Zip Files and Image Compression: Why Zipping a JPEG Barely Helps
What zipping does, why it saves little for photos already compressed and when it's still useful.
Send a folder of photos as a zip, and it stays almost the same size. That surprises many people.
What zipping does
A zip file uses lossless compression. It finds repeated patterns in data and stores them more compactly. It works well on text, spreadsheets and some documents, which contain a lot of repetition.
Why photos barely shrink
JPEG files are already compressed. JPEG compression has removed most of the redundancy in the picture, so there are few patterns left for zip to find.
| File type | Typical saving from zipping |
|---|---|
| Plain text | large |
| Word documents and spreadsheets | moderate to large |
| Uncompressed images (BMP) | large |
| PNG | small |
| JPEG, WebP, HEIC | very small, often a few percent |
| MP4 video, MP3 audio | very small |
What zipping is still good for
- Bundling many files into one for easy sending.
- Keeping folder structure intact.
- Protecting with a password, in some tools.
- Compressing text and documents in the same folder.
What actually reduces photo size
- Resize to smaller dimensions.
- Lower the JPEG quality a little.
- Crop out unneeded area.
- Convert to a more efficient format, where compatible.
These change the picture data itself, so the effect is much bigger than zipping.
A combined approach
To send many photos:
- Resize and compress each photo to a sensible size.
- Then zip the folder for convenience.
The zip won't make it much smaller, but it keeps everything together and avoids many attachments.
Email limits still apply
A zip of 20 MB attachments is still 20 MB. Check the size limit of your email service and the recipient's. Cloud share links are an alternative.
Rules of thumb
- Don't expect zip to shrink photos.
- Shrink the photos first.
- Zip only for bundling.
Batch processing
Compressing a whole set of photos in one go, with a quality or target size, is the most effective way to shrink a folder before you send it.
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