HTTP Compression Test
About the HTTP Compression Test
The HTTP Compression Test checks whether a web server compresses its responses before sending them to browsers, and which algorithm it uses. When you submit a URL, the tool sends a request advertising support for common encodings via the Accept-Encoding header and inspects the response's Content-Encoding header to confirm whether the server replied with Gzip, Brotli, Deflate, or no compression at all. Compression is one of the highest-impact, lowest-effort performance wins available, since it shrinks HTML, CSS, JavaScript, and JSON payloads dramatically before they travel over the network.
Behind the scenes, text-based assets compress extremely well because they contain repeated tokens, whitespace, and predictable structure. Gzip is the long-standing baseline supported everywhere, while Brotli, developed by Google, typically achieves 15 to 25 percent smaller output for static text and is now supported by all modern browsers. The tool reports the compressed transfer size against the original uncompressed size so you can see the exact byte savings and the resulting compression ratio for the resource you tested.
Common use cases include verifying a new CDN or reverse proxy configuration, diagnosing why a page feels slow despite small visual content, and confirming that compression survives behind load balancers or caching layers like Cloudflare or Nginx. It pairs naturally with a Website Speed Test for end-to-end load timing and a Security Headers Checker when you are auditing overall server response configuration.
A practical tip: not every response should be compressed. Already-compressed binary formats like JPEG, PNG, WebP, and MP4 gain little and can even grow slightly, so servers should compress only text-based MIME types. Also be aware of the historic BREACH attack, which is why some teams disable compression on responses that mix secrets with reflected user input. If the tool shows no compression on large HTML or JavaScript files, enabling Brotli or Gzip is usually the single best place to start optimizing.
Frequently asked questions
- What is the difference between Gzip and Brotli?
- Both compress text-based responses, but Brotli generally produces 15 to 25 percent smaller files than Gzip at comparable speeds and uses a built-in dictionary tuned for web content. Gzip is universally supported, while Brotli is supported by all current browsers.
- Why does my image or video show no compression benefit?
- Formats like JPEG, PNG, WebP, and MP4 are already compressed internally, so applying Gzip or Brotli adds little and can slightly increase size. Servers should only compress text MIME types such as HTML, CSS, JavaScript, JSON, and SVG.
- How does the server know which compression to use?
- Your browser sends an Accept-Encoding header listing the algorithms it supports, and the server replies with a Content-Encoding header naming the one it chose. This negotiation is what the test inspects.
- Can compression be a security risk?
- In rare cases, the BREACH attack exploited compression on HTTPS responses that combined secret tokens with reflected user input. Most sites mitigate this with per-request CSRF tokens rather than disabling compression entirely.
Measure website load time
Check HTTP protocol version support
Check SSL certificate validity and expiration
Analyze HTTP response headers
Check security headers configuration
Follow redirect chains