What is a CBZ file?

CBZ is a container: a wrapper that holds streams encoded by something else. It is used for handing a finished file over.

The extension is .cbz, and the full name is Comic Book Archive (ZIP). Both matter less than what the file can hold, which is what the rest of this page is about.

It is published in full, so anyone can implement it from the document rather than by inspection, which is why it turns up in so many programs and why files written twenty years ago still open. A published specification is not the same thing as a royalty-free one: where a format wraps a codec, the patent licensing is a separate question the standard does not settle.

CBZ stores a directory: names, folders and the structure between them. That is what lets it stand in for a folder in an email attachment.

CBZ is not limited to one page, which matters when converting to something that is: one conversion produces one file holding a single page — the first one, unless the page is something the converter lets you pick.

Calibre, YACReader and ComicRack read it, and so do most programs of the same kind.

If a file will not open, the format is rarely the problem — it is more often that the program predates it. Converting to something older is the reliable way past that, and it is what the rest of this site is for.

No browser reads it.

That is the single most common reason to convert it: not that the format is bad, but that the place you want to show the file cannot read it.

CBZ is meant to be handed over rather than worked in. Editing one is possible and rarely pleasant; the sane approach is to change the source and export again.

That is not a simplification. A CBZ is an ordinary ZIP archive containing one image per page, and renaming it to .zip lets any archive tool open it. There is no encoding, no proprietary structure and no digital rights management — which is exactly why comic readers standardised on it.

The consequence is that everything you know about ZIP applies. The images inside are stored, not re-compressed by the container, so a CBZ of JPEG pages is about the size of those JPEGs added together. Extracting gives you a folder of pictures. Nothing is lost in either direction, because nothing was transformed in the first place.

There is no manifest, no index, no metadata saying which image is page one. Readers sort the filenames and show them in that order, which works perfectly until somebody names the pages 1.jpg through 12.jpg — at which point page 10 follows page 1 and the reader is not wrong, the naming is.

Zero-padding is the entire fix: 001.jpg, 002.jpg, and so on. It is worth checking before anything else when a CBZ reads out of sequence, and it is the one thing to get right when assembling one from a folder of scans.

CBR is the same idea in a RAR archive, and it was common for years because RAR compressed slightly better on the hardware of the time. It has one problem that has only grown: RAR is proprietary. The compression algorithm is not open, free tools have to reverse-engineer it, and support outside Windows has always been patchy.

ZIP is an open format that every operating system reads without installing anything. Since neither container meaningfully compresses images that are already compressed, the size advantage was small and is now irrelevant, while the compatibility difference is not. Converting a CBR to a CBZ costs nothing and removes a dependency.

Dedicated readers handle it best because the format is designed around how comics are read: two-page spreads, right-to-left order for manga, fitting a page to the screen height rather than its width. Calibre, YACReader and ComicRack on the desktop, and any of a dozen readers on a tablet.

Where it fails is everywhere else. No browser displays a CBZ, no e-reader firmware that predates the format will touch it, no email client previews it, and a colleague sent one will see an unfamiliar extension and assume it is broken. Converting to PDF is the standard answer to all of those, and it is the main reason anybody converts a comic archive at all.

The pages become pages, in order, and the file opens on anything. What is added is a document structure the CBZ never had; what is lost is the reader’s ability to treat the pages as images — no automatic two-page spread, no right-to-left mode, no fitting to screen height in the way a comic reader does it.

Size is the other consideration. A CBZ of high-resolution scans is large, and a PDF wrapping the same images is larger still by the overhead of the document structure. Neither format makes the images smaller; if the archive needs to shrink, that has to happen to the images before they go into either container.

EPUB is the format for reflowable text, and reflow is precisely what a comic must not do. A fixed-layout EPUB can hold a comic properly and several publishers ship them that way, with real page structure, metadata and support on commercial e-readers.

CBZ is simpler and more honest about what it is: pictures, in order. It has no metadata standard worth the name, no table of contents and no author or series information that every reader agrees on — some read a ComicInfo file placed inside the archive, and it is a convention rather than a specification. For a personal library that is fine. For distribution, the lack of metadata is the format’s real weakness.

Name the files with zero padding, put them in a folder, ZIP the folder’s contents rather than the folder itself, and change the extension. That is genuinely the whole process, and it is why the format has survived thirty years without a specification.

Two details save trouble later. Keep the images in one format and one resolution across the archive, because readers scale pages independently and a mixed set produces pages that visibly jump in sharpness. And put the cover first as 000 or 001 — there is no cover field, so the first file is the cover by convention, and that convention is what a library view will display.