On a recent Google Webmaster Help video loaded onto YouTube, Google explained how accurate site:domain.com is for determining the number of pages in the Google index.
Mr Matt Cutts explains how site: queries are not accurate Google simply tries to estimate the amount of pages in an index and does not give an exact figure. In fact it is only accurate for the first three digits, something revealed by Cutts in this video.
For ranking purposes it is not always a great idea to check the site: query as it does not return accurate results. However, for an idea of a more general rank it can be useful.
Here are a couple of examples:
For a Webandrank site: query it returns 119 results.
For a Apple site:query you get 11,500,000 results. In site: queries you will only ever see the first three numbers the rest will always be 0.
For a further example a Facebook site: query will return 1,330,000,000 results, again showing that after the first three numbers it is 0’s only.
In the video Mr Matt goes on to explain that ‘Yes we do have a pretty good estimate but no it’s not perfect, we know it’s not perfect and rather than putting a ton of engineering resources into making site: queries incredibly accurate we spend those engineering cycles on other stuff and then only show three significant digits so people have an idea of the total number of pages but it’s not exact’.
This is definitely a useful piece of knowledge for any SEO firms out there who use site: queries in understanding rank authority. Google does what it can but it looks as if we will not be getting accurate data result anytime soon unless you use the Google Webmaster.