The Organic traffic metric in Ahrefs is an estimation of how many clicks from google your target website, subfolder or URL gets each month.

NOTE: Remember to treat the organic traffic estimations in Ahrefs as precisely that: estimations.

While these estimates don’t, and can’t, show you exactly how much organic traffic a website gets, they work incredibly well for comparison. For example, it’s fantastic for learning if your competitors’ websites get more or less organic search traffic than your own.

Although sometimes our estimations can have discrepancy, it is highly consistent. As such, you can use the following formula to estimate your competitor's traffic:

The number that Ahrefs shows is an estimate, not the actual search traffic data of a website. It’s not possible for all the variables in our traffic estimation formula to be 100% accurate. Learn why

While Ahrefs shows an estimate of the search traffic of a website, Google Analytics shows total traffic (direct, paid, social, etc). So these two figures are likely to be quite different.

For most keywords, we use the third-party data sources to estimate averaged CTR values. This is then applied based on SERP position to estimate the amount of clicks each google result receives.

Overall clicks per search can be found in Keywords Explorer:
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We will, of course, tell you that we have the best quality data ;)
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On a more serious note, we once tried all the leading total traffic estimation tools and did some experimenting to see which was the most accurate. Read all about it