Many people try to wrap their heads around how Google determines what an “authoritative” website is, but the truth is that there is no single “authority metric”. Instead, Google uses a range of undisclosed metrics to calculate what’s authoritative and it may even vary depending on the query.
PageRank – the original authority metric
PageRank used to be Google’s single authority metric, which was based on backlinks to pages. Links to pages were counted and a PageRank score was calculated for that particular page. It didn’t just award pages with a lot of backlinks; it also attempted to calculate the importance of those links. For example, a page with only a few backlinks from other authoritative pages could have a stronger PageRank than a page with many more links from non-reputable sources.
But PageRank didn’t instantly mean a page would gain the top spot on Google; rather it was just a small part of Google’s ranking algorithm.
How is Authority calculated today?
Of course, the above is still relevant to how Google assesses a domain’s authority; links and content are some of the most important ranking factors. However, Google’s RankBrain system, which uses artificial intelligence, involves over 200 major ranking signals, so it is safe to say there isn’t just one major “authority” factor.
Google themselves have not provided any specifics on what exactly goes into calculating authority, but they have confirmed that authority is on a per-page basis as opposed to being site-wide or domain based. This is to avoid false assumptions about individual pages within popular domains; for example, blogs on Tumblr or WordPress aren’t automatically awarded rank based on the domain’s overall popularity and authority.
There are tools available that claim to calculate page and domain authority, but when it comes down to it, these aren’t Google’s own metrics. They may offer a guideline or guesstimate, but that’s all they are; how they think Google might score your page.