According to this story in the Boston Globe, the US’s “tax gap” is between $312 billion and $353 billion for individuals (not corporations). This gap consists of underreporting of income, underpayment of taxes, and nonfilers. While the IRS’s enforcement efforts reduce this gap somewhat (to around $250 billion), the US still has a noncompliance rate of about 15%.
The biggest culprit is underreporting of income, generated from business income.