Google has always been reticent to reveal just how much the advertising arm of YouTube generates but eMarketer has taken a leap and are forecasting some big numbers for 2013. They predict that YouTube’s gross ad revenues will rise 51.4% to $5.6 billion in 2013, accounting for 11.1% of Google’s total income. Even after YouTube has paid out a share of the profits to content ad partners (they keep a 45% cut of advertising), their net revenue is still expected to be around $1.96 billion this year, up 65.5% compared to 2012’s $1.18 billion.

eMarketer are also estimating that YouTube’s net ad revenue in the U.S. will reach $1.08bn in 2013, with $850m coming from video ads. That would give YouTube a 20.5% share of all U.S. video advertising revenues for the year, a market worth $4.15 billion.

Google bought YouTube for $1.65bn in 2006. It has since grown to be the second largest search engine in the world (after Google) and the largest video portal. YouTube confirmed today that 80% of views come from outside the U.S.