The net tax gap (which accounts for late payments made either voluntarily or as a result of enforcement action) averaged $406 billion per year from 2008-2010. About 16 percent of taxes went unpaid. This was equivalent to 2.8 percent of annual gross domestic product (GDP) and 18 percent of annual federal revenues during that period.
Payroll taxes — paid on the wages and salaries of employees and used to finance social insurance programs such as Social Security and Medicare — and self-employment taxes accounted for about 19 percent of evasion, but 39 percent of taxes paid.
https://www.brookings.edu/blog/up-fr...f-tax-evasion/