How do the world's greatest givers stack up?

We looked to answer that question when we created a list of the 20 most generous people in the world with our partners at Wealth-X, a firm that does research and valuations on ultra-high net worth individuals.

Everybody on our list created a billion-dollar fortune during their career, including Bill Gates and Warren Buffett, who started the Giving Pledge to encourage billionaires to give away half or more of their wealth. Some have donated so much that they're no longer billionaires.

We ranked them based on their total charitable giving to date. All told, these 20 philanthropists have given away nearly $110 billion.