Donald Trump is $1 billion poorer since becoming president, according to Forbes latest list of the world's billionaires.

The magazine's newest ranking, released Monday, estimated that Trump's net worth has fallen from $4.5 billion last year to $3.5 billion today, dropping him 220 spots on the magazine's list of billionaires, from No. 336 last year to No. 544.

Although Trump resigned from his businesses when he entered office, his slide isn't due to his having sold any of his businesses because he still owns them.

Rather, Trump's net worth is down because about 40 percent of his wealth comes from real estate he owns in midtown Manhattan, including Trump Tower and eight other buildings within about a mile.

"Midtown Manhattan real estate is down; therefore, so is Donald Trump's fortune," the magazine said in a statement.

Trump has disputed Forbes' estimates in the past, particularly regarding the worth of his brand which -- unlike real estate -- is more difficult to value. During the campaign he said that he was worth $8.7 billion and said last year in a personal financial disclosure form filed with the federal government that his net worth was "in excess of $10 billion."

According to its methodology, Forbes bases its billionaires list on a "snapshot of wealth" taken on Feb. 17 by valuing assets using stock prices and international exchange rates. It acknowledges some fortunes may have changed considerably even since then and that sorting out one's net worth is more complex when it is tied to a family fortune.

"We list individuals rather than multigenerational families who share large fortunes, though we include wealth belonging to a billionaire's spouse and children if that person is the founder of the fortune," the magazine says.

All told, Forbes said it was a record year to be rich, as the number of billionaires it counted climbed 13 percent to 2,043 from 1,810 last year. The magazine attributed the rise in part to surging stock markets and higher commodity prices.

Microsoft founder Bill Gates leads the list for the fourth year in a row, with a fortune of $86 billion, $11 billion more than last year. Investor Warren Buffett claimed the number two spot and Amazon chief executive Jeffrey P. Bezos was third. (Bezos owns The Washington Post).

Forbes has tracked Trump's net worth since 1982 and last year he attained a personal high on the list with a ranking of 121. But since then Midtown New York real estate values have been sliding, which is a large chunk of his fortune.

A recent breakdown of the president's wealth valued his New York City real estate at $1.7 billion, his real estate outside New York at $630 million, his golf clubs at $620 million, his cash and personal assets at $270 million, and his brand businesses at $230 million.

As the New York's real estate market has slowed, Forbes began downgrading Trump's net value last year, saying in October that his net worth was down to $3.7 billion.

Forbes isn't the only publication tracking Trump's wealth to conclude that he is likely worth much less than he says he is. Fortune and Bloomberg have previously offered their own lower estimates.