A recent Mother Jones article attempts to answer this question with help from the Innocence Project, the Center on Wrongful Convictions and experts in the field.
Extrapolating from the 281 known DNA exonerations in the US since the late 1980s, a conservative
estimate is that 1 percent of the US prison population, approximately 20,000 people, are falsely convicted.
In fact, since the late 1980s there have been as many as 850 exonerations nationwide, according to
University of Michigan law professor Samuel Gross, a leading researcher in the field. Many of them float under the radar, Gross says, unlike the highly publicized DNA exonerations.
Read the full article and see a map
of how many people have been exonerated in each state since 1989.