Skype's owners, led by private equity firm Silver Lake, are set to earn more than three times their investment, for a total capital gain of more than $5 billion, on the sale of Skype to Microsoft Corp, a source familiar with the situation said.

The gain is particularly large considering the short amount of time the investors have owned internet phone service Skype -- around 18 months -- since buying a majority stake from eBay. That is a short turnaround for private equity which typically invests with a time horizon of three to five years.