File compression: when it helps and by how much

How shrinking works

Compression reduces size by representing repeated material briefly. Files where the same content recurs often shrink the most. Plain text documents, spreadsheets and program source hold a lot of repetition and compress well, while photos and video are already compressed and have little repetition left, so they stay the same size. A log file repeating the same lines hundreds of times shrinks to a fraction of its size, whereas data close to random barely shrinks at all. Encrypted files also look random from the outside, so compression does nothing for them.

Lossless and lossy

There are two kinds. Lossless returns exactly the original when unpacked; lossy discards information people rarely notice to reach far smaller sizes. Documents and programs cannot tolerate a single changed character, so they always use lossless. Recompressing an already lossy file with another lossy pass saves little space and can only lower quality.

• Documents, spreadsheets, source: lossless, shrink considerably
• Photos: mostly already in lossy formats
• Video: lossy by default, recompressing gains little
• Audio: split between lossless and lossy formats

Bundling many files into one

The other purpose of compression is bundling rather than size. Sending hundreds of files individually is slow because each is handled separately, while one bundle transfers faster and preserves the folder structure. With very many small files, a size reduction appears as well. When choosing a format, what matters is whether the recipient can open it. ZIP is the safest choice, since most operating systems can extract it without extra software.

Adding a password

Archives can be password protected, but the strength varies widely by format and settings. Older schemes are easily broken, and file names sometimes remain visible even when contents are protected. For genuinely sensitive material, relying on an archive password alone is unwise. If you add a password, choose the AES option your archiver offers, and share the password through a different channel from the file. Putting the file and its password in the same email defeats the purpose.

Care when unpacking

With archives from unknown sources, the danger is running what is inside rather than unpacking itself. Files that look like documents can be executables, so check the full extension. Unpacking also spills the original folder structure and can overwrite existing files, so extracting into an empty folder is the safer habit. Be wary of archives nested inside archives, too, because some are crafted to expand to an abnormally huge size when unpacked.

Time matters more than compression level

Archivers usually let you choose a compression level. Higher levels make the file a little smaller but take much longer, and the saving is often smaller than expected. For a folder mostly holding already-compressed photos or video, a store mode that only bundles without compressing is fastest and gives almost the same size. When you need to send something quickly, leaving the default or a fast level is the practical choice.

Split archives and size limits

When email attachments or uploads have a size limit, a split archive divides one large archive into several parts. If even one part is missing or damaged, however, the whole thing often cannot be extracted, and the recipient needs software that supports the format. For very large transfers, a cloud share link is often simpler than splitting. The parts carry numbers at the end of their names, and it helps to tell the recipient that every part must sit in the same folder for extraction to work.

A checklist before sending

Checking a few things before sending an archive greatly reduces the need to send it again. The last item is tedious but worth doing.

• See whether the contents are already compressed
• Confirm the recipient can open the format
• For sensitive material, use AES and send the password separately
• Unpack it yourself once to confirm it opens