Updated October 16, 2026. Quick answer: Form 1120 is the U.S. Corporation Income Tax Return. The instructions say all domestic corporations must file an income tax return whether or not they have taxable income, unless exempt or filing a special return. The general due date is the 15th day of the 4th month after the tax year ends, and the late filing penalty is 5% of the unpaid tax for each month or part of a month.
More IRS forms for employers and business owners, read line by line: Form 720 instructions.
S corporation return (Form 1120-S) for 2025, read line by line: Form 1120-S lines 23 to 28, Form 1120-S Schedule B questions 1 to 3 and Form 1120-S Schedule B questions 6 to 11.
Partnership return (Form 1065) for 2025, read line by line: Form 1065 lines 1a to 3 and Form 1065 Schedule B questions 1 to 3.
More IRS forms and schedules, read line by line: Form 7004 instructions, Form 8886 instructions and Form 8275 instructions.
What Form 1120 is
The form’s title is “U.S. Corporation Income Tax Return”. The instructions state what it is for: “the income, gains, losses, deductions, credits, and to figure the income tax liability of a corporation.”
A corporation that has made the S election files a different return, and a business owner choosing between the two will find the comparison on the S corporation election calculator page.
Key numbers at a glance
The percentages and dollar figures that decide the penalty and the tax line are collected here.
| Item | As printed |
|---|---|
| Late filing penalty | “A corporation that does not file its tax return by the due date, including extensions, may be penalized 5% of the unpaid tax for each month or part of a month the return is late, up to a maximum of 25% of the unpaid tax.” |
| Minimum penalty | “The minimum penalty for a tax return required to be filed in 2026 that is more than 60 days late is the smaller of the tax due or $525.” |
| Tax rate line | “Multiply taxable income (page 1, line 30) by 21% (0.21). Enter this amount on line 1a.” |
Who must file Form 1120
The instructions begin with the broad rule and then add the entity cases that surprise owners.
| Situation | As printed in the instructions |
|---|---|
| Domestic corporations | “Unless exempt under section 501, all domestic corporations (including corporations in bankruptcy) must file an income tax return whether or not they have taxable income.” |
| Entities electing corporate treatment | “A domestic entity electing to be classified as an association taxable as a corporation must file Form 1120, unless it is required to or elects to file a special return listed under Special Returns for Certain Organizations.” |
| Multi-owner LLC | “If an entity with more than one owner was formed as an LLC under state law, it is generally treated as a partnership for federal income tax purposes and files Form 1065” |
| Single-member LLC | “The LLC can file a Form 1120 only if it has filed Form 8832 to elect to be treated as an association taxable as a corporation.” |
The decision on how to form the business is covered on the page on whether you need an LLC. For an owner who sold a practice, the corporate history matters too, as on the page on selling a practice that used to be a C corporation.
A corporation filing as a special organization uses another form: “Domestic corporations must file Form 1120, unless they are required, or elect to file a special return.”
When Form 1120 is due
The due date is counted from the end of the corporation’s tax year, not from a fixed calendar date.
| Item | As printed in the instructions |
|---|---|
| General rule | “a corporation must file its income tax return by the 15th day of the 4th month after the end of its tax year.” |
| June 30 fiscal year | “a corporation with a fiscal tax year ending June 30 must file by the 15th day of the 3rd month after the end of its tax year.” |
| Dissolved corporation | “A corporation that has dissolved must generally file by the 15th day of the 4th month after the date it dissolved.” |
| Weekend or holiday | “If the due date falls on a Saturday, Sunday, or legal holiday, the corporation can file on the next business day.” |
| Extension | “File Form 7004, Application for Automatic Extension of Time To File Certain Business Income Tax, Information, and Other Returns, to request an extension of time to file.” |
The extension request has its own timing: “Generally, the corporation must file Form 7004 by the regular due date of the return.”
The penalty for filing late
The late filing penalty here is a percentage of the unpaid tax. The instructions print it: “A corporation that does not file its tax return by the due date, including extensions, may be penalized 5% of the unpaid tax for each month or part of a month the return is late, up to a maximum of 25% of the unpaid tax.”
There is also a floor for a long delay: “The minimum penalty for a tax return required to be filed in 2026 that is more than 60 days late is the smaller of the tax due or $525.” And there is an exception: “The penalty will not be imposed if the corporation can show that the failure to file on time was due to reasonable cause.”
