World War I began on July 28, 1914, with the declaration of war by Austria-Hungary on Serbia, and hostilities between the Allied and Central Powers continued until the signing of the Armistice on 11 November 1918, a period of four years, three months, and 14 days. The aggregate direct war costs of all the belligerents amounted to about £90 billion. Casualties in the land forces amounted to more than 37 million; in addition, close to ten million deaths among the civilian populations were caused indirectly by the war.
It is not surprising that sources vary on the exact numbers killed, injured and missing due to the difficulty of keeping and maintaining paper records in a war zone, however these figures are a good guide to the scale of the numbers involved.