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. Form 7004 extends the filing date only by six months. Payment of any balance is still due on the original date, otherwise interest and the late payment penalty run from that date.
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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. A calendar-year corporation that expects to owe $500 or more must also make estimated tax installments on 15 April, 15 June, 15 September and 15 December. A fiscal-year corporation pays on the 15th day of the 4th, 6th, 9th and 12th months of its year.
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. Interest runs at the federal short-term rate plus 3 percentage points, set each quarter. The underpayment rate is 7% for the fourth quarter of 2026. A return more than 60 days late also carries a minimum penalty of $525 for returns due after 31 December 2025, or 100% of the tax due if that is less.
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
- Check state filing dates, which can differ from the federal 15 April date
Documents you'll need
- Year-end trial balance, income statement and balance sheet
- Fixed asset and depreciation schedule with Form 4562 support
- Prior-year return and any net operating loss carryforward schedule
- Payroll, officer compensation and related-party loan records
- Records of estimated tax payments made through EFTPS
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)
Never miss a United States deadline
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