These figures are based on the censuses carried out by these countries in 1910 and 1911, and I have focused on the male population for the 15-19 and 20-24 age cohorts because those were the primary age groups that would see service in the war. I have not included data for Imperial Russia simply because there is no accurate government data available for the immediate prewar years. The only census was carried out in Russia in 1897.

Sources: B. R. Mitchell, International Historical Statistics: Europe, 1750-2005, 6th ed. (New York: Palgrave, 2007) and U.S. Bureau of the Census, Historical Statistics of the United States, Colonial Times to 1970 (Washington: USGPO, 1975)

Percentage of Pre-1914 Male Population Aged 15-24 that were casualties

I was interested to make this calculation, even though there are some flaws with this. Using the prewar age cohort population of young men in these countries, I want to get a rough idea of the casualties suffered by this age group, and since this age group suffered high casualties (See French Losses per Annual Mobilization Class.), I used total casualties for these countries--yes, not total casualties for this age cohort--and calculated the percentage (total casualties/size of age cohort). This gives a rough idea of the impact of the war on young men in these countries.

Sources: See above and Encyclopedia Britannica.