Warren Buffett was once the richest man in the world. Then he gave it all away

Warren Buffett could have been the world's richest man if he didn't make one costly decision almost 20 years ago.

The 93-year-old's net worth is today estimated to be around $US128.9 billion ($189.9 billion) making him the world's 10th richest person, according to Forbes.

But the Berkshire Hathaway chief executive could have been sitting on a fortune almost three times that amount if he hadn't made a historic pledge in 2006 to give all of his money away.

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Over the course of 18 years, Buffett has donated more than $81.6 billion worth of his Berkshire Hathaway shares to charity.

In fact, a late June donation of $7.8 billion bumped Buffet down two spots on Forbes' rich list, giving him his lowest ranking in decades.

At the time of the Buffett's announcement, he owned 474,998 class A shares, then worth around $63.8 billion.

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If he'd kept hold of the stock, his net worth would be around $293 billion, outpacing the world's richest man Elon Musk by $60.8 billion, No.2 Jeff Bezos by $114.2 billion and No.3 Bernard Arnault by $151.3 billion.

Instead, Buffett has been working to give away almost 99 per cent of his fortune by making billions of dollars worth of donations to philanthropic foundations each year.

A large chunk of his fortune has gone towards a trust that funds the Bill and Melinda Gates Foundation - a non-profit focused on tackling poverty and strengthening healthcare in developing countries.

Buffett has also gifted billions to his three children - Susan, Howard and Peter - who manage their own charities, as well as the Susan Thompson Buffett Foundation, which was named after his late wife.

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He has since burned his Berkshire stake down to 207,963 class A shares with the intent to keep donating stock to the five foundations each year until he dies.

"This pledge will leave my lifestyle untouched and that of my children as well," Buffett wrote in his pledge letter.

"They have already received significant sums for their personal use and will receive more in the future.

"They live comfortable and productive lives. And I will continue to live in a manner that gives me everything that I could possibly want in life."

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