Question 1

What does this tool do?

Accepted Answer

It compresses files to the GZIP (.gz) format using the browser's native CompressionStream API. The output .gz file preserves the original filename in its header so GZIP decompressors (gunzip, 7-Zip, WinRAR, macOS Archive Utility) restore the original name automatically. For multiple files, you can combine them into a single .tar.gz archive (TAR + GZIP).

Question 2

What compression level does it use?

Accepted Answer

CompressionStream uses the browser's built-in zlib implementation, which is typically level 6 (default). The browser API doesn't expose level control, so we display 'Default (browser-managed, ~level 6)' in the stats. This gives a good balance between speed and ratio for most file types.

Question 3

Why are some files barely smaller after compression?

Accepted Answer

Already-compressed formats (JPEG, PNG, MP4, ZIP, XLSX, GZIP) don't compress further — they're already entropy-coded. Text files (TXT, CSV, JSON, XML, HTML, source code) typically shrink 60-90%. Random binary data won't compress at all.

Question 4

What's the maximum file size?

Accepted Answer

There's no hard limit, but very large files (> 500MB) will use lots of memory because GZIP compresses the whole file at once. For multi-GB files, consider splitting first. The browser also has a per-tab memory cap (~2-4GB) that limits total batch size.

Question 5

Can I compress multiple files at once?

Accepted Answer

Yes. Drag-drop or select multiple files — each becomes a separate .gz file preserving its original name. For a single output, enable 'Combine as .tar.gz' to bundle all files into a TAR archive then GZIP-compress it into one .tar.gz file.

Question 6

What extra features does this tool have compared to others?

Accepted Answer

10 extras: (1) Drag-drop file input. (2) Batch compress multiple files at once. (3) Compression level display (browser-managed, ~level 6). (4) Before/after size comparison per file. (5) Aggregate stats — total saved bytes, total ratio. (6) Progress bar during batch compression. (7) Custom output filename. (8) .tar.gz support — combine TAR + GZIP into one archive. (9) History (localStorage — last 10 batch summaries). (10) Shareable URL with options (tar.gz toggle, custom filename).

Question 7

Is my file uploaded anywhere?

Accepted Answer

No. All GZIP compression runs in your browser via the native CompressionStream API. File contents never leave your device. Only batch summaries (filenames + sizes) are saved to local history.

Question 8

Does it work offline?

Accepted Answer

Yes. CompressionStream is a browser-native API available in all modern browsers (Chrome 80+, Firefox 113+, Safari 16.4+). Once the page loads, no network access is required.

GZIP Compressor

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