Why does this need a server, unlike most tools here?

This tool sends your URL to Tool4U’s server to inspect the destination response headers. Use a public URL without private tokens or credentials; the destination receives a request from that server.

Why does compression matter?

A compressed response transfers far fewer bytes over the network, which speeds up page loads — an uncompressed text/HTML/JSON response on a production site is usually a missed easy win.

Does every response need to be compressed?

No — already-compressed formats (JPEG, PNG, MP4, WOFF2) gain little to nothing from further compression; this matters most for text-based responses (HTML, CSS, JS, JSON, SVG).

Compression Checker

Also known as: Website Compression Test

Check whether a URL’s response is served compressed (gzip/brotli) and with what Vary header.

URL

The URL is fetched from our server (an arbitrary cross-origin fetch isn't possible directly from your browser) — the target site sees this server's request, not your IP, and nothing about the check is stored.

Read the compression response correctly

Headers from the final response

The check follows redirects and reads Content-Encoding and Vary from the final response. An encoding other than identity is reported as compressed. The tool does not calculate a compression ratio or test every resource on a page.

One request is not every visitor’s response

Results describe the response to Tool4U’s server request. They may differ from a logged-in browser session or another request. A missing encoding header means this response did not advertise compression, not that compression is disabled everywhere on the site.

This check uses a server

The URL you submit is sent to Tool4U’s server, which requests the destination. Use a public URL and leave out private tokens or credentials. This check is not an offline or local-only tool.

Interpret a compressed response

Content-Encoding: br
Vary: Accept-Encoding

The response advertises Brotli compression and varies by Accept-Encoding. This does not measure bytes saved or prove the page’s images, CSS and scripts use the same encoding.

Related tools

How it works

1
Enter a URL

Including the protocol.
2
Content-Encoding and Vary read

From the final response, after redirects.
3
See whether it’s compressed

And which encoding (gzip, br, deflate) is used.

FAQ

Why does this need a server, unlike most tools here?: This tool sends your URL to Tool4U’s server to inspect the destination response headers. Use a public URL without private tokens or credentials; the destination receives a request from that server.
Why does compression matter?: A compressed response transfers far fewer bytes over the network, which speeds up page loads — an uncompressed text/HTML/JSON response on a production site is usually a missed easy win.
Does every response need to be compressed?: No — already-compressed formats (JPEG, PNG, MP4, WOFF2) gain little to nothing from further compression; this matters most for text-based responses (HTML, CSS, JS, JSON, SVG).