In Capital and Ideology, Piketty calculates that 19th-century European states (such as France and the UK) collected about 7% of their national income in taxes (

In Capital and Ideology, Piketty calculates that 19th-century European states (such as France and the UK) collected about 7% of their national income in taxes (most of which was spent on defense, courts, infrastructure such as roads, etc). 19th-century European states did a lot more than ancient ones, so I think <5% of national income is a reasonable estimate. Suffice it to say that at such a low level of taxation you wouldn't expect many people to be taxed directly.