What percentage of the world population served in ww2?

Statistics on American, German and Japanese forces are not hard to find, but they often refer to those under arms at certain times (e. g., at the start or end of the war) rather than the grand total. A total of 12,209,238 Americans were in military service by September 2, 1945, representing 9% of the 131,028,000 U. S. population.