What is Gzip Decompress?
Gzip Decompress turns gzip-compressed data back into its original content. Gzip is everywhere in web development: HTTP responses sent with Content-Encoding: gzip, compressed log files, .gz downloads, and payloads that APIs, message queues and cookies store as Base64-encoded gzip (strings starting with H4sI). This tool decodes any of these and shows the original text, with JSON displayed as a navigable tree. Paste the data as Base64 or hex, or open a .gz file from your computer. The tool also recognizes zlib data (Base64 starting with eJ, hex starting with 78), which is often mislabeled as gzip, and reads the original file name and date when the gzip header includes them.
How to use Gzip Decompress
- Paste gzip data into the left panel as Base64 (it usually starts with H4sI) or as hex (it starts with 1f8b). You can also click Open .gz file, or drag a file onto the tool.
- The decompressed content appears instantly on the right. JSON is shown as a tree you can expand, collapse and search.
- Check the line under the workspace for the format, the compressed and original sizes, and the original file name.
- Copy the result, or click Save to download the decompressed file under its original name.
Key features
- Decompress gzip from Base64, Base64url, hex or data: URLs
- Open or drag and drop .gz files
- Also handles zlib / deflate data (eJ… / 789c…)
- Shows the original file name and date from the gzip header
- JSON output shown as a searchable tree
- Binary content shown as a hex preview, with Save to download it
- Protected against zip bombs with a 100 MB output limit
- 100% client-side: private and works offline
Common use cases
- Reading Base64 gzip payloads (H4sI…) stored in databases, cookies, JWT claims or message queues
- Inspecting compressed API or webhook bodies captured from logs
- Opening .gz log files or JSON exports without installing gunzip
- Debugging Content-Encoding: gzip responses copied from browser dev tools
- Decoding zlib-compressed data from Python's zlib or Node's deflate
Technical details & standards
Decompression uses the browser's built-in DecompressionStream, so it is fast and the data never leaves your device. The input format is detected from its magic bytes: 1f 8b for gzip (RFC 1952) and a valid zlib header such as 78 9c for zlib (RFC 1950). Base64 may be standard or URL-safe, with or without padding and line breaks; hex may include spaces, line breaks or a 0x prefix; data: URLs are accepted too. Output is streamed and capped at 100 MB, so a small, highly compressed "zip bomb" cannot freeze the page. Content that is not valid UTF-8 text (for example a tar archive inside a .tar.gz) is shown as a hex preview and can be saved as a file. Files made by concatenating several gzip members are not supported.
Frequently asked questions
What does H4sI mean at the start of a string?
H4sI is how the gzip header bytes 1f 8b 08 look when Base64 encoded. Any string starting with H4sI is almost certainly gzip-compressed data in Base64, and this tool can decompress it.
How do I decompress a .gz file online?
Click Open .gz file, or drag the file onto the tool. The contents appear immediately, and Save downloads the decompressed file with its original name. Nothing is uploaded; the file is read and decompressed in your browser.
Can it open .tar.gz or .tgz files?
It removes the gzip layer and gives you the .tar archive inside, shown as a hex preview that you can save. Extracting the individual files from the tar archive needs an archive tool.
What is the difference between gzip, zlib and deflate?
All three use the same deflate compression. Gzip wraps it with a header (file name, date) and a CRC-32 checksum, and zlib uses a shorter header with an Adler-32 checksum. This tool detects gzip and zlib automatically from their headers.
Why does it say the data is corrupted or truncated?
The compressed data is incomplete or was changed: for example, a Base64 string that was cut off, a binary file opened and saved as text, or extra characters added at the end. Copy the full value again and make sure nothing was trimmed.
Is there a size limit?
Decompressed output is limited to 100 MB, which also protects your browser from zip bombs (tiny files that expand to gigabytes).
Is my data safe?
All processing happens entirely in your browser. Your data never leaves your device and is never sent to or stored on a server.
Does it work offline?
Yes. Once the page has loaded, most tools keep working without an internet connection.