I disagree. The loopholes are not from the progressive part of the tax code, so making it a flat rate doesn't simplify anything. I assume you are talking about a flat tax, which is a personal income tax only, but event still that is difficult. The complexity is from defining "income". A big source of complexity is about companies operating in other countries. You still have to define rules about a company car vs a personal car; and a business expense and a personal expense; and income that is entirely in another country vs domestic income. These problems don't go away with a flat tax despite it's proponents unquestioned belief that it does.