Paying late is a separate charge: “A corporation that does not pay the tax when due may generally be penalized 1/2 of 1% of the unpaid tax for each month or part of a month the tax is not paid, up to a maximum of 25% of the unpaid tax.”
Illustration, not an IRS example: for $10,000 of unpaid tax and a return filed 3 months late, the filing penalty at 5% a month is 3 months times 5%, which is 15%, and $10,000 times 15% is $1,500.
The income tax line
The tax itself is figured on Schedule J. The instructions give the formula for the main line: “Multiply taxable income (page 1, line 30) by 21% (0.21). Enter this amount on line 1a.”
Illustration, not an IRS example: for taxable income of $100,000, the computed amount is $100,000 times 21%, which is $21,000. The result is the Schedule J, line 1a figure before any other tax on that schedule is added.
Key lines and schedules
Page 1 builds from income through deductions to taxable income and then to the tax and payment lines.
| Page 1 line | Wording on the form |
|---|---|
| 4 | “Dividends and inclusions (Schedule C, line 23)” |
| 11 | “Total income. Add lines 3 through 10” |
| 12 | “Compensation of officers” |
| 23 | “Pension, profit-sharing, etc., plans” |
| 27 | “Total deductions. Add lines 12 through 26” |
| 29b | “Special deductions (Schedule C, line 24)” |
| 30 | “Taxable income. Subtract line 29c from line 28.” |
| 31 | “Total tax (Schedule J, line 12)” |
| 35 | “Amount owed. If line 33 is smaller than the total of lines 31, 32, and 34, enter amount owed” |
The schedules behind page 1 are listed on the form by title.
| Schedule | Title on the form |
|---|---|
| Schedule C | “Dividends, Inclusions, and Special Deductions” |
| Schedule J | “Tax Computation and Payment” |
| Schedule K | “Other Information” |
| Schedule L | “Balance Sheets per Books” |
| Schedule M-1 | “Reconciliation of Income (Loss) per Books With Income per Return” |
| Schedule M-2 | “Analysis of Unappropriated Retained Earnings per Books” |
Schedule C feeds two page 1 lines. The instructions say “Complete Schedule C and enter on line 4 the amount from Schedule C, line 23, column (a).” Schedules L, M-1 and M-2 have a small-corporation exception: “Corporations with total receipts (page 1, line 1a plus lines 4 through 10) and total assets at the end of the tax year less than $250,000 are not required to complete Schedules L, M-1, and M-2 if the “Yes” box on Schedule K, question 13, is checked.”
A larger corporation reports differently: “Corporations with total assets nonconsolidated (or consolidated for all corporations included within the consolidated tax group) of $10 million or more on the last day of the tax year must file Schedule M-3 (Form 1120) instead of Schedule M-1.”
Line 23 is the corporation’s own plan deduction. Owner decisions about which plan to set up are compared on the business owner retirement page.
Who signs
The signature rule reads “The return must be signed and dated by:” “The president, vice president, treasurer, assistant treasurer, chief accounting officer; or” “Any other corporate officer (such as tax officer) authorized to sign.”
Which revision applies
The edition read for this page is Form 1120 (2025) and the Instructions for Form 1120 (2025), which are the revisions irs.gov served when they were read on 2026-10-16. Check the revision printed on the form you are filing, because a later revision can change the due dates, the penalty amounts and the schedules.
Common mistakes
- Assuming a corporation with no income has no return. The instructions say domestic corporations must file an income tax return whether or not they have taxable income.
- Counting the due date from the wrong month. The general rule is the 15th day of the 4th month after the end of the tax year, and the instructions give a different month for a June 30 fiscal year.
- Treating a single-member LLC as a corporation filer by default. The instructions say the LLC can file a Form 1120 only if it has filed Form 8832 to elect to be treated as an association taxable as a corporation.
Related: S corporation election calculator, do you need an LLC, selling a practice that used to be a C corporation, business owner retirement, and late S corporation election.
Sources
- Form 1120 (2025), U.S. Corporation Income Tax Return: https://www.irs.gov/pub/irs-pdf/f1120.pdf
- Instructions for Form 1120 (2025): https://www.irs.gov/pub/irs-pdf/i1120.pdf
Read from the IRS documents linked above on October 16, 2026. General information, not tax advice. Your own facts decide the outcome, and a preparer, the account provider or the IRS is the right place to confirm anything consequential.