Gzip and Brotli Compression: How to Check It Is Working

Gzip and Brotli compress text files so they travel faster, and you can confirm they are working by looking for a Content-Encoding header in the server response. A page without compression sends far more data than it needs to, which slows loading, especially on mobile connections.

How compression works

The browser says which formats it supports, and the server replies with a compressed version and a Content-Encoding header naming the method, such as gzip or br for Brotli. The browser then unpacks it.

What to compress

• HTML pages.
• CSS and JavaScript files.
• JSON, XML and plain text, including sitemaps.
• SVG images.

What to leave alone

Formats that are already compressed, such as JPEG, PNG, WebP and video, gain little. Reduce those through image optimisation instead, using the image compressor and WebP converter.

How to check it

Enter a page into the HTTP header checker and look for the Content-Encoding line. Then test the effect on speed with the PageSpeed checker.

If compression is missing

Enable it in your hosting control panel, server configuration, caching plugin or CDN. Most modern hosts offer it. Check again afterwards, because layers such as a CDN can change the result.

Related speed work

Compression sits alongside caching and a CDN, as explained in hosting, caching and CDN for speed and Cache-Control headers.

Frequently asked questions

Next steps

For a full speed review, contact me here.

More in this series (HTTP header questions): start with Free HTTP Header Checker, then explore:

• X-Robots-Tag Explained: How to Noindex PDFs and Other Files With HTTP Headers
• Cache-Control Headers: What They Do and How They Affect SEO
• Security Headers and SEO: HSTS, CSP and the Headers Worth Setting
• Canonical in the HTTP Header: How to Canonicalise PDFs and Non-HTML Files