1. Bill Gates $79.2bn (Microsoft)
3. Warren Buffett $72.7bn (Global investor)
4. Amancio Ortega $64.5bn (Zara and other fashion chains)
5. Larry Ellison $54.3bn (Oracle data storage technology)
6. Charles Koch $42.9bn (Industrialist)
7. David Koch $42.9bn (Industrialist)
8. Christy Walton $41.7bn (Walmart retail giant)
9. Jim Walton $40.6bn (Walmart retail giant)
10. Liliane Bettencourt $40.1bn (L'Oreal cosmetics firm)
The funds required to make it into the top 20 in the list have fallen by $2bn to $29bn this year.
The founders and chief executives of US technology firms dominated the list again, accounting for six of the top 20 richest men in the world.
The first woman on the rich list is Christy Walton, the widow of John Walton, one of the heirs to the Wal-Mart retail empire.
She has been declared the richest woman in the world for five of the last six years by Forbes.
Facebook's Mark Zuckerberg moves up five places in the rankings to number 16, with a net worth of $33.4bn. This is the first time he has been included in the list of the world's 20 richest people.
The youngest billionaire in the world is 24 year-old Evan Spiegel, the co-founder of messaging app Snapchat, with a fortune of $1.5bn.
Silicon Valley has produced 23 new billionaires this year including co-founders of car-hailing service Uber, Travis Kalanick and Garrett Camp, and their first employee, Ryan Graves.
Elizabeth Holmes, the founder of blood-testing firm Theranos, has debuted on the global list as the youngest self-made woman at age 31, with a fortune of $4.5bn.