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; no scheduled sunset).
Unlike partnerships and S corporations, C corporations face double taxation.
Late filing penalty: 5% per month, max 25% (minimum $525 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).
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; no scheduled sunset) |
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 $525 for returns required to be filed in 2026 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
Question types to practice on Form 1120:
- 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?
March 15
April 15
June 15
October 15
How can a C corporation obtain an extension to file Form 1120?
File Form 7004 by the original due date
File Form 4868 by the original due date
Call the IRS and request an extension
No extension is available
Which statement best describes C corporation taxation?
Pass-through like partnerships
Tax at entity level; shareholders taxed on dividends
No federal income tax
Shareholders not taxed on dividends
Sections you finish are checked off in the contents.