The estimated total casualties at the Battle of Antietam were around 22,000, making it the bloodiest single-day battle in American history.

This staggering figure includes both Union and Confederate soldiers who were killed, wounded, or missing as a result of the fierce fighting on September 17, 1862. The high number of casualties highlighted the brutal nature of the Civil War and the intense combat experienced during this battle. The heavy losses deeply affected both armies and underscored the war's human cost, contributing to the growing public awareness and opposition to the conflict in both the North and South.