Illinois Form IL-1120 — Corporation Income and Replacement Tax Return 2025-2026

Get the blank Form IL-1120 (PDF)  ·  Checked line by line against the official Illinois form, last verified October 14, 2026  ·  How we verify

What is Form IL-1120?

Illinois starts from federal taxable income (Form 1120, line 30), makes additions and subtractions to reach base income, apportions that income to Illinois using the sales factor if the business operates in more than one state, subtracts any Illinois net loss deduction, and then applies both tax rates to the resulting net income.

S corporations do not use this form: they are subject only to replacement tax and file Form IL-1120-ST. Partnerships file Form IL-1065.

Who Must File Form IL-1120?

File Form IL-1120 if you are a corporation that has net income or loss as defined in the Illinois Income Tax Act, or that is qualified to do business in Illinois and must file a federal income tax return, whether or not it has net income. A limited liability company that files as a corporation for federal purposes is treated as a corporation.

Unitary business groups use Schedule UB for combined apportionment. Political organizations and homeowners’ associations that file federal Form 1120-POL or 1120-H, cooperatives filing 1120-C and settlement funds filing 1120-SF also file Form IL-1120.

Who does not file this form: S corporations (they file Form IL-1120-ST), REMICs, and organizations exempt under IRC section 501(a), although unrelated business taxable income is reported on Form IL-990-T.

E-filing mandate: any taxpayer required to file its federal return electronically must file its Illinois return electronically.

How to File Form IL-1120

• Register with the Department (MyTax Illinois or Form REG-1) before filing. Your identification numbers are your FEIN and your Illinois account number.
• Start from your federal return. Enter federal taxable income from federal Form 1120, Line 30 on Line 1 and attach a copy of the federal return.
• Complete the steps in order: income or loss (Step 2), base income (Step 3), apportionment to Illinois if needed (Step 4), net income (Step 5), replacement tax (Step 6), income tax (Step 7) and refund or balance due (Step 8).
• Pay electronically. Use MyTax Illinois, a credit card at tax.illinois.gov/payments/creditcard, or an ACH credit. If you must mail a payment, complete Form IL-1120-V and write your FEIN, the tax year ending and “IL-1120-V” on the check.
• Sign. The president, vice president, treasurer or another authorized officer must sign; a paid preparer must also sign and enter a PTIN.
• To correct a filed return, file Form IL-1120-X, not another IL-1120.

When Is Form IL-1120 Due?

Form IL-1120 is due on or before the 15th day of the 4th month following the close of the tax year, which matches the federal due date. A corporation with a June 30 year-end files by the 15th day of the 3rd month, and a cooperative by the 15th day of the 9th month. Illinois grants an automatic extension to file of 7 months (8 for a June 30 year-end) whether or not you ask. An additional extension is granted only if the IRS grants one beyond the Illinois date, equal to the federal extension plus one month, and you attach the federal approval. The tax itself must be paid in full by the original due date; interest runs from the day after that date. If you must make quarterly estimated payments, they are due on the 15th day of the 4th, 6th, 9th and 12th months of the tax year.

Where to Mail Form IL-1120

If a payment is enclosed, mail Form IL-1120 to: Illinois Department of Revenue, PO Box 19038, Springfield IL 62794-9038. If no payment is enclosed, mail it to: Illinois Department of Revenue, PO Box 19048, Springfield IL 62794-9048. Most corporations file electronically.

How to Complete Form IL-1120 Line by Line

Step 1 — Identify Your Corporation (Boxes A–Y)

Enter the legal name, mailing address, FEIN (Box N), the parent’s FEIN if you are in a federal consolidated group (Box O), NAICS code (Box P) and Secretary of State charter number (Box Q). Check boxes for a first or final return, unitary filers, foreign insurers, and the apportionment formula that applies (sales, insurance, financial organizations, transportation, federally regulated exchanges). Box S is the business income election to treat all nonbusiness income as business income, Box T the accounting method, Box W an annualized income election on Form IL-2220, and Box X Public Law 86-272 protection.

Step 2 (Lines 1–9) — Figure Your Income or Loss

Step 3 (Lines 10–23) — Figure Your Base Income or Loss

Subtractions include interest from U.S. Treasury and other exempt federal obligations (Line 10), River Edge Redevelopment Zone and High Impact Business subtractions (Lines 11–14, Schedule 1299-B), the contribution subtraction (Line 15), foreign dividends (Line 17, Schedule J), the Illinois special depreciation subtraction (Line 18) and related-party expenses (Line 19). Line 23 is base income or loss.

Step 4 (Lines 24–34) — Income Allocable to Illinois

Complete this step only if the corporation does business inside and outside Illinois. Nonbusiness income or loss is allocated on Schedule NB (Line 24), and business income (Line 27) is apportioned using sales:

A unitary business group completes Schedule UB, and the new Finnigan rules apply to the sales factor.

Step 5 (Lines 35–39) — Net Income

Steps 6–7 (Lines 40–49) — Replacement Tax and Income Tax

Neither line can be negative.

Step 8 (Lines 50–67) — Refund or Balance Due

Lines 50–55 apply any foreign insurer reductions to get net replacement tax (Line 52) and net income tax (Line 55).

Common Mistakes to Avoid

• Using the wrong year’s form. The 2025 form is for tax years ending on or after December 31, 2025 and before December 31, 2026; using the wrong form delays processing.
• Filing IL-1120 for an S corporation. It files Form IL-1120-ST.
• Treating the extension as a payment extension. Tax is due on the original date.
• Skipping the federal return attachment. Attach a copy of the federal return with Line 1.
• Sending a computer printout instead of the IDOR form. Only approved forms and approved software output are accepted.
• Filing a second IL-1120 to correct errors. Use Form IL-1120-X.
• Missing estimated payments. A liability expected to exceed $400 requires quarterly estimated payments.

Frequently Asked Questions

What are the Illinois corporate tax rates? A 2.5% replacement tax and a 7.0% income tax, both applied to net income.

When is Form IL-1120 due? The 15th day of the 4th month after year-end (3rd month for a June 30 year-end).

Is there an automatic extension? Yes, 7 months (8 months for a June 30 year-end), granted whether or not you request it, but tax is still due on the original date.

Do S corporations file IL-1120? No. They file Form IL-1120-ST and pay replacement tax only.

Do I need to attach my federal return? Yes, attach a copy of the federal return with Line 1.

How do I amend an Illinois corporate return? File Form IL-1120-X for each tax year you want to change.

Final Review Before Filing

• This is the right form for the corporation (not an S corporation or a partnership).
• Line 1 matches federal Form 1120, Line 30, and the federal return is attached.
• Apportionment (Step 4) is completed if the business operates in more than one state.
• Both the 2.5% replacement tax and 7.0% income tax are computed on Line 39.
• Estimated payments are included on Line 61 and the penalty (Form IL-2220) on Line 59 if applicable.
• An authorized officer signed the return.
• Payment is made electronically, or Form IL-1120-V accompanies the check.

General information from the official form and its instructions, not advice for your circumstances. See the full disclaimer.