Form 1120 — C-Corporation Annual Return
Annual federal income tax return for US C-corporations. Due 15 April for calendar-year filers (15th day of the 4th month after fiscal year-end). 6-month automatic extension via Form 7004 — extension is to file, not to pay.
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Who this applies to
- All US C-corporations
- LLCs that elected C-corp tax treatment via Form 8832
- Foreign corporations with US source income
What to file
Form 1120 with full P&L, balance sheet, schedules for tax credits, depreciation, NOLs and any state-required attachments.
How to file
Electronically via authorized e-filers (most CPAs / tax software). Paper filing accepted but discouraged. Use Form 7004 for 6-month extension by 15 April.
Payment due
Tax owed must be paid by 15 April even if filing extended. Underpayment of 90% of liability triggers Section 6655 estimated tax penalty.
Penalties for missing this deadline
Late filing penalty: 5% per month (max 25%) of unpaid tax. Late payment penalty: 0.5% per month. Combined penalties typically capped. Plus interest at the federal short-term rate + 3% (currently ~8%).
Filing checklist
- Reconcile book income to taxable income (Schedule M-1 or M-3)
- Apply 21% federal corporate income tax
- Calculate any tax credits (R&D §41, foreign tax credit, etc.)
- Reconcile estimated tax payments made during the year
- File electronically or paper
- Pay any balance via EFTPS, IRS Direct Pay or wire
Common mistakes to avoid
- Filing late assuming extension covers payment (it does NOT)
- Missing R&D credit by not filing Form 6765
- Misclassifying QBI / Section 199A (S-corp/partnership only, not C-corp)
- Forgetting state corporate income tax (separate filing in 44+ states)
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