The Global Slavery Index 2013 says India has the highest number of people living in conditions of slavery at 14 million.
But Mauritania has the highest proportional figure with about 4% of its population enslaved.
The report's authors hope it will help governments tackle what they call a "hidden crime".
The index was compiled by Australian-based rights organisation Walk Free Foundation using a definition of modern slavery that includes debt bondage, forced marriage and human trafficking.
"A lot of governments won't like hearing what we have to say," WFF chief executive Nick Grono told the French news agency Agence France-Presse.
"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."
The organisation'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.
India, China, Pakistan and Nigeria have the highest numbers of people enslaved, the charity said.
Together with five other countries, they account for three-quarters of the total estimated number of people in modern slavery worldwide.
The report said India's ranking was mostly due to the exploitation of Indians citizens within the country itself.
While the highest proportion of slaves is in Mauritania, with many people inheriting slave status from their ancestors, Haiti is second in the index and Pakistan is third.
The new survey has the backing of world figures including former US Secretary of State Hillary Clinton, ex-British Prime Minister Tony Blair.
Mrs Clinton said that although the index was not perfect, it provided a starting point, according to 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."

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The Hartlepool fighter, celebrating her 21st birthday, secured a 17-15 victory.
She told BBC Sport: "I was only 12 when I walked into a boxing ring, I loved it ever since and now I'm world champion."
Earlier, Nicola Adams had to settle for flyweight silver after being outpointed 14-10 by China's Ren Cancan.
This was Savannah Marshall's second successive world final appearance after taking a silver medal as an 18-year-old at Barbados in 2010.
Marshall, who had qualified for the Olympics on Friday, nudged 7-5 ahead at the halfway stage but had to hold on through a tough third round when an excellent uppercut from Vystropova gave her a bloody nose.
With Marshall still holding her two-point advantage heading into the final round, Vystropova came on strong again but a neat combination in the dying seconds from the Briton helped edge her to an historic win.
"It hasn't even sunk in that I'm going to the Olympics yet. It's crazy," Marshall added.
Leeds boxer Adams started slowly in her bout against Cancan, who she had also lost to Ren in the 2010 final in Bridgetown, and trailed by two points after the first round.
Although Adams enjoyed increasing success with her right hand in the remaining three rounds, Ren scored consistently to stay ahead.
The 29-year-old was unhappy with the judges' verdict and told BBC Sport: "I thought the scoring was a bit wrong, but that's boxing for you. "
I believe I won it but didn't get the decision."
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The Yorkshire fighter is now aiming for revenge at this summer's London Olympics, having secured her place in the Great Britain Olympic squad by reached the semi-finals in Qinhuangdao.
"This hasn't dented my confidence. It's been a great experience and I've had an amazing time," added Adams.
"I qualified for the Olympics and that's what I came to do. In London, it'll be on my own turf so the score will be different then.
"I'll be going for gold. I'll get a bit fitter and have a bit more technical ability."
Women's boxing will feature at the Olympics for the first time at London 2012.
Nearly 30 million people around the world are living as slaves, according to a new index ranking 162 countries.
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British middleweight Savannah Marshall won the gold medal at the Women's World Boxing Championships in China, after defeating Azebaijan's Elena Vystropova in the final in Qinhuangdao.