From Wikipedia
The total number of casualties in World War I, both military and civilian, were about 37 million: 16 million deaths and 21 million wounded. The total number of deaths includes 9.7 million military personnel and about 6.8 million civilians. The Entente Powers (also known as the Allies) lost 5.7 million soldiers and the Central Powers about 4 million.

British and German wounded, Bernafay Wood, 19 July 1916.