when is the illinois income tax rate become effective
Illinoisâs corporate income tax is charged at a flat rate of 7% of federal taxable income, with adjustments. Returns are due on the 15th day of the 3rd month after the end of the corporationâs tax year. For corporations with tax years following the calendar year, this means March 15th. (For the sake of comparison, the Illinois personal income tax rate is a flat 5% of federal adjusted gross income, with adjustments.)