The usual answer is just under 620,000. Professor Hacker of Binghamton says the number was probably closer to 750,000. Here is the interview.
The population was about 31 million in 1860. Today it’s about 310 million. So, if you could imagine a war today that produced 7.5 million deaths, you get some indication of how devastating the war was on the population, on the economy, on the institutions of 19th century America.
It is hard to imagine America sustaining commitment to a war today that would cost these many lives. There must have been an extraordinary zeal to sustain such a cost.
Paying for the blight of slavery at the cost of the equivalent of 7.5 million lives demonstrated the level to which Americans were willing to go to rid this country of two centuries plus of bondage.