About 11%. The Confederacy had a pre-war population of just over 9,000,000, of which about 3,500,000 were slaves and another 132,000 were free people of color. Out of 5,500,000 white people, about half - or 2,750,000 - were male. Of those, at least 290,000 - probably a bit more - died in Confederate service. If you further break it down based on military age (17-55 by the end of the war) and fitness for service, it's probably closer to 25%.