Form 1120: The Corporate Income Tax Return and Who Must File It
Published November 7, 2026
Form 1120 is the federal income tax return for a domestic corporation. The 2025 Instructions for Form 1120 explain who must file it. They also cover the due date, how the tax is figured, and estimated tax. This post is general information. It is not advice about a specific corporation.
Who must file
Unless a corporation is exempt under section 501, every domestic corporation must file an income tax return. This applies whether or not the corporation has taxable income.
Domestic corporations file Form 1120, unless they are required, or choose, to file a special return. The instructions list those special returns for certain organizations.
An entity that elects to be taxed as a corporation also files Form 1120. It must attach a copy of Form 8832 to Form 1120 for the year of the election. See Form 8832: Choosing How Your LLC Is Taxed.
When the return is due
A corporation generally files by the 15th day of the 4th month after its tax year ends. For a calendar-year corporation, that is April 15.
A new corporation with a short-period return generally files by the 15th day of the 4th month after the short period ends. A dissolved corporation generally files by the 15th day of the 4th month after it dissolved.
How the tax is figured
Line 1a is the income tax. The instructions say to multiply taxable income from page 1, line 30, by 21% (0.21). Line 3 covers the corporate alternative minimum tax, figured on Form 4626 when it applies. Line 5 covers tax credits.
The accounting method matters because it affects taxable income. The method must clearly reflect income. The cash method and the accrual method are both allowed. So is any other method the rules authorize. See Cash or accrual? How a small business chooses its accounting method.
Estimated tax
A corporation that does not make estimated tax payments on time may owe a penalty. The instructions direct corporations to Form 2220, Underpayment of Estimated Tax by Corporations. Form 2220 shows whether a penalty applies.
The paid preparer signature
The signature area has a box for the paid preparer. If the corporation checks "Yes," the IRS may discuss the return with the preparer who signed it.
The instructions say this applies only to the individual named in the Paid Preparer Use Only section. It does not apply to the firm.
C corporations and S corporations
A C corporation pays tax at the corporate level. Shareholders can be taxed again on dividends they receive. An S corporation is different. It passes its results to shareholders on Schedule K-1. For those rules, read Form 7203: S Corporation Stock and Debt Basis.
Common questions
Does a corporation with no income still file? The instructions say yes, unless it is exempt under section 501. The return reports the facts even when the result is a loss or zero tax.
What about a new corporation? A short-period return has its own due date rule. Read the short-period rules in the instructions before filing.
Is this the same as Form 1120-S? No. An S corporation files Form 1120-S and passes its results to shareholders. The S corporation post explains that return.
Who signs the return? The return must be signed. Keep a signed copy with the workpapers that support each figure.
Checklist
- Confirm the corporation must file Form 1120, or has elected to file it.
- Note the due date: the 15th day of the 4th month after the tax year ends.
- Check that estimated tax payments were made on time.
- Keep the records that support taxable income and each credit claimed.
Sources: Sources: IRS, Instructions for Form 1120 (2025), U.S. Corporation Income Tax Return (https://www.irs.gov/pub/irs-pdf/i1120.pdf) This is general information, not advice about your specific return.
