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About File Gzip Compression Checker Online
This tool fetches a URL and reports whether the server is sending gzip-compressed content, what the compression ratio is, and what bandwidth is saved versus uncompressed delivery. It also detects brotli (br) and zstandard (zstd) when supported.
Compression is one of the cheapest performance wins on the web: most text content (HTML, CSS, JS, JSON) compresses 70-80%, dramatically reducing transfer sizes and load times. A site that doesn't enable compression is leaving free performance on the table.
Use it to verify your CDN or web server is configured correctly, audit a competitor's site, or quantify the savings from enabling compression before pitching the change to a team.
How to use this tool
How to inspect a file's gzip compressibility
Upload any file
Drop the file you want to analyse into the File field. accept is */* — text, binaries, archives, images, anything. Size is capped at the configured maximum (decoded base64 length is enforced).
What the tool checks
It detects whether the file starts with gzip magic (0x1f 0x8b), tries to gunzip it, and (whether or not the upload was already gzip) re-gzips the underlying payload at a fixed zlib level for reference.
Press Run
Output returns originalBytes, isGzipCompressed, optional gzipHeaderExtraFlags / gzipHeaderXflByte (if the file IS gzip), uncompressedBytes, referenceZlibLevel, gzipBytes, ratio, and savingsPercent.
Read the note
Zlib levels 1–9 are NOT stored inside .gz files — the gzip header XFL byte only records the compressor's MODE (fastest / normal / maximum). "Reference re-gzip size" is what zlib at this tool's fixed level would have produced; it's a sanity baseline, not a guarantee of best-possible compression.