Create ZIP File

What DEFLATE can and cannot squeeze

ZIP archives use an algorithm called DEFLATE, which finds repeated byte sequences and replaces later copies with short back-references, then re-codes the result so common symbols use fewer bits. That works brilliantly on anything with structure and repetition, and hardly at all on anything that has already been through a compressor.

Expect large savings on plain text, CSV exports, log files, JSON, XML, source code, SQL dumps, BMP images and uncompressed WAV audio. A 40 MB CSV of transaction records routinely drops below 5 MB, because the same field names, dates and formats repeat thousands of times.

Expect almost nothing on JPEG and PNG photos, MP4 and MOV video, MP3 and AAC audio, or existing ZIP, RAR and 7z files. Those formats already removed their own redundancy. Zipping 200 MB of holiday photos will usually give you a 199 MB archive, and the half hour your device spent trying was wasted.

That is still a perfectly good reason to zip them. A ZIP bundles many files into one, which is what most email clients, upload forms and ticketing systems actually want.

Choosing a compression level

The dropdown maps to DEFLATE levels. Store writes the files into the archive with no compression at all and is the fastest option by a wide margin. Low is level 3, Normal is level 6, and Maximum is level 9.

The returns fall away sharply at the top of the scale. Moving from level 6 to level 9 typically buys you one or two per cent smaller output while taking substantially longer, because the encoder searches much harder for matches it mostly does not find. Level 6 is the default in almost every ZIP implementation for good reason.

Use Store deliberately when your files are photos, video or audio. You get a bundle in seconds instead of minutes and the size is effectively identical.

A worked example

Say you are sending an accountant twelve months of exported records: twelve CSV files totalling 18 MB, two PDF statements at 1.2 MB each, and a folder of forty scanned receipt JPEGs totalling 46 MB.

At Normal, the CSVs collapse to perhaps 2 MB, the PDFs shrink slightly since PDFs often contain compressed streams already, and the JPEGs move almost unchanged. The archive lands somewhere near 50 MB, nearly all of it the photos. At Store it would be about 67 MB. At Maximum it would be about 49.5 MB after a much longer wait. In this case Normal is clearly right, and the real saving would come from shrinking the scans first with Compress Image to KB before you zip anything.

A ZIP is a container, not a lock

This is the single most important thing to understand before you email an archive. Creating a ZIP does not protect the contents. Anyone who receives the file can open it and read everything inside. The tool has no password option, and even the password feature in desktop ZIP software varies wildly in strength depending on which encryption method it uses.

If the contents are genuinely sensitive, do not rely on the archive format. Send them through a channel you trust, or encrypt them properly with dedicated software before zipping.

The archive also carries file names in plain text in its directory listing. Someone can read the names of everything inside without extracting a single byte, so name your files with that in mind.

Practical notes and limits

Compression happens on your own device using the fflate library. Everything runs in your browser, so your files never leave your device. The archive is assembled and handed straight to your downloads folder.

Two behaviours are worth knowing. First, if you add two files with the same name, the second is automatically renamed with a numeric suffix rather than silently overwriting the first. Second, this builds a flat archive: the files you add sit at the top level with no folder structure, because browsers hand over a plain list of files rather than the directory tree they came from.

Memory is the real ceiling. Every file is read into the tab in full, then the compressed output is built alongside it, so peak usage is well above the total size of what you selected. The page warns once your selection passes 100 MB. On a phone, large batches will often fail by reloading the tab instead of showing an error, so use a desktop browser for anything substantial.

To check what ended up inside, or to open an archive someone sent you, use Extract ZIP.

Questions

Why is my ZIP the same size as the files I put in?

Because those files were already compressed. JPEG, PNG, MP4, MP3 and PDF all have compression built in, so DEFLATE finds nothing left to remove. This is normal and not a fault with the tool.

Can I add a password?

No. This tool creates a standard unencrypted archive. Anyone with the file can open it.

Can I add folders rather than individual files?

The archive is built flat from the files you select. If you need a folder structure preserved, create the ZIP with your operating system's built-in right-click option instead.

How many files can I add at once?

There is no fixed count. The limit is your device's memory, since everything is held in the browser tab while the archive is built. Watch for the warning that appears past 100 MB.

Will the ZIP open on Windows, macOS and Linux?

Yes. It is a standard ZIP archive with DEFLATE compression, which every desktop operating system opens natively without extra software.