At a glance
ということは means "so that means..." or "in other words...". It starts sentence B and shows that B is a conclusion the speaker draws from A. A is often something another person just said.
A。ということは、B。
A。ってことは、B。（casual）
明日は祝日です。ということは、銀行は休みですね。 Tomorrow is a national holiday. That means the banks are closed.
How to form it
ということは is a connector. It does not change form, but it has a casual version.
| Form | Where it goes | Register |
|---|---|---|
| ということは | Start of sentence B, after sentence A | Neutral: speech and writing |
| ってことは | Start of sentence B, after sentence A | Casual speech |
Sentence B is the speaker's own inference, so it often ends with ね, んですね, でしょう, かもしれない, or a question such as ～ということですか.