Why zipping photos does not make them smaller (and what does)
The folder was 180 MB. The ZIP is 178 MB. Nothing is broken: ZIP is the wrong tool for making photos smaller.
What ZIP actually does
A ZIP file is a container. For each file it stores a small header, the data, and a checksum (CRC-32), plus a table of contents at the end. The data part is either:
- Stored (method 0): copied as-is.
- Deflated (method 8): run through the same lossless algorithm used by gzip, which replaces repeated runs of bytes with short back-references.
Deflate is excellent on text, logs, CSV, source code and uncompressed formats like BMP or WAV, where the same words and patterns repeat. It typically shrinks those by 60 to 90%.
Why photos do not shrink
A JPEG has already thrown away detail your eye does not notice and then packed the rest with Huffman coding. PNG already uses Deflate internally. WebP and HEIC use even stronger video-codec methods. The output of a good compressor is close to random noise, and noise cannot be compressed again. Try it with any photo:
| Content | Typical ZIP saving |
|---|---|
| Plain text, CSV, JSON, logs | 60 to 90% |
| BMP or uncompressed TIFF | 30 to 70% |
| PNG screenshots | 0 to 5% |
| JPEG, WebP, HEIC photos | 0 to 3% |
| MP4, MOV video | 0 to 1% |
Switching to 7z, RAR or “maximum compression” changes the result by a percent or two at most, and can even make the archive slightly larger because of the extra headers.
That is also why the batch download on this site's image compressor uses the stored method: deflating already-compressed photos would cost time and battery for no gain. The ZIP is just a convenient envelope.
What really shrinks a set of photos
- Resize. A 12-megapixel photo shown on a laptop screen needs about 2 megapixels. Capping the long side at 1920 px removes about 75% of the pixels before anything else.
- Re-encode at a sensible quality. JPEG quality 75 to 80, or WebP, roughly halves the size again with little visible change.
- Then bundle. Zip the results if the recipient wants one file. That last step is about convenience, not size.
Together, steps 1 and 2 typically turn a 4 MB phone photo into 300 to 600 KB, which is the 85 to 90% saving people hope ZIP will give. The batch guide walks through doing a whole folder at once without uploading anything.
When zipping photos is still useful
- Sending many files through a service that accepts one attachment.
- Keeping folder structure and file names together.
- Checking nothing was damaged in transit: every file has a CRC-32 checksum, and
unzip -t photos.zipverifies all of them. For a stronger check of a single file, compare a SHA-256 with the hash generator.
Zipping does not remove the GPS tag or any other metadata inside the photos. If the pictures are going to strangers, strip location data first.
Related
What actually makes a photo smaller · Why zipping photos barely makes them smaller (interactive lesson) · Lossless vs lossy compression · Why a ZIP can be larger than the originals · gzip vs ZIP · Compress many photos at once without uploading them · Compress an image without uploading it · Resize an image without uploading it · Photo too large to email · Remove GPS from a photo · Image compressor (single or batch)
Questions
Why is my ZIP file the same size as the photos?
JPEG, PNG, WebP and HEIC are already compressed, so ZIP's lossless compression finds almost nothing to remove. Expect 0 to 3% savings at most.
Will 7z or RAR compress photos better than ZIP?
Only by a percent or two. No lossless archiver can do much with already-compressed images; resizing and re-encoding is what makes them smaller.
How do I make a folder of photos smaller?
Resize them (for example to 1920 px on the long side) and re-save as JPEG quality 80 or WebP, then zip the results if you want a single file. That typically saves 85 to 90%.
Does zipping remove EXIF or GPS data?
No. A ZIP stores each photo byte for byte, location tags included. Re-encoding the picture, for example with a canvas-based compressor or ImageMagick -strip, removes them.
Does ZIP reduce photo quality?
No. ZIP is lossless: unzipping gives back exactly the same bytes. That is why it cannot make them smaller either.
Published · updated .