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Before comparing the three formats, there is a more useful fact to establish — because it decides whether this comparison matters to you at all.
JPG, PNG, PDF, MP4, MP3, DOCX and XLSX are already compressed. Compression works by finding repetition and encoding it shorter; those formats have already removed the repetition. Running a second compressor over the result finds almost nothing left to squeeze.
Put twenty photos in a ZIP and the archive will be perhaps 1–2% smaller than the originals. Sometimes it is larger, because the archive adds its own structure. The same is true of a folder of PDFs, and of video, which is the most heavily compressed thing most people own.
This matters because “zip it to email it” is the single most common reason people reach for an archiver — and for exactly those files, it is the one thing that does not work. If a 40 MB PDF will not fit in an email, zipping it produces a 39 MB ZIP. Compressing the PDF itself — re-encoding its images at a sensible resolution — produces a 4 MB PDF. That is a different operation with a different result.
Archives genuinely shrink text: source code, CSV, logs, XML, uncompressed images like BMP and TIFF, databases. A folder of CSV files can lose 80–90%. If that is what you have, read on.
ZIP dates from 1989 and is built into Windows, macOS, Linux, iOS and Android. Nobody has to install anything, and that single fact outweighs most technical considerations.
Its compression is the weakest of the three, typically by a meaningful margin on large sets of files. It compresses each file independently, which costs ratio but has a real benefit: you can extract one file without processing the whole archive, and a corrupted ZIP usually still yields the files before the damage.
On encryption, be careful. ZIP supports two schemes: the original ZipCrypto, which is cryptographically broken and should be treated as no protection at all, and AES-256, which is sound but is not what every tool produces by default. If you encrypt a ZIP, confirm which one you got.
One more limitation: ZIP encrypts file contents but leaves the file names readable. Anyone who has the archive can see what is in it without the password.
7z is open, free, and compresses harder than both others — typically 20–40% smaller than ZIP on compressible content, more on large collections of similar files.
Two things explain the gap. It uses LZMA2, a more modern algorithm with a much larger search window. And it supports solid archives: instead of compressing each file separately, it treats the whole set as one continuous stream, so repetition between files gets compressed too. A folder of a thousand similar HTML files benefits enormously.
Solid archiving has a cost, and it is the mirror image of ZIP’s advantage: to extract one file, the compressor has to work through everything before it. For a large archive that is slow, and damage early in the file can cost you everything after it.
Its encryption is AES-256 and, unlike ZIP, it can encrypt the file names too — the archive reveals nothing without the password.
The drawback is support. Windows and macOS do not open 7z without installing something. For a recipient you do not know, that is a real obstacle.
RAR compresses better than ZIP and roughly comparably to 7z, sometimes winning on certain content. It supports solid archives, AES-256, and has one genuinely distinctive feature: recovery records, redundant data stored inside the archive that can repair a damaged file. For something written to physical media or transferred over an unreliable link, that is real value.
But RAR is proprietary. Extracting is free; creating RAR archives requires a licence. Given that 7z matches it on compression, is free and open, and handles encryption at least as well, there is little reason to choose RAR for something new. Its continued presence is mostly inertia and existing archives.
This is worth separating out, because the usual instinct is wrong. When something is too large to send:
Doing it the other way round — zip first, discover it saved nothing, split the ZIP into parts — produces a package the recipient has to reassemble, for no reduction at all.