The index, compiled by the Walk Free Foundation (WFF), defined modern slavery to comprise of human trafficking, forced labour, and practices such as debt bondage, forced marriage, and the sale or exploitation of children; and found that slaves existed in all of the 162 countries surveyed.
"Today some people are still being born into hereditary slavery, a staggering but harsh reality, particularly in parts of West Africa and South Asia," the report said, as cited by Reuters.
"Other victims are captured or kidnapped before being sold or kept for exploitation, whether through 'marriage', unpaid labour on fishing boats, or as domestic workers. Others are tricked and lured into situations they cannot escape, with false promises of a good job or an education," the report added.
The countries with the highest prevalence of modern slavery were Mauritania (0.4 percent), Haiti (0.2 percent), Pakistan (0.012 percent), India (0.012 percent), Nepal (0.01 percent), Moldova (0.01 percent), Benin (0.01 percent), Ivory Coast (0.01 percent), Gambia (0.01 percent) and Gabon (0.01 percent).
In terms of total numbers, the countries with the most people in modern slavery were estimated to be India (13.95 million), followed by China (2.95 million) and Pakistan (2.1 million).
In India, "by far the largest proportion of this problem is the exploitation of Indian citizens within India itself, particularly through debt bondage and bonded labour", the report said.
"In Mauritania, kids are born into slavery," explained WFF chief executive Nick Grono to AFP, describing Mauritania as a nation with "deeply entrenched hereditary slavery", where "people in slavery may be bought and sold, rented out and given away as gifts".
The WFF's estimate of 29.8 million slaves worldwide is higher than other attempts to quantify modern slavery. The International Labour Organisation estimates that almost 21 million people are victims of forced labour.
The new survey has the backing of world figures including former US Secretary of State Hillary Clinton, ex-British Prime Minister Tony Blair.
Although Clinton admitted that the index was not perfect, it provided a starting point, she told the Associated Press.
"I urge leaders around the world to view this index as a call to action, and to stay focused on the work of responding to this crime," she said.
"Modern slavery is an issue that's getting an awful lot of attention. It's illegal everywhere and it's absolutely abhorrent," added Grono.
"I think once we start pointing out the scale of the problem on a country by country basis, policy makers will react."
"A lot of people are very surprised to hear that slavery still exists...What modern slavery is is a situation that reflects all of the characteristics of slavery of past centuries.
"People are controlled by violence. They are tricked or they are forced into jobs or situations where they are economically exploited. They live on no pay or base subsistence pay and they're not free to leave."
"Those governments that want to engage with us, we will be very open to engaging and looking at ways in which we can better measure the issue of modern slavery," Grono added.