How do sites without access to a site's analytical data, determine a site's organic traffic?

[Deacyde]

I've recently used a organic traffic checker that showed you your traffic compared to each google algo update. I was interested in how they derived the organic traffic totals for each month, without having access to our site's google analytics? I've since compared the data to historical google analytics data and it's not wrong, isn't 100% match either but isn't far from fact.

So if they're predicting or making a guess, it's rather spot on, site crawlers and SERPs snapshots only provide so much info, I'm just wondering where they get the rest from and how?

[ChrisAshton]

Exactly where they get it will depend on which tool you're using but if they haven't got your GA login info, it's probably pulling from something like SEMrush or a similar tool.

From my understanding, these tools use reasonably accurate extrapolation to give you those numbers. For example, if they have 1 million people with their toolbar installed and tracking their usage data, they can then make some educated assumptions and suggest that if you're getting X number of visits 1 million people, your actual numbers will be approximately Y, based on a complex algorithm to factor in certain elements, I'm sure.