That's $13.7 billion with a B:

In a conversation at the GMIC mobile conference this week, Y Combinator co-founder Paul Graham gave the most recent stats on the seed stage incubator. Of the 511 companies that had passed through YC prior to its most recent Summer 2013 class, 306 had valuations tied to them. The total value of those companies is now $13.7 billion, up $2 billion since Graham’s last update on the number in June.

At the 500 Startups PreMoney conference in June, Graham told me that there were 285 YC companies with post-money valuations, compared to 306 now. (Check out the video below.) At that time, the total amount that YC companies had raised was $1.7 billion.

Those statements echoed data that Graham had previously shared on Hacker News.

A number of YC companies have also raised additional rounds of funding in the past several months. Twitch (formerly Justin.tv) raised $20 million, E La Carte raised $13.5 million, Clustrix raised $10 million, and Instacart brought on $8.5 million, among others.

And, finally, there have been a few notable exits since June. YC alum Xobni was acquired by Yahoo, Cue was bought by Apple, and Lanyrd got purchased by Eventbrite.

But the bulk of the increase probably comes from a major change at the top. Like many private equity investors, the lion’s share of Y Combinator’s investment value is concentrated in just a few high-flying startups. Back in June, Graham told me that the top 10 companies accounted for $8.6 billion of its $11.6 billion total valuation.

Y Combinator wouldn’t break out what the valuation of that top 10 is right now, but it seems likely that some major change in the value of those companies is the biggest reason for the increase. 

Airbnb and Dropbox and Stripe, oh my!

April 2014. Dropbox + Airbnb are worth $10 billion each. Geez.