This CBZ to CBR conversion does not run here
Some old comic apps asked for CBR, which is a RAR of the page images.
A browser cannot create a RAR archive, so this conversion does not produce a CBR.
What to do instead
CBZ opens in current comic readers. Keep the CBZ, or use CBR to CBZ if you are going the other direction.
The page that does work is CBR to CBZ.
CBZ to CBR without the extra promises
CBZ to CBR sits on this site because CBZ and CBR are not interchangeable, even when they look similar in a file list. Some old comic apps asked for CBR, which is a RAR of the page images. CBZ is the practical comic copy because ZIP is widely readable. People convert CBR to CBZ so a reader that has no RAR support can open the comic.
A browser cannot create a RAR archive, so this conversion does not produce a CBR.
Read this about CBZ before you upload
CBZ is a comic stored as pictures inside a ZIP. Most comic readers open it. There is no upscaling of the art. You get the pictures that were in the archive.
Keep the original archive until you have flipped through the pages.
Read this about CBR before you send the download
The download has to be CBR, because that is the format the next step named. People convert CBR to CBZ so a reader that has no RAR support can open the comic. Unpacking to CBZ is the direction that works here.
Password-protected comics fail. Page images should be checked after unpacking.
Keep, check, then delete
People lose time when they convert before they know which rule failed. Size, pixel dimensions, codec, font encoding, and layout are different rules. CBZ to CBR only addresses the CBZ to CBR step. Some old comic apps asked for CBR, which is a RAR of the page images. A browser cannot create a RAR archive, so this conversion does not produce a CBR. CBZ opens in current comic readers. Keep the CBZ, or use CBR to CBZ if you are going the other direction. The file stays in your browser and is not uploaded. The limit is 100 MB. Keep the original until the download has been opened in the program that will use it. Do the test with a short or simple file first when the real file is large. A one-page PDF, a single photo, or a one-minute clip will show you whether the output is the kind of file you needed. After that test looks right, run the real file once. Keep both the CBZ original and the CBR download until the receiving person or the upload form accepts it. Convert only a book you own and are allowed to convert. CBZ to CBR moves chapter text. A fixed illustrated layout is simplified, and DRM-locked files fail. Kindle formats can often be read here and not written. Keep the original ebook until the new file opens in the reader you actually use.
Here is the decision for CBZ to CBR. You start with CBZ and you stop when a real program accepts CBR. Page images can be packed and unpacked in the browser. Creating a new CBR needs WinRAR. This site will not pretend to write one. If those two sentences describe a tradeoff you do not want, do not convert. Renaming the extension by hand will not do this job. Running the tool twice on its own output will not do it either.
CBZ opens in current comic readers. Keep the CBZ, or use CBR to CBZ if you are going the other direction.
A related step, if you still need one, is EPUB to MOBI.
Other jobs start from the homepage, where each tool is named for the conversion it actually does.