The net worth of the richest Americans grew by 13 per cent in the past year to $1.7-trillion (U.S.), Forbes magazine said on Wednesday, and a familiar cast of characters once again populated the top of the magazine's annual list of the U.S. uber-elite, including Bill Gates, Warren Buffett, Larry Ellison and the Koch brothers. The average net worth of the 400 wealthiest Americans rose to a record $4.2-billion, the magazine said. Collectively, this group's net worth is the equivalent of one-eighth of the entire U.S. economy, which stood at $13.56-trillion in real terms according to the latest government data.