20.1 Form 1120 Overview
Key Takeaways
- Form 1120 is the U.S. Corporation Income Tax Return filed by C corporations.
- For calendar-year corporations, Form 1120 is due April 15 (April 15, 2026 for TY 2025).
- A 6-month extension to October 15 is available by filing Form 7004.
- Estimated tax payments are due the 15th of the 4th, 6th, 9th, and 12th months of the tax year.
- C corporations are separate taxable entities subject to a flat 21% federal rate (TCJA, made permanent by OBBBA).
- Unlike partnerships and S corporations, C corporations face double taxation.
- Late filing penalty: 5% per month, max 25% (minimum $510 if more than 60 days late).
Why This Matters for the Exam
Form 1120 is the core C corporation return. The exam tests due dates, estimated tax timing, extension rules, and the fundamental concept of C corporation taxation.
Exam Note: For the July 1, 2026 - February 28, 2027 testing window, you are tested on C corporation rules as of December 31, 2025 (Tax Year 2025), including changes from the One Big Beautiful Bill Act (OBBBA, signed July 4, 2025).
Expect at least 3-4 questions on Form 1120 basics.
What Is Form 1120?
Form 1120 (U.S. Corporation Income Tax Return) is filed by C corporations to report:
| Item | Description |
|---|---|
| Gross income | All income from operations |
| Deductions | Business expenses, depreciation, etc. |
| Tax credits | Research credit, foreign tax credit, etc. |
| Tax liability | Computed at 21% flat rate (TCJA, made permanent by OBBBA) |
Who Must File?
| Entity | File Form 1120? |
|---|---|
| Domestic C corporation | Yes |
| LLC electing C corp treatment | Yes |
| S corporation | No (File 1120-S) |
| Tax-exempt organization | No (File 990) |
| Foreign corporation (U.S. income) | File 1120-F |
C Corporation as a Separate Entity
Unlike pass-through entities, a C corporation is a separate taxpayer:
| Comparison | C Corporation | Pass-Through (S Corp/Partnership) |
|---|---|---|
| Entity-level tax | Yes (21% — permanent) | No |
| Shareholder/partner tax | On dividends | On distributive share |
| Double taxation | Yes | No |
| Tax return | Form 1120 | Form 1120-S / 1065 |
Filing Deadlines
| Return Year | Original Due Date | Extended Due Date |
|---|---|---|
| Calendar year (Tax Year 2025) | April 15, 2026 | October 15, 2026 |
| Fiscal year | 15th of 4th month after year end | 6 months later |
June 30 fiscal-year quirk: Corporations with a June 30 fiscal year end use the 15th day of the 3rd month (September 15) under a transitional rule preserved by current law; for all other fiscal years, the 15th day of the 4th month applies.
Estimated Tax Payments (Form 1120-W)
Corporations expecting tax of $500+ must pay quarterly estimates:
| Installment | Due Date (calendar year) |
|---|---|
| 1st | April 15 |
| 2nd | June 15 |
| 3rd | September 15 |
| 4th | December 15 |
Fiscal-year corps use the 15th day of the 4th, 6th, 9th, and 12th months of their tax year. Underpayment penalty (Form 2220) uses the federal short-term rate + 3%.
Extension: Form 7004
| Extension Detail | Information |
|---|---|
| Form | Form 7004 |
| Length | 6 months automatic |
| Due date | Same as original return |
| Tax payment | Still due by original date |
Important: An extension extends time to file, not time to pay. Estimated tax must be paid by the original due date.
Key Schedules
| Schedule | Purpose |
|---|---|
| Schedule C | Dividends received deduction (DRD) |
| Schedule J | Tax computation and credits |
| Schedule K | Other information questions |
| Schedule L | Balance sheet |
| Schedule M-1 | Book-to-tax reconciliation |
| Schedule M-2 | Retained earnings reconciliation |
| Schedule M-3 | Detailed reconciliation (large corps with $10M+ assets) |
Late Filing Penalties
| Penalty | Rate |
|---|---|
| Late filing | 5% per month, max 25% (min $510 for 2025 if >60 days late) |
| Late payment | 0.5% per month, max 25% |
| Combined max | 47.5% (with interest) |
Real-World Scenario
Scenario: A calendar-year C corporation realizes on April 10, 2026 that it cannot complete its 2025 return by April 15.
- Action: File Form 7004 by April 15, 2026 for an automatic 6-month extension.
- Tax payment: Estimate and pay any tax due by April 15, 2026.
- New deadline: October 15, 2026.
- If tax underpaid: Interest and possible penalty on underpayment.
On the Exam
Expect 3-4 questions on Form 1120, typically:
- Due Date Questions: "When is Form 1120 due for a calendar-year corporation?"
- Estimated Tax Questions: "When are the quarterly estimates due?"
- Extension Questions: "What form extends the filing deadline?"
- Entity Questions: "How are C corporations taxed differently than S corporations?"
The key is to remember: Due April 15 (calendar year). Extend via Form 7004 to October 15. Estimates due Apr/Jun/Sep/Dec 15. C corp = separate taxpayer = double taxation.
What is the original due date for a calendar-year C corporation Form 1120?
How can a C corporation obtain an extension to file Form 1120?
Which statement best describes C corporation taxation?