ZIP is for bundling, not usually for shrinking
This surprises people, so it is worth stating plainly. Zipping a folder of photos, videos, MP3s or PDFs will save you almost nothing — often under two percent. Those formats are already compressed, and there is no redundancy left for ZIP to squeeze out.
What ZIP genuinely solves is the other problem: turning forty separate attachments into one. Most email clients, forms and messaging apps make sending a single file easy and sending forty files painful.
When zipping does shrink things
ZIP works by spotting repetition, so it performs well on anything stored as plain, uncompressed data:
- Very well — text, CSV, JSON, XML, log files, source code, and uncompressed images like BMP or TIFF. Savings of 70 to 95 percent are normal.
- Reasonably — WAV audio, some older document formats, databases.
- Barely at all — JPG, PNG, GIF, MP4, MOV, MP3, PDF, DOCX, XLSX, PPTX. All of these are compressed already.
If your folder is mostly photos and you actually need it smaller, tick the shrink option above. Reducing the photos first and then bundling them is what produces a real saving — zipping alone will not.
Splitting for email
Gmail caps attachments at 25MB and Outlook at 20MB. Rather than working out by hand which files fit together, choose a split size and the tool packs your files into as many parts as it takes, each one under the limit.
Each part is a normal, independent ZIP file. Whoever receives them can open each one on its own — there is no need to have all the parts present, which is the usual annoyance with the multi-volume archives that desktop software produces.
Related things people need
Since ZIP barely shrinks files that are already compressed, the real savings come from compressing them first. Run photographs through the image compressor, documents through the document compressor, and video through the video compressor before bundling.
If the goal is one attachment rather than one small attachment, merging photos into a PDF is often tidier than zipping them.
What ZIP compression actually does
ZIP uses an algorithm called DEFLATE, which finds repeated sequences of bytes and replaces later occurrences with short references back to the first. A document containing the word "agreement" two hundred times stores it once and refers back to it, which is why text compresses so dramatically.
This explains precisely why ZIP does nothing for a folder of photographs. JPG, MP4, MP3, PDF and Office files have already had their repetition removed by their own compression. There is nothing left for DEFLATE to find, and the result is a file within a percent or two of where it started.
It is not a fault, and it is not this tool failing. It is arithmetic. Compression cannot remove redundancy twice.
What compresses well
- Text, CSV, JSON, XML, logs, source code — routinely 70 to 95 percent smaller. Highly repetitive by nature.
- Uncompressed images such as BMP and TIFF — often 50 to 80 percent.
- WAV audio — commonly 10 to 30 percent, since audio has some but not much redundancy.
- Databases and older document formats — variable, often substantial.
And what does not: JPG, PNG, GIF, MP4, MOV, MP3, AAC, PDF, DOCX, XLSX, PPTX. All compressed already.
The real reason to zip things
Bundling, not shrinking. Most email clients, upload forms and messaging apps make sending one file straightforward and sending forty files tedious or impossible. A ZIP turns forty attachments into one, preserves the filenames, and unpacks identically on Windows, macOS, iOS, Android and Linux with no extra software.
That is worth doing even when the size is unchanged, and it is why the tool reports honestly when nothing was saved rather than implying something went wrong.
Splitting to fit a limit
When the total is simply too large for an email, splitting is the practical answer. Choose a maximum size and the files are packed into as many parts as needed, each one under the limit.
Each part here is a complete, independent archive. That differs from the multi-volume archives desktop software produces, which are fragments of a single file and useless unless every part is present. Those are a common source of frustration when one email goes astray. Independent parts avoid that entirely: whoever receives them can open each on its own.
If a single file is larger than the split size, it gets a part to itself and the tool tells you so, since it cannot be divided any further.
On password protection
ZIP's built-in password protection is deliberately not offered here. The original ZIP encryption is trivially broken, and while the newer AES-based variant is genuinely secure, support for it is inconsistent enough that recipients often cannot open the file.
More importantly, offering it would suggest a level of protection most people would overestimate. For anything genuinely sensitive, use a tool built for encryption rather than a ZIP password.
Questions
Are my files uploaded?
No. The archive is assembled inside your browser, on your device. Nothing is transmitted, which matters when you are bundling contracts, records or personal documents.
Can the recipient open it?
Yes. These are standard ZIP files. Windows, macOS, iOS, Android and Linux all open them without extra software.
Can I password-protect it?
Not here. ZIP's built-in password protection is weak by modern standards and would give false reassurance. For anything genuinely sensitive, use a tool built for encryption rather than a ZIP password.
Why is my ZIP the same size as the originals?
Because the files inside were already compressed. This is normal and not a fault. You have still gained the thing you came for, which is one file instead of many.