How 7Z Works
7z is the archive format of the free, open-source 7-Zip program, and it is built to deliver very high compression. Its core design idea is to separate archive structure from compression method, so each block of data in an archive can use a different method, or a chain of methods, chosen to suit that data. This flexibility lets 7z apply the best available compression to each part of an archive rather than forcing one algorithm on everything.
By default, 7z uses the LZMA and LZMA2 filters, which employ a large dictionary and sophisticated modeling to find and eliminate redundancy far more aggressively than ZIP's older DEFLATE. Like RAR, 7z also supports solid archiving, compressing grouped files as one continuous stream so it can exploit similarities across files. The combination of LZMA and solid mode is why 7z routinely produces the smallest archives of the mainstream formats, often beating both ZIP and RAR.
Crucially, 7z is free and open-source. Unlike RAR, which needs the commercial WinRAR to create archives, anyone can make and open 7z files with the free 7-Zip tool (and many other archivers support it). It also offers strong AES-256 encryption. So 7z effectively gives you RAR-class (or better) compression without any licensing cost, which is its central appeal.
7z vs ZIP vs RAR
The three sit on a clear spectrum. ZIP wins on universal compatibility, it opens everywhere with no software, but compresses the least. RAR compresses tightly and has nice reliability features, but creating it needs paid software and it is not natively supported. 7z usually compresses the most and is completely free and open, but, like RAR, it is not built into operating systems, so the recipient needs 7-Zip or a compatible tool to open it.
The practical decision is about who opens the file. If you want the smallest possible archive and both ends can install 7-Zip, choose 7z. If you need a file that anyone can open with zero setup, choose ZIP. 7z is the power user's choice for space; ZIP is the safe choice for sharing.
When to Use 7Z
Use 7z when maximum compression with no licensing cost is the goal: archiving large folders to save disk space, packing big downloads where smaller size matters, or backing up data where every megabyte counts and you control the software. Its free, open nature makes it ideal for these. When you instead need a recipient to open the archive instantly on any device, convert or re-pack to ZIP, trading some size for universal compatibility.
Limitations
7z's main limitation is compatibility: it is not built into Windows or macOS, so opening a 7z file requires installing 7-Zip or a compatible archiver, which makes it less convenient than ZIP for sharing with non-technical recipients. Its strong compression also means higher CPU and time to compress at maximum settings, and solid mode makes single-file extraction slower. These are modest costs for its excellent free compression, but they are why 7z is a power-user format rather than the universal default.
