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Gzip Checker

Test whether a URL is served compressed, with which method, and how many bytes it saves. Stylesheets and scripts on the page are checked too.

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Gzip Checker - results and guide

What the Gzip Checker checks

  • Whether the response is compressed: gzip, Brotli, Zstandard or none
  • Original size, transferred size and the percentage saved
  • How much gzip would save when compression is off
  • The Vary: Accept-Encoding header
  • Compression of the CSS and JavaScript files the page loads
  • An unusually low compression ratio

What is gzip compression?

Text compresses very well. HTML, CSS and JavaScript are full of repeated words and patterns, and a compression method such as gzip typically shrinks them by 60 to 80% before they are sent. The browser says which methods it understands in the Accept-Encoding request header, the server picks one, compresses the response and labels it with Content-Encoding. The browser unpacks it in a few milliseconds.

Brotli (shown as br) is a newer method that usually beats gzip by 15 to 20% on text and is supported by every current browser. Zstandard (zstd) is newer still.

Why compression matters

Smaller responses arrive sooner, which improves the time to first paint and Largest Contentful Paint, especially on mobile networks. It also lowers bandwidth costs. Enabling it is one of the cheapest speed wins there is: a few lines of server configuration, or a switch in a CDN.

Only text needs it. JPEG, PNG, WebP, video and WOFF2 fonts are already compressed, and compressing them again wastes CPU. The usual mistake is a rule that covers HTML but not text/css and application/javascript, which this tool checks for. To see the total weight of a page, use the Web Page Size Checker.

How to use the Gzip Checker

  1. Enter the address of the page you want to check. You can leave out https://.
  2. Press Check compression. The page is fetched from our servers, the way a search engine crawler would fetch it.
  3. Read the findings from top to bottom: problems first, then warnings. Each one says what to change.

Common problems and how to fix them

Compression is off (Apache)

Enable mod_deflate and add: AddOutputFilterByType DEFLATE text/html text/css application/javascript application/json image/svg+xml. For Brotli enable mod_brotli.

Compression is off (Nginx)

Set gzip on; and list the types in gzip_types, including text/css and application/javascript. HTML is compressed by default once gzip is on.

HTML is compressed but CSS and JS are not

The rule lists too few content types, or static files come from a different host or CDN where compression is not enabled.

Frequently asked questions

How do I know if gzip is enabled?
Enter the URL above. If the response carries a Content-Encoding header with gzip, br or zstd, compression is on.
Is Brotli better than gzip?
For text, yes: files are usually 15 to 20% smaller. Serve Brotli where possible and keep gzip as the fallback.
Does compression affect SEO?
Indirectly. It makes pages load faster, and speed feeds into Core Web Vitals, which Google uses as a ranking signal.
Should images be gzipped?
No. Common image formats are already compressed. The exception is SVG, which is text and compresses well.

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