How to use the Website Compression Test
- Enter a page URL or just a domain and click Test compression.
- Read the verdict: Brotli, gzip, Zstandard or no compression.
- Compare transferred bytes per
Accept-Encodingvalue and see how small the page could be with gzip 9 or Brotli 11. - Check the CSS and JavaScript list for files that are sent uncompressed, then follow the recommendations.
What does HTTP compression do?
Browsers send an Accept-Encoding header listing the compression formats they understand — today usually gzip, deflate, br, zstd. A correctly configured server picks one, compresses the response and labels it with Content-Encoding. For text such as HTML, CSS, JavaScript, JSON and SVG this typically cuts transfer size by 60–80 %, which directly improves load time, Largest Contentful Paint and mobile data usage.
What this test checks
- Each encoding separately: the page is requested with
identity,br,gzip,zstdand a browser-like combined header, and the real transferred bytes are measured. - Potential savings: the uncompressed page is compressed on our server with gzip 6, gzip 9, Brotli 5 and Brotli 11, so you can see what you are leaving on the table.
- Linked CSS and JavaScript: up to 12 stylesheets and scripts from the page are checked, because it is common for HTML to be compressed while static files are not.
- Vary header:
Vary: Accept-Encodingtells caches to store compressed and uncompressed versions separately.
Enabling compression
| Server | Gzip | Brotli |
|---|---|---|
| Nginx | gzip on; gzip_types text/css application/javascript application/json image/svg+xml; | ngx_brotli module: brotli on; brotli_types …; |
| Apache | mod_deflate: AddOutputFilterByType DEFLATE text/html text/css application/javascript | mod_brotli: AddOutputFilterByType BROTLI_COMPRESS … |
| Node / Express | compression middleware | Pre-compressed .br files or a CDN |
| Cloudflare, Vercel, Netlify | Automatic for text content types | |
What not to compress
JPEG, PNG, WebP, AVIF, video, fonts in WOFF2 and ZIP files are already compressed — running gzip over them wastes CPU and can even make them larger. Compress text formats only. For the files themselves, minify first (JavaScript, HTML, CSS), then let the server compress them.
Our requests identify as DevPackBot and fetch only the URL you enter plus its linked CSS and JS files. Sites behind bot protection may block the test.
Tools that pair well with the Website Compression Test
- HTTP Header Checker — Fetch any URL and see exactly what the server sends back — status code, redirect chain and every response header, with a plain-English explanation of the ones that matter for SEO, caching and security.
- SEO Checker & Site Audit — Paste a URL to run up to 70 on-page SEO checks plus a full site audit: Lighthouse speed, security headers & SSL, structured data, broken links, robots.txt & AI crawlers, accessibility basics, tracking & consent, SPF/DKIM/DMARC and domain expiry — rolled up into one site score with prioritised fixes.
- JSON Minifier — Remove all unnecessary whitespace from JSON, validate it at the same time and see exactly how many bytes you save — before and after gzip.
- XML Minifier — Compact XML documents, feeds, sitemaps and SOAP payloads by removing indentation and comments — with a well-formedness check so nothing breaks.
Frequently asked questions
How do I check if gzip is enabled on my website?
Enter your URL above. If the row for
gzip shows “Gzip” as the server’s encoding and a much smaller byte count than identity, gzip is enabled.Is Brotli better than gzip for websites?
Usually yes — Brotli files are 15–25 % smaller and every modern browser supports it. Keep gzip enabled as a fallback for older clients.
What is Zstandard (zstd)?
Zstandard is a newer, very fast compression format. Chrome and Firefox now accept
Content-Encoding: zstd, and some CDNs have started serving it. Brotli and gzip remain the most widely deployed.Why does the test show different sizes than my browser’s DevTools?
DevTools includes headers and may load a different variant (for example a logged-in page or a CDN edge in another region). The body size of the same URL and encoding should be close.
Does compression help SEO?
Indirectly: smaller responses load faster, which improves Core Web Vitals and user experience — both of which matter for rankings. Google’s PageSpeed Insights flags missing text compression as an issue.