Before Yahoo announced their plans to buy Tumblr for $1.1 billion, some analysts wondered if and to what end the fact that Tumblr is full of porn would affect that decision. Apparently, not too much, as Yahoo CEO Marissa Mayer made the thing official this morning. Still, some Tumblr users worry that in an attempt to clean up their new acquisition, Yahoo may go to war on Tumblr’s vibrant adult community.

But just how much porn is on Tumblr? A lot. We know that. But until now, we’ve never had any real figures. Data from SimilarGroup, obtained by TechCrunch, suggests that more than 11% of Tumblr’s top 200,000 domains are very NSFW.

The analysis looked at the 200,000 most-popular blogs on Tumblr and found that 22,775 would be considered “adult.” That works out to 11.4%. Yes, Marissa Mayer just bought a network that’s more than 1/10th porn.

SimilarGroup also looked at traffic to said adult Tumblr blogs and found that they account for 16.6% of Tumblr’s traffic. Not only that, but just over 22% of referral traffic to Tumblr comes from adult websites. 8% of outbound Tumblr referrals lead straight to porn.

Tumblr has a porn empire. Is that going to be a problem for Yahoo?

Maybe, but Mayer isn’t suggesting that quite yet.

“The width and breadth of content on Tumblr is what’s exciting and has allowed it to reach more users,” said Mayer this morning when asked about Tumblr’s “not-brand-safe” content (read: porn).

In order to deal with it, Mayer said that Yahoo would need to have good targeting for ads. Maybe that will be the strategy going forward – smart ad targeting that will make sure that the porny bits stay away from the non-porny bits. But this study proves that adult content isn’t just a fringe element of Tumblr’s massive blog network – it’s a big part of it.