GZIP Compression

What is Gzip Compression?

Gzip compression is a method websites use to make files smaller before sending them to the visitor's browser. Smaller files download faster, so pages load quickly - especially on mobile networks or slower connections.

When you share a big folder, you often compress or “zip” it to reduce its size. Gzip compression does something similar for web pages in real time. The server compresses the content, and your browser automatically uncompresses it after download the zipped file. All of this happens behind the scenes, so visitors don’t need to install anything or click anything to uncompress files at their end.

Gzip works best for text-based files such as: HTML,CSS,JavaScript and JSON/XML (data).These file types often contain lots of repeatable text patterns, which makes them easy to compress thereby reducing the file size significantly.

Your browser and the web server use HTTP headers to “agree” on compression:

• The browser says what type of compression it supports via Accept-Encoding (for example: gzip or br).
• If the server sends a compressed response, it includes Content-Encoding: gzip in the response headers.

If you see Content-Encoding: gzip or Content-Encoding: br (for Brotli), it usually means compression is enabled and working.If you don’t see a Content-Encoding header for text resources, your website may be sending larger, uncompressed files which may lead to slower load times and higher bandwidth usage.

How the Gzip Compression Test Works

When you enter a URL, our tool checks whether the server or CDN is sending compressed responses for text-based resources such as HTML, CSS, and JavaScript. This is done by inspecting the HTTP response headers and validating whether compression is applied correctly or not.

Here are the main checks we perform:

• Checks browser support request: Modern browsers tell servers which compression formats they support using the Accept-Encoding request header (commonly gzip, br).
• Confirms compression is enabled: If compression is active, the server should respond with Content-Encoding: gzip (or Content-Encoding: br for Brotli).
• Verifies eligible content types: Compression should typically apply to text-based MIME types like text/html, text/css, application/javascript, application/json,image/svg+xml etc.
• Detects common misconfigurations: We look for issues like compression missing on CSS/JS, compression only working on the homepage, redirects where compression disappears, or responses that appear uncompressed.

If the tool reports that Gzip is not enabled, it usually means the server isn’t compressing responses, the CDN isn’t configured to compress, or the content type/rules aren’t set to include the files that should be compressed.

Do’s and Don’ts of Gzip Compression

Gzip compression is one of the easiest speed wins, but it should be applied thoughtfully. Use the guidelines below to compress the right files, avoid wasted CPU, and keep caching behavior correct.

Common Reasons Gzip is Not Working

If Gzip compression is disabled or only working on some pages of your website, it usually comes down to server/CDN configuration, incorrect content types, or conflicting rules in your delivery stack. Most issues are easy to fix once you know where to look.

1. Compression isn’t enabled on the origin server: If you control Apache, Nginx, or IIS, Gzip may simply be off by default or not configured for the file types you serve. Enabling that configuration on the server level should fix the problem.
2. CDN compression is disabled: Some CDNs require you to toggle compression on, and others compress only specific MIME types. You may see HTML compressed but not CSS/JS. In this case, you have to configure your CDN to compress all file types with Gzip.
3. Wrong or missing MIME types: Servers often decide what to compress based on "Content-Type". If JS, CSS, or JSON are sent with an unexpected type (or as plain text), compression rules may not match.
4. Files are too small to compress: Many servers won’t compress responses under a certain file size because the savings are minimal. That can make compression look “inconsistent” across resources.
5. Brotli is enabled but Gzip is not (or vice versa): Modern setups may serve "Content-Encoding: br" to supported browsers.If you’re only looking for Gzip, you might miss that compression is still active but it's not Gzip but Brotli.
6. Redirects lead to a different configuration: If your URL redirects (HTTP → HTTPS, non-www → www, or to a different host), the final destination might be the one that actually needs compression enabled.
7. Reverse proxy / load balancer conflicts: Stacks with Nginx + Apache, Varnish, or other proxies can accidentally disable compression, strip headers, or apply different rules depending on the route.
8. Already-compressed or non-eligible resources: Images, videos, and archives usually won’t show Gzip because they’re already compressed.That’s expected behavior and not much of a problem.
9. Misconfigured caching headers: If a cache stores an uncompressed version and serves it back, you may not see "Content-Encoding" consistently. Proper use of "Vary: Accept-Encoding"helps prevent this issue.
10. Server rules exclude certain paths or file extensions: Some setups compress only HTML and forget about assets like .css, .js, or API responses (/api/ endpoints returning JSON).

If you’re unsure where the problem is, start by testing a few different resource types (HTML page, CSS file, JS file).If only some compress, it’s usually a rule/MIME-type issue. If nothing compresses, it’s likely a server/CDN setting or misconfiguration which needs to be fixed.

Once Gzip compression is enabled correctly, you should see Content-Encoding: gzip on eligible responses and noticeably smaller transfer sizes. That translates into faster loads, especially for first-time visitors and mobile traffic.