Startup and Private Valuations: Is Stack Exchange valued above $1 billion yet?

I have no inside knowledge of any of this. However, let's play with numbers and reasonable assumptions and see where that gets us.

Stack Exchange raised 3 institutional rounds. A of 6, B of 12 and C of 40. Let's do some math based on non-anomaly startups, with what are probably typical ranges of VC ownership, based-on stage.

In the simplest form, let's look at where VCs like to be. A rounds usually cost 20-35%, as per the typical VC model. For simplification purposes, let's say rounds are priced based on how much founders are willing to sell 20-35% of their company for. So, the A round places s

I have no inside knowledge of any of this. However, let's play with numbers and reasonable assumptions and see where that gets us.

Stack Exchange raised 3 institutional rounds. A of 6, B of 12 and C of 40. Let's do some math based on non-anomaly startups, with what are probably typical ranges of VC ownership, based-on stage.

In the simplest form, let's look at where VCs like to be. A rounds usually cost 20-35%, as per the typical VC model. For simplification purposes, let's say rounds are priced based on how much founders are willing to sell 20-35% of their company for. So, the A round places stack exchange valuation $18 - $30 million post.

Then, they raised another 12 million, where VCs usually like to own 10-20%. So, that $12 million would have a $60-$120 post.

The C round investors would like to own 5-10% of the company. $40 million would mean a $400-$800 million post.

Let's sanity check this. Let's pretend Jeff and Joel were equal partners with a 15% employee pool. That might mean that they, on a combined basis, still own 65.4-49.05% of the business, in this oversimplification. On the high side, that seems a little high, but I don't know enough about the business. All in all, it doesn't offend the sensibilities that SE could be worth $400-$800 million, but my gut is telling me it's closer to $400.

What is a stack exchange? Boston' trading floor?