When compressing saves space on a Mac, and when it saves nothing
Zip helps with text and hurts with media. Which formats compress, which do not, and the macOS aware way to do it.
Compressing a folder feels like it should save space and often saves none at all. Whether it helps is entirely determined by what is inside, and knowing which is which saves a lot of pointless waiting.
What compresses and what does not
| Content | Typical saving |
|---|---|
| Text, code, logs, CSV | 70 to 95 percent |
| Uncompressed audio, WAV and AIFF | 30 to 60 percent |
| RAW photos | 10 to 30 percent |
| Office documents | Little, they are already zipped |
| JPEG, PNG, HEIC | Almost nothing |
| MP4, MOV, H.264 and H.265 video | Almost nothing |
| Already compressed archives | Nothing, sometimes worse |
The rule underneath: anything already using compression internally has had its redundancy removed, and compressing again cannot find what is not there. That is why zipping a folder of photos returns a file the same size.
The right command on a Mac
ditto -c -k --sequesterRsrc --keepParent MyFolder MyFolder.zip
ditto -x -k MyFolder.zip ./restored/ditto understands Mac metadata and resource forks, which a plain zip can lose. The Archive Utility in Finder, from the right click Compress option, uses the same underlying approach and is fine for everyday use.
Where compression genuinely helps
- Log and data archives. Text compresses enormously and is often kept for years.
- Source code archives. Same reason, and it keeps thousands of small files as one item.
- Old email exports and databases. Text heavy and rarely opened.
- Folders of many small files. Even without compression, one file is easier to move and store than fifty thousand.
The better alternative for media
For photos and video, the way to save space is re-encoding rather than compressing: exporting video at a sensible bitrate, or converting old formats to modern codecs, genuinely reduces size where zip cannot. That is a quality decision rather than a lossless one, which is the important difference.
For finished work generally, removing derived data before archiving saves far more than compression ever will, which is the point of archiving old projects properly.
Common questions
Does zipping files save space on a Mac?
It depends entirely on the content. Text and code compress by 70 to 95 percent, while photos, video and already compressed archives gain essentially nothing because their redundancy is already removed.
What is the best way to compress a folder on a Mac?
ditto -c -k --sequesterRsrc --keepParent, which preserves Mac metadata and resource forks. The Finder's own Compress option is equivalent for everyday use.
Why does zipping photos not make them smaller?
Because JPEG, HEIC and PNG already use compression internally. A second pass cannot find redundancy that has already been removed, so the archive is the same size as the originals.
How do I actually reduce the size of videos?
Re-encode rather than compress: export at a sensible bitrate or convert to a modern codec. That reduces size genuinely, at a quality cost, which zip cannot do at all.