What Check GZIP Compression does
Check GZIP Compression sends a server-side request that advertises compression support and inspects the actual response headers. It tries HEAD first and falls back to GET when the server responds that HEAD is unsupported, then reports whether Content-Encoding indicates a compressed representation.
The tool recognizes any non-identity encoding, so it can report br or another value as well as gzip. Content-Length is displayed only when the response provides a valid header and describes that selected representation. Because caches, CDNs, user agents, MIME types, and request methods can affect compression, one response is not a complete delivery audit.
How to use Check GZIP Compression
- Enter the full public URL of the page or resource.
- Select Check compression to send a request that advertises compression support.
- Review the final status, compressed flag, Content-Encoding, Content-Length, and Vary values.
- Repeat with production diagnostics for important resource types and cache paths if delivery behavior differs.
Important details
Content-Encoding determines the result
The compressed flag is true when the server returns a non-empty Content-Encoding other than identity or none. A missing header is reported as uncompressed for this request.
Vary explains cache selection
A Vary value containing Accept-Encoding tells shared caches that compressed and identity representations can differ. The tool displays the header but does not validate a CDN's complete cache configuration.
Content-Length is not a savings calculation
The number comes from the response header when available. PagesTools does not fetch both compressed and uncompressed bodies, so it cannot calculate a percentage reduction or verify the declared byte count.
Practical uses
- Confirm whether a deployed HTML, CSS, or JavaScript URL advertises compressed delivery.
- Inspect Vary after changing a reverse-proxy or CDN compression rule.
- Record response-header evidence while investigating unexpectedly large transfers.
Privacy and limitations
The URL is sent to PagesTools, whose server requests the public resource with compression advertised. The destination and intermediaries can log that request; the JSON report is returned with no-store cache headers.
This is one server-side HEAD or fallback GET response. It does not compare encoded and identity bodies, calculate savings, validate every asset, execute JavaScript, or reproduce all browser, geography, CDN, and cache variants.