Gates ranked richest American by Forbes

Microsoft co-founder Bill Gates has topped Forbes' list of the 400 richest Americans for the 21st straight year.

The list, released on Monday, was largely unchanged for 2014 and showed the rich getting richer. The combined wealth of those on the list rose 13 per cent to $US2.29 trillion ($A2.48 trillion), helped by a stronger US stock market.

Gates' net worth totalled $US81 billion, up $US9 billion from 2013.

Investor Warren Buffett, the head of Berkshire Hathaway Inc, remained in second place at $US67 billion. Oracle Corp co-founder Larry Ellison also kept his No. 3 spot with $US50 billion.

Brothers Charles and David Koch, co-owners of Koch Industries Inc, stay tied for fourth with $US42 billion each.

There were 27 new members of the list, including WhatsApp co-founder Jan Koum in the 62nd spot. Facebook announced plans to buy the mobile messaging app for $US19 billion in February.

The biggest gainer is Facebook CEO Mark Zuckerberg, No. 11, whose net worth grew $US15 billion since 2013's list to $US34 billion.

The net worth of America's wealthiest people has risen in the years since the financial crisis, widening the gap between the exceptionally well-to-do and the rest of the country. The average net worth of a Forbes 400 member is $US5.7 billion, up from $US5 billion in 2013.

TOP 10 PEOPLE ON 2014'S FORBES 400 LIST:

* Bill Gates, $US81 billion

* Warren Buffett, $US67 billion

* Larry Ellison, $US50 billion

* Charles Koch, $US42 billion

* David Koch, $US42 billion

* Christy Walton & family, $US38 billion

* Jim Walton, $US36 billion

* Michael Bloomberg, $US35 billion

* Alice Walton, $US34.9 billion

* S. Robson Walton, $US34.8 billion