Because of poor record keeping there is no accurate number
Estimates range from 500,000 to 1.5 million.
Most agree those estimates are too high
The War Dept reports:
1861 - 326,768
1862 - 449,439
1863 - 464,646
1864 - 400,787
Jamie is right, he usually is, The North could substain heavyer casualties and replinish, the South could not. A fact the U.S. Grant took full advantage of.
At the end, the south was trying to defend its homes with 14,15 and 16 year olds: old men and the walking wounded.
One of my grandfathers was the Captian of a Company of 15,16 and 17 year olds that fought and were captured at Lake Blakely in Baldwin County AL. two days after Lee surrendered.
An OR reports "Capt. Pitts and his company of boys fought bravely".