Pioneering American philanthropist Andrew Carnegie once set the standard for giving back: "No man can become rich without himself enriching others," he said. "The man who dies rich dies disgraced."

Scores of the world's wealthiest people have taken to his philosophy, donating their riches to hundreds of causes.

We worked with our friends at Wealth-X, a firm that does research and valuations on ultra-high net worth individuals, to compile a list of the 20 most generous people in the world, ranked by total charitable giving to date.

Since you need to have a lot to give a lot, Wealth-X also determined each person's Generosity Index, which is the ratio of their lifetime donations to their current net worth. Some people, like retail magnate Chuck Feeney, known as the "James Bond of philanthropy," have indices over 100% because they have given away more money than they currently have.

With a total combined donation amount of $106.8 billion, these 20 individuals are the top philanthropists in the world.