Reach is an Alexa Web Search metric that measures the number of people who have the Alexa toolbar or extension installed and who saw your content. If your website has a Reach score of 5,000, this means that 5,000 people out of a sample of 1 million, who have the toolbar or extension installed, saw your site yesterday.
Use a summary chart to best visualize your Reach per Million Users data. This type of chart compares the current value to a previous time period.
This isn’t a perfect metric as it includes a skewed sample of users. But, used over time it's still a good way to understand trends and compare sites.
Data for Alexa's reach, rank, and search analytics are collected from millions of users who have the Alexa toolbar or extension installed. As a result, Alexa is able to collect, anonymize, and track web traffic over time.