Below is a quote from the linked site. For a populaton of 5,000,000, you would include everyone from infants to old folks, so anywhere from 10 to 15% is a reasonable estimate. Stan
"For the Confederates, figures are even more in dispute. Estimates of total Confederate enlistments run from 600,000 to 1,400,000. Many Confederate scholars count 6oo,oco total individuals. After considering the numerous surveys made, perhaps 750,000 individuals would be reasonably close. Thus it can be said that Federals, counting Negro troops, outnumbered the Confederates about three to one in number of individuals."