Introduction to OpenCalais

The free OpenCalais service and open API is the fastest way to tag the people, places, facts and events in your content.  It can help you improve your SEO, increase your reader engagement, create search-engine-friendly ‘topic hubs’ and streamline content operations – saving you time and money.

OpenCalais is free to use in both commercial and non-commercial settings, but can only be used on public content (don’t run your confidential or competitive company information through it!). OpenCalais does not keep a copy of your content, but it does keep a copy of the metadata it extracts there from.

To repeat, OpenCalais is not a private service, and there is no secure, enterprise version that you can buy to operate behind a firewall. It is your responsibility to police the content that you submit, so make sure you are comfortable with our Terms of Service (TOS) before you jump in.

You can process up to 50,000 documents per day (blog posts, news stories, Web pages, etc.) free of charge.  If you need to process more than that – say you are an aggregator or a media monitoring service – then see this page to learn about Calais Professional. We offer a very affordable license.

OpenCalais’ early adopters include CBS Interactive / CNET, Huffington Post, Slate, Al Jazeera, The New Republic, The White House and more. Already more than 30,000 developers have signed up, and more than 50 publishers and 75 entrepreneurs are using the free service to help build their businesses.

You can read about the pioneering work of these publishers, entrepreneurs and developers here.

To get started, scroll to the bottom section of this page. To build OpenCalais into an existing site or publishing platform (CMS), you will need to work with your developers. 

Why OpenCalais Matters

The reason OpenCalais – and so-called “Web 3.0” in general (concepts like the Semantic Web, Linked Data, etc.) – are important is that these technologies make it easy to automatically connect the people, companies and concepts in your content to the related content on the rest of the Web.

So when you’re writing about Twitter's new Promoted Tweets offering, you can be automatically connected to the other stories about Twitter's Promoted Tweets without having to embed links along the way.

Creating standardized metadata is about revealing the connections between people, companies, concepts and events and forging connections to relevant and related content automatically – streamlining your editorial processes and saving you time and expense along the way.

Ultimately, this new set of technologies is driving the next wave of innovation in digital media, and has the potential to inspire yet another “boom” similar to what we saw with SEO and SEM.

As innovators like MediaCloud, and (three more OpenCalais early adopters) lead the way, we will see more and more publishers, entrepreneurs and developers learning how to work with the new tools.

Why OpenCalais is Free

OpenCalais is a strategic initiative from Thomson Reuters to support the interoperability of content across the digital landscape.

Our goal with this initiative is not to make money, but rather to make it easy for folks to categorize and tag their content in a uniform and consistent fashion that complies with Semantic Web standards.

Offering a de-facto standard for making content interoperable in this fashion ultimately benefits Thomson Reuters, as it enables us to track themes, memes and trends on the Web, and to potentially do things like link out to relevant content that helps provide context to our readers, customers and other constituents.

The value exchange to us is in the metadata, in the growing body of interoperable content, and in the ability to support innovation, experimentation and the continued evolution of the Web.

We are fully committed to OpenCalais, and we offer the API for both commercial and non-commercial purposes precisely to inspire creativity and enterprise by a new wave of innovators and entrepreneurs.

There is no plan to someday "drop the other shoe" and charge folks for the basic service. 

How That Helps You

Ultimately, OpenCalais provides start-up entrepreneurs, publishers and institutions like libraries, museums and universities with the ability to forge a path into the future of digital media.

In addition to being a source for inspiration and innovation, the free service is a time- and cost-saving tool. It’s the fastest way to tag the people, places, facts and events in your content – and the easiest way to get the metadata you need in order to get truly creative with your user experience and user interface.

As our partners will attest, OpenCalais can help you:

  • Improve your SEO as well as search and navigation within your own site
  • Enhance your content with free open data assets from the Linked Data cloud
  • Increase your reader engagement with greater personalization, ‘related reading’ sidebars, ‘more by this author’ widgets, search-engine-friendly ‘topic hubs’ and more.
  • Streamline your editorial processes and content operations to save both time and money.

We are also thrilled that our price point has made OpenCalais a favorite with open source platforms like the Drupal Community, public research initiatives like DocumentCloud, and Ushahidi / Swift River – a leading real-time platform for crisis communications used in disaster recovery efforts around the world.

How to Get Started

There are a number of easy ways to get started.

 If you are on WordPress, try the easy-to-install Calais Tagaroo plug-in, which automatically tags your content as you type. It can also fetch images from Flickr and videos from Google Video, which you can select for inclusion in your post, or disregard.

 *Note: You must have a hosted site – or your own server – where you have access to install WordPress plugins in order to use Tagaroo. Blogs hosted on won’t work with Tagaroo.

If you want to manipulate your search results appearance in Google and Yahoo!, you can try Calais Marmoset, which is simple javacode you embed in your site pages. Marmoset will collect the metadata from your page (in the form of RDFa) and hand it over to Google Rich Snippets> and Yahoo! Search Monkey so that you can customize the way your search results appear.



 If you want to extract metadata from Web pages using URLs, then try our Semantic Proxy service at


If you are using the popular open source platform Drupal, you can find a complete Calais Collection of modules for easy integration.

If you are building a new site from the ground up, consider usingOpenPublish, a free Content Management System based on Drupal. 

OpenPublish bakes-in OpenCalais from the ground up to “semantify” your site and automate the creation of ‘related reading’ widgets, ‘topic hubs’ and more. OpenPublish comes from Phase2 Technology and Thomson Reuters, and now comes with the option of fee-based support and hosting from Drupal founder Dries Buytaert’s company, Acquia.

To build OpenCalais into an existing site or publishing platform (CMS), you will need to work with your developers. Developers can find the resources and information they need on the OpenCalais documentation pages, and – for general education on the Semantic Web - at Semantic Universe.



Heard the buzz about OpenCalais but still have some fundamental questions? Start with this blog post for a big picture overview.

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