Ever been wondered why there's a lot of conflicting advice about longest page title allowed in search results pages (SERPS)?
 
The short answer is different search engines have different limits and these limits keep changing. The current official guidelines, as of April 2013, are:
  • W3C recommends a maximum of 64 characters for page titles.
  • Bing recommends a title around 65 characters long.
  • Yahoo recommends a maximum title length of 67 characters (although this advice is obsolete since Bing now supplies Yahoo's search results) 
  • Google don't have any guidance for content publishers, and now limits the title on the visible (typographic) width displayed in the browser. For example, both these titles are 41 characters long, but one is much wider when displayed in a browser:
    • WWW2012 - WORLD WIDE WEB CONFERENCE 2012
    • HTML 5 - proper use of the title element
 
To see how these play out, we searched for "patent 7143296" in the top three search engines:
  • Bing - some titles up to 71 characters are displayed in full, but titles with a large visible width (for examples, titles all in CAPS) are truncated at a much lower limit (around than 50 characters in some cases)
  • Google - some titles up to 71 characters are displayed in full, but titles with a large visible width (for examples, titles all in CAPS) are truncated at a much lower limit (around than 40 characters in some cases)
  • Yahoo - results are now provided by Bing (since mid-2010)
 
This table shows how the maximum title length in the major search engines has changed over time:
YearGoogleBingYahoo
2007 66 chars 65 chars 120 chars
2008 66 chars 65 chars 72 chars
2009 71 chars 65 chars 72 chars
2010 71 chars 67 chars 65 chars
2011 71 chars 70 chars Uses Bing results, so inherits Bing limits
2012 Visible width up to 70 chars 70 chars  
2013 Visible width up to 70 chars Visible width up to 71 chars  
These limits have changed over time, and are likely to keep on changing as the search engines store more pages and optimize retrieval speeds. It's worth noting that the data volumes to store page titles are not trivial. As of Apr 2010 Google had indexed 13 billion web pages, so that's 13 billion page titles to store, with multiple redundant copies needed to provide backup in the event of disk failure.
 

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Update: Orginally posted April 2010, updated April 2013