ex. 2

A neutral pattern, common in conversation and writing, used when the speaker connects facts to a conclusion or an explanation. In the 'no wonder' sense it typically follows どうりで or なるほど after the speaker learns a reason. In the conclusion sense it often follows a conditional such as なら and restates what the facts imply. Unlike はずだ, which expresses expectation, わけだ presents the conclusion as following naturally from what is already known.

• というわけだN1「というわけだ」 adds a nuance of quoting or summarizing; it often follows 「だから」 or 「つまり」 to organize the preceding text into a conclusion, or to explain the background. When attaching to nouns or な-adjectives, it can connect directly or add 「だ」. When drawing a conclusion, it is often interchangeable with 「わけだ」.
• ということだN3The 'in other words' sense of 「ということだ」 is close to 「わけだ」 and they are often interchangeable, but it focuses on summarizing or paraphrasing. It also has a hearsay sense ('apparently/I heard that') and does not express 'no wonder'. After nouns or な-adjectives, attach 「だ」 or connect directly.
• はずだN4「はずだ」 focuses on inference based on common sense, meaning 'should be', and can also express 'no wonder' after confirmation, in which case the two are often interchangeable. However, the 'was supposed to' sense seen in 「はずだった」 cannot be replaced with 「わけだ」.
• ことになるN4「ことになる」 mainly expresses a decision or arrangement made by others, an organization, or objective circumstances—a usage 「わけだ」 does not have. The two are close only when calculating that 'the result equals...', where they are often replaceable. 「ことになる」 does not express 'no wonder'; it attaches to verbs, not directly to nouns.
• わけではないN3「わけだ」 affirms the conclusion drawn, while 「わけではない」 politely negates an inference, meaning 'it's not that...'; it is often followed by 「が」 or 「ただ」 to explain the actual situation, and also expresses partial negation—they are not interchangeable. After nouns, do not attach 「の」.

わけだ draws a conclusion or explanation from known facts. By contrast, はずだ expresses what the speaker expects to be true, ことになる describes an outcome or arrangement that results, and ということだ restates a conclusion or reports hearsay. というわけだ wraps up a preceding explanation as a summary, and わけではない denies an assumed conclusion ('it doesn't mean that').