Does anyone know where to find a source that I can reference with this information?
I've heard of a report by the US war department but I can't actually find somewhere to reference this from.
I'm looking for a reliable academic source.
http://www.pbs.org/greatwar/resources/casdeath_pop.html
This is an incredibly good source, I hope it aids you. In short: Death Tolls 42,188,810 for Allied Powers (Britain, France, Russia, etc.) 22,850,000 for Central Powers (Germany, Austria-Hungary, etc.) 66,038,810 for Both (Central Powers and Allied Powers combined)
It's easy to see why the Allied Powers lost more men, as simply put- they had more men to lose. While the Central Powers simply had Austria-Hungary, Germany, Bulgaria, and the Ottoman Empire (Though they were all extremely powerful) the Allied Powers had MANY more. This isn't even counting Britain's extra cannon fodder from the Middle East.
Hope it helped!