For more than a century, it has been accepted with a grain of salt that about 620,000 Americans died in the conflict, with more than half of those dying off the battlefield from disease or festering wounds.
All along, however, historians sensed that number underrepresented the death toll.
Nor had any historian undertaken the mammoth task of devising and executing a new count.
That was until December, when historian J David Hacker published a paper that used demographic methods and sophisticated statistical software to study newly digitised US census records from 1850 to 1880.
Story: BBC News | Photo: BBC News