A costly presidential campaign, a lawsuit and a rocky real estate market have seen US President Donald Trump take a US$600-million hit to his personal fortune, Forbes magazine says.

Forbes, which maintains a ranking of the 400 richest Americans based on public records of their holdings and assets, estimated Mr Trump’s fortune at $US3.1 billion ($A4 billion) in one of the few reductions in wealth on the list.

While most people on the Forbes rankings increased their wealth thanks to strong gains in the stock market – Mr Trump dropped from 156th on the list last year to to 248th in the latest estimates.

Most of the decline in Mr Trump’s wealth was attributed to declining values of his high-end Manhattan real estate holdings, Forbes reported.

“A tough New York real estate market, particularly for retail locations; a costly lawsuit and an expensive presidential campaign all contributed to the declining fortune of the 45th President,” Forbes said.

Trump spent $US66 million on his presidential campaign and $US25 million to settle a lawsuit regarding his Trump University, which offered a course in real estate investing that New York state authorities alleged was fraudulent.

While most of Trump’s real estate is in New York City, a few of his hotels and other buildings in the western United States gained in estimated valued.

Mr Trump boasted of his personal wealth when he  announced in he was running for the White House in 2015, saying he had $US9.2 billion in assets and $US8.7 billion in net worth.

“I’m not doing that to brag,” he said at the time. “Because you know what? I don’t have to brag.”

In June, The Atlantic cited filings by U.S. Office of Government Ethics as saying Mr Trump had received tens of millions of dollars in income from the golf courses and resorts whose profile he boosted during frequent visits since taking office.

Microsoft boss Bill Gates led the Forbes 400 for the 24th consecutive year, with an estimated fortune of $89 billion, $8 billion higher than estimated last year.

Of the top 10, all of whom were men. Facebook founder Mark Zuckerberg saw his net wealth grow the most, adding $15.5 billion over 12 months to $71 billion.

Wal-Mart Stores heir Alice Walton was the highest placed woman with a net worth of $38.2 billion, ranking at No 13 – the same position as last year.

The combined value of the top 400 hit $2.7 trillion, $300 billion higher than last year. New entrants now need at least $2 billion to make the list.

– With AAP