Cache-Control Headers: What They Do and How They Affect SEO

Cache-Control headers tell browsers and networks how long they may reuse a file instead of fetching it again. Used well, they make repeat visits faster. Used badly, they show visitors outdated pages. They are not a ranking factor, but they support site speed, which matters for visitors and for the experience metrics search engines look at.

The main headers

• Cache-Control: sets how long a response may be reused, for example max-age.
• ETag and Last-Modified: let a browser ask whether a file has changed, so the server can reply that nothing is new.

Static files

Images, scripts and style sheets with versioned names can be cached for a long time, because a new version gets a new file name. This saves repeat downloads and speeds up pages.

HTML pages

Pages change more often, so use short cache times or require the browser to check for updates. A long cache on HTML can leave visitors seeing yesterday’s content.

Mistakes to avoid

• Long caching on pages that change frequently.
• Caching error pages by accident.
• No cache headers at all on large static files.
• Different settings from a plugin, server and CDN that conflict.

How to check your headers

Use my HTTP header checker on a page and on one of its images, then compare the cache lines. Check speed results with the PageSpeed checker.

Where caching fits in speed work

Caching works alongside hosting and a CDN, as covered in hosting, caching and CDN for speed.

Frequently asked questions

Next steps

For a 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