Through Dec. 11, Buffett's net worth—at least on paper—shot up by $12.7 billion to $59.1 billion in 2013 from $46.4 billion at the start of the year, according to Wealth-X, the wealth research firm. That works out to a paper gain of $1.5 million an hour.

Warren Buffett gained more wealth than any other U.S. billionaire, adding $37 million a day, according to one study.

Gates is still the richest man in America. But he was the number two gainer in dollar terms this year, with his paper wealth soaring by $11.5 billion to $72.6 billion. Casino tycoon Sheldon Adelson was third, with an $11.4 billion gain to $35.3 billion.

The gains were fueled almost entirely by rising stock prices, which boosted the value of company shares held by founders and corporate chiefs. And the wealth was spread throughout all industries, suggesting that the broad stock market gains have been the chief wealth driver this year—rather than industry-specific gains in tech, energy or other sectors.

(Read more: Millionaires become the 'family bank')

The top 10 billionaire gainers saw their wealth increase by a combined $101.8 billion—more than the GDP's of Puerto Rico or Slovakia.

Here is the full list.