The billionaire has given away more than $46 billion since 2000.
The Chronicle of Philanthropy calculated the giving since 2000 of the 10 richest people in America, and calculated their charity as a share of their total wealth.
Buffett topped the list, giving away more than $46 billion since 2000. That worked out to 71 percent of his $65.5 billion fortune.
The billionaires' wealth levels were from Forbes as of the end of 2016. Many of those fortunes are now higher. The Chronicle's calculations were based on the Philanthropy 50 reports, data from the IRS and information supplied by some of the donors.
Gates has given away $18 billion, or 22 percent of his $81 billion fortune, to charity since 2000. Yet he and Melinda Gates have given away more than twice that since they created their foundation in 1994. And last month, he donated another $4.6 billion in Microsoft stock.
Michael Bloomberg ranked third, with 10 percent of his $45 billion fortune going to charity, That too is understated, since it doesn't count his giving before 2000 or possible anonymous gifts.
The Google guys, Sergey Brin and Larry Page , rank fourth and fifth, respectively, with Brin giving $37.5 billion – or 6 percent of his fortune, and Page giving $38.5 billion, or 4 percent of his fortune.
Jeff Bezos ranked last of the top 10. While he briefly became the world's richest man this summer, he has given less than 0.1 percent of his net worth to charity. The Chronicle said his total giving since 2000 is $68 million, compared with a fortune of $67 billion as of 2016 and more than $80 billion today.
Neither Bezos nor Amazon responded to requests for comment. But Bezos seems to be working on ramping up his philanthropic plans.
In June he took to Twitter to ask users for suggestions on how to pursue a giving strategy that was "more at the intersection of urgent need and lasting impact." In August, he thanked the public for all their ideas and said the responses have "already changed my thinking about how to approach this." He added that there would be "more to come."