Things are on the up for US billionaires with more than half of them adding to their net worth in a year which once again saw Bill Gates as the richest of them all, Forbes magazine said.

Mr Gates, founder of Microsoft, was the richest person in the United States for the 17th year in a row with an estimated worth of $US54 billion, up $US4 billion from last year.

Second on the list was investor Warren Buffett, worth $US45 billion, up $US5 billion.

Oracle founder Larry Ellison's net worth was unchanged, in third place, at $US27 billion.

Christy Walton, heir to Walmart founder Sam Walton's fortune, was in fourth place with $US24 billion, up $US2.5 billion.

In stark contrast to the 2009 Forbes 400 list, 217 of the tycoons are now worth more than they were a year ago. Just 84 of the billionaires had a decrease in wealth.

In 2009 a whopping 314 members of the exclusive club had experienced a decrease in their wealth.

This year the top 10 gained $US24.9 billion. The same group had shed almost $US40 billion last year.

The Forbes 400 are worth a combined $US1.37 trillion, compared with $US1.27 trillion last year.

There are no new entries or drop-offs in the top 10 but the list shuffled.

The Koch brothers, heads of energy and manufacturing giant Koch Industries, now rank above three other members of the Walton family and New York Mayor Michael Bloomberg, founder of news and information company Bloomberg LP, who round out the top 10.

Facebook's Mark Zuckerberg, number 35, saw the largest relative increase, more than tripling his fortune to $US6.9 billion from $US2 billion.

Facebook co-founders Dustin Moskovitz and Eduardo Saverin join the list with fortunes of $US1.4 billion and $US1.15 billion respectively.

Born eight days after Mr Zuckerberg, 26-year-old Mr Moskovitz is the world's youngest billionaire.

The oldest tycoon on the list is David Rockefeller Sr, 95, worth $US2.4 billion.

Media magnate Oprah Winfrey comes in at number 130 and is one of 42 women on the list.

- Reuters