What an XML sitemap is, when you need one, and which tags matter

A sitemap is a file that tells search engines which pages and files on a site exist and how they relate. Google reads it to crawl the site more efficiently and to learn which pages matter, when a page was last updated, and whether it exists in other languages.

Variants carry extra detail. A video sitemap can include running time and ratings, an image sitemap shows where images sit on a page, and a news sitemap adds a headline and publication date.

A sitemap is a request to be found, not a promise. Google says it helps Google discover URLs but doesn't promise that every listed item will be crawled and indexed.

Google says that if a site's pages are properly linked, it can usually find most of the site without one. It names three cases where a sitemap helps. One is a large site, where it's hard to make sure every page is linked from at least one other page. Another is a new site with few links from other sites. The third is a site with a lot of video and images, or one that appears in Google News.

You may not need one if the site has 500 pages or fewer that you want in search results, and every important page can be reached by following links from the home page. The site should also have little media or news content to show in Search. Google adds that in most cases a site still benefits from having one. If the site runs on WordPress, Wix or Blogger, check first, because the CMS may already publish one.

The file opens with <urlset> and the protocol namespace, then has one <url> entry per page, each with a <loc> holding the full address. Those are the only required parts. Each <loc> must start with the protocol, such as https, and be less than 2,048 characters, and every URL in one file must come from a single host.

The file must be UTF-8. Ampersands, quotes and angle brackets inside values must be entity-escaped, so an & in a query string is written &.

<lastmod> is the one worth getting right. It must be the date the page itself last changed, in W3C Datetime format such as YYYY-MM-DD, not the date the sitemap was generated. A sitemap that stamps every URL with today's date tells crawlers nothing.

<changefreq> and <priority> carry less weight. The protocol calls changefreq a hint and not a command, so crawlers may visit an hourly page less often and a yearly page more often. Priority runs from 0.0 to 1.0 with a default of 0.5, only compares pages within your own site, and isn't likely to change where your pages rank. Setting every URL to high priority doesn't help. Support for all optional tags varies by search engine.