A CBR is a RAR archive with the extension changed. A CBZ is a ZIP archive with the extension changed. Neither format knows anything about comics, and neither one compresses your pages—the JPEGs or PNGs inside are already compressed, and archive formats leave them essentially as they found them.

That matters more than it sounds, because it means converting CBR to CBZ does not have to touch your artwork at all. The job is to open one container and write the same files into another one. Every page can come across byte for byte.

Why CBZ instead of CBR

CBR is the older convention and it is on the way out, for a practical reason: RAR is a proprietary format, and support for it is patchy. ZIP is everywhere—macOS opens it natively, every reader handles it, and archival tooling treats it as a first-class format.

If you keep a library for the long term, CBZ is the safer container.

What can go wrong

Re-encoding. A converter that decodes each page and writes a new JPEG gives you a smaller file and permanently softer artwork. You usually cannot tell from a thumbnail. You can tell on lettering, and on flat color where JPEG blocking shows up first.

A rebuilt ComicInfo.xml. Metadata you fixed by hand can come back reordered, re-encoded, or with unknown fields dropped, depending on whether the tool parsed and rewrote the file or copied it.

Password-protected archives. An encrypted RAR cannot be read without the password. A tool that offers to handle it anyway is doing something you should be suspicious of. CBZ Forge refuses these rather than attempting them.

How to check what you got

Rename a copy of the output from .cbz to .zip, open it, and compare a page against the same page from the original. If the file sizes match exactly, the image was copied. If the new one is smaller, it was re-encoded.

That is the check worth doing once on any converter before you run it across a shelf of books.