Microsoft Corp. founder Bill Gates, who has gifted $28 billion to his charitable foundation, gave some simple advice to Australia’s richest man Andrew Forrest on his crusade to end modern slavery: find a metric to quantify it.

“Global modern slavery is hard to measure and Bill’s a measure kind of guy,” Forrest, 51, the founder of iron ore producer Fortescue Metals Group Ltd. who formed the Walk Free anti-slavery charity last year, said in a phone interview. In McKinsey & Co. “management speak, if you can’t measure it, it doesn’t exist,” he said.

Spurred by Gates, the world’s second-richest person, Forrest plans to publish the Global Index of Modern Slavery in August, ranking the existence and efforts to fight involuntary servitude in 160 countries. Slavery generates about $32 billion in profits a year, according to the International Labour Organisation, with activities such as sex trafficking and mining so-called conflict minerals used in electronics including laptops and mobile phones.

“When it comes to the superwealthy, they hide behind the parapet sometimes with tough social issues,” said Simon McKeon, chairman of the Melbourne office of Macquarie Group Ltd., Australia’s biggest investment bank, who was named 2011 Australian of the Year for his charitable efforts. “When someone like Andrew says I don’t feel right that we still have slavery, then that’s music to my ears.”

Branson Joins

Forrest, who has a $4.3 billion fortune, according to the Bloomberg Billionaires Index, and Virgin Group Ltd. Chairman Richard Branson in December officially started Walk Free in Myanmar, urging 25 top global companies and governments to stop using forced labor.

“Richard has indicated that he will be fully supporting Walk Free,” Forrest said. “He’ll be joining in and helping in any way possible.”

Branson and Forrest were among 12 new signatories who in February announced they had joined the Giving Pledge initiative started by Warren Buffett, chairman of Berkshire Hathaway Inc., Gates and his wife, Melinda. Forrest and his wife Nicola pledged to donate A$2.5 billion ($2.6 billion).

“Andrew is quite an individual, and I can see similarities between him and Branson,” said Peter Rudd, resources and mining manager at Altitude Private Wealth in Melbourne who has more than 20 years of experience in the sector. “If they can achieve something on the charitable side, all the better.”

Fortescue fell 0.8 percent to A$3.95 at the close of trading in Sydney today. They’ve fallen 15 percent this year. The benchmark S&P/ASX 200 Index gained 0.8 percent.