The world's rich lost nearly 20 percent of their total wealth in 2008 as volatile markets wiped out two years of growth, a Merrill Lynch study showed on Wednesday.

The total value of the wealth of people with net assets of more than $1 million, excluding their main home and consumables, dropped to $32.8 trillion -- below 2005 levels, the 13th annual Merrill Lynch World Wealth Report showed.

The number of people with more than $1 million in net assets fell 14.9 percent, while the number of people with fortunes of more than $30 million fell by a quarter, the study showed.