The surging price of Microsoft shares returned US tech tycoon Bill Gates back to the top of Forbes’s world’s billionaires list, with his $US76 billion ($A85.61 billion) beating out Mexico’s Carlos Slim’s $US72 billion.

The annual list, released on Monday, counted 1645 men and women as billionaires, with an average wealth of $US4.5 billion and a collective wealth of $US6.4 trillion, up $US1 trillion from a year ago.

Gates, co-founder of the US software firm, showed his staying power at the top – the world’s richest man for 15 of the past 20 years, according to Forbes – despite spending recent years giving away large sums of money to global health and anti-poverty programs.

Gates owns about 4.4 per cent of Microsoft, making up less than 20 per cent of his total fortune.

But the company’s share price has risen 25 per cent over the past year, and, along with gains in other assets, he has added $US7 billion to his fortune since a year ago, according to Forbes.

Slim, with a hand in everything from telecommunications (America Movil) to mining, finance and industry (Grupo Carso), to retailing and real estate across the Americas region, was worth $US1 billion less than a year ago, hit in part by sagging markets in South America.

In third was Spain’s Amancio Ortega, whose pockets have filled with profits from fashion: his hugely successful Inditex garment empire, parent of popular chains Zara, Pull & Bear, and Bershka.

More recently, Ortega heavily invested in real estate in Europe and the United States; his worth was put at $US64 billion.

A familiar cast of mega-wealthy filled out the rest of the top 10: US investment guru Warren Buffett ($US58.2 billion); software group Oracle’s founder and chief executive Larry Ellison ($US48 billion); US industrialists and brothers Charles and David Koch (each with $US40 billion); Las Vegas casino king Sheldon Adelson ($US38 billion); Walmart heiress Christy Walton ($US36.7 billion) and her brother Jim Walton ($US34.7 billion).

Together the $US507 billion held by the top 10 is larger than the entire size of the economy of Norway, or Belgium or Poland in 2012.