Because of the catastrophic nature of the Battle of Antietam, exact numbers of casualties were virtually impossible to compile. The sources for these figures are The Official Records of the War of the Rebellion and the Antietam Battlefield Board.
Casualties include three categories: 1) dead; 2) wounded; and 3) missing or captured. In general terms, casualties of Civil War battles included 20% dead and 80% wounded. Of the soldiers who were wounded, about one out of seven died from his wounds. Over 2/3 of the 622,000 men who gave their lives in the Civil War died from disease, not from battle.
The combat and the resulting casualties at Antietam were concentrated at some key areas of the battlefield. This chart lists the approximate times, numbers of soldiers engaged, and the casualties at these locations. These are estimates and the total does not equal 22,720 because there were additional casualites in other areas of the field.