I realise that this is a fairly broad question, but I am trying to get a sense of the ratio between the total population and the size of the military that societies could support at different times and places throughout European history. My main interest would be comparing ancient Rome to dark ages Britain and Scandinavia (Vikings) and then to medieval and late medieval Britain and France. I'd be interested in how the social structure and economic setup of different societies effects the 'efficiency' through which they are able to support the largest military for a given population size.
Extremely rough approximates guestimates
Ancient times 1%. Industrial 10% Industrial-Crisis maximum - 20%
20% is around ALL the military age males. Ancient period that gives 1 military age male in 20.