Income tax in the United States increase complexity, irrespective of the system's flatness or lack thereof. Income tax is also levied by most U.S. states and many localities on individuals, corporations, estates, and trusts. These taxes are in addition to federal income tax and are deductible for federal tax purposes. State and local income tax rates vary from 1% to 16% of taxable income. Some state and local income tax rates are flat (single rate) and some are graduated. State and local definitions of what income is taxable vary highly. Some states incorporate the federal definitions by reference. Taxable income is defined separately and differently for
State income tax particularly with respect to partnerships. Most states require taxpayers to make quarterly payments of tax not expected to be satisfied by withholding tax. All states impose penalties for failing to file required tax returns and/or pay tax when due. In addition, all states impose interest charges on late payments of tax, and generally also on additional taxes due upon adjustment by the taxing authority. Forty-three states impose a tax on the income of individuals, sometimes referred to as personal income tax. State income tax rates vary widely from state to state. The states imposing an income tax on individuals tax