Create a ZIP archive

That depends entirely on what you put in. Text, spreadsheets, code and documents compress well — often to a third of their size or less, because they are full of repetition. Photographs, video, music and PDFs full of images barely move at all, because they are already compressed and there is no repetition left for a second pass to find.

So a folder of JPEGs will produce a ZIP roughly as large as the JPEGs were, and that is not a failure. The archive is still doing the job you wanted: turning forty files into one that can be attached to an email or uploaded to a form that only accepts a single file.

7Z and RAR both squeeze more out of the same data. They also both need software the person at the other end may not have, and an archive that cannot be opened is worth considerably less than one that is slightly larger. ZIP has been built into every desktop operating system for twenty years.

If size genuinely matters more than convenience — a large upload over a slow connection, say — converting the finished ZIP to 7Z afterwards will save more. That conversion runs on our server rather than in your browser, because no browser ships an LZMA encoder, and the page says so before you start.

JPEG, PNG, MP3, MP4, PDF and every Office document are already compressed internally. Zipping them produces an archive roughly the size of the sum of its parts, because there is no redundancy left for the compressor to find.

That is not a failure and it is worth expecting rather than being surprised by. The reason to zip such files is bundling — one attachment instead of forty, with the folder structure preserved — and bundling is a perfectly good reason on its own. Where compression genuinely pays is text, source code, CSV, logs and uncompressed images.

A ZIP stores the path of every file inside it, so a nested folder tree comes out the other end arranged exactly as it went in. Selecting a folder rather than a list of files is what preserves that.

It is why zipping is the reliable way to send anything where the arrangement matters — a website, a project, a set of images organised by date. Dragging forty loose files into an email delivers forty loose files; a ZIP delivers the structure.

Unix file permissions, ownership and symbolic links have no dependable home in a ZIP, so a folder of scripts archived here and extracted on a server frequently arrives without the executable bit set. macOS resource forks travel as a `__MACOSX` folder that appears as clutter on Windows.

For documents, images and anything a person opens by hand, none of this matters. For moving a project to a Unix machine it does, and `.tar.gz` is the format that preserves it — which is a real limitation rather than a preference.

The archive is built inside your browser tab, so the files never leave the machine. Given that people zip tax documents, scanned identity papers, client work and photograph collections, that is a meaningful difference from the usual arrangement of handing everything to a website in exchange for a download link.

It also means there is no size ceiling beyond your own memory and no queue. A folder of several hundred files costs processor time locally rather than an upload, which is usually the faster of the two by a wide margin.