The Illinois Income Tax is imposed on every corporation earning or receiving income in Illinois. The tax is calculated by multiplying net income by a flat rate. The Illinois Income Tax is based, to a large extent, on the federal income tax code.
Use the Tax Rate Database to determine the income and replacement tax rates.
The starting point for the Illinois Corporate Income Tax Return is federal taxable income, which is income minus deductions. Next, the federal taxable income is changed by adding back certain items (e.g., state, municipal, and other interest income excluded from federal taxable income) and subtracting others (e.g., interest income from U.S. Treasury obligations). The result is base income.
You must file Form IL-1120 if you are a corporation that
NOTE: If you are a cooperative, Form IL-1120 is due on the 15th day of the 9th month following the close of the tax year regardless of when your tax year ends.
You are not required to file Form IL-505-B, Automatic Extension Payment, in order to obtain this automatic extension. However, if you expect tax to be due, you must use Form IL-505-B to pay any tentative tax due in order to avoid interest and penalty on tax not paid by the original due date of the return. An extension of time to file your Form IL-1120 does not extend the amount of time you have to pay your Illinois tax liability.
Corporations (other than S corporations) who can reasonably expect their income and replacement tax liability to be more than $400 must make quarterly estimated payments. Estimated Payments are due on the 15th day of the 4th, 6th, 9th, and 12th months of the tax year. Corporations who mail estimated tax payments must complete Form IL-1120-ES, Estimated Income and Replacement Tax Payment for Corporations.
NOTE: If payments are made by an electronic method do not complete Form IL-1120-ES.
If you need to correct or change your return after it has been filed, you must file Form IL-1120-X, Amended Corporation Income and Replacement Tax Return. Returns filed before the extended due date of the return are treated as your original return for all purposes.
For more information see Form IL-1120-X Instructions. You should file Form IL-1120-X only after you have filed a processable Illinois Income Tax return. You must file a separate Form IL-1120-X for each tax year you wish to change.
Do not file another Form IL-1120 with “amended” figures to change your originally filed Form IL-1120.