The bar is getting higher every year. This year, one will need 10-figure asset to be included in the top 400 wealthiest Americans list, as compiled by Forbes magazine.

That's how much it took this year to make Forbes magazine's annual list of the 400 richest Americans, the first time everyone on the list was worth at least $1 billion.

Bill Gates (right) and Warren Buffett topped this year's list of the 400 richest Americans.

The collective net worth of the country's wealthiest people rose $120 billion to $1.25 trillion.

Bill Gates, founder of software leader Microsoft Corp. (Charts), topped the list as usual, with a $53 billion net worth, while his friend and bridge partner Warren Buffett, who runs diversified holding company Berkshire Hathaway Inc. (Charts), was second with $46 billion.

Gates and Buffett are also the world's richest people, Forbes has said. Mexican tycoon Carlos Slim ranked third with $30 billion, the magazine said in March.

Buffett in June agreed to donate 85 percent of his fortune, valued at the time at $37 billion, to the Bill & Melinda Gates Foundation and four family charities.

Third place among Americans went to casino magnate Sheldon Adelson, the chief executive of Las Vegas Sands Corp. (Charts), with $20.5 billion - up $9 billion from a year earlier.

Forbes estimated he has made nearly $1 million an hour over the last two years. Shares of Las Vegas Sands have more than doubled since going public in Dec. 2004.