Guides

What Is File Compression?

File compression shrinks data by removing redundancy. How much it shrinks, and whether anything is lost, depends on the method used.

Lossless vs lossy

• Lossless compression shrinks data so it can be perfectly reconstructed afterward. Archive formats like RAR, ZIP, and 7z use lossless methods, since you need your original files back exactly.
• Lossy compression discards some data permanently in exchange for smaller size, accepting some quality loss. This is how formats like JPEG and MP3 achieve much smaller files than their raw sources.

Redundancy is the key

Compression works by finding and removing redundant, repeated patterns in data. A file with lots of repetition, like plain text with common words or an image with large blocks of the same color, compresses well because there's a lot of redundancy to eliminate.

Dictionary and entropy coding, briefly

At a simplified level, many compressors build a kind of internal dictionary of patterns they've already seen and replace repeat occurrences with shorter references to that dictionary. On top of that, entropy coding assigns shorter codes to more common symbols and longer codes to rare ones, similar in spirit to Morse code using short signals for common letters. Combined, these techniques squeeze out redundancy without losing information.

Why some files barely shrink

Files that are already compressed — JPEG photos, MP3 or MP4 media, and formats that use lossy compression internally — have already had most of their redundancy removed. Running them through an archive tool again offers little to no further benefit, and can occasionally make the file slightly larger due to container overhead.

Compression vs speed trade-off

Stronger compression settings generally take more time and processing power to search for redundancy more thoroughly. That's why archive tools offer adjustable compression levels rather than a single fixed setting: you're trading time for size.

Real-world uses

• Shrinking files for email or upload limits.
• Reducing storage space for backups and archives.
• Speeding up transfers over slower network connections.
• Bundling many small files into one container for easier handling — see what is an archive file.

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