Miss your C-Corp tax deadline and you’re looking at automatic penalties—5% of your unpaid tax per month, up to 25%. Add interest on top of that. Then there’s the estimated tax penalty if you didn’t pay quarterly. The costs add up fast.
C-Corporation tax deadlines aren’t complicated, but they are unforgiving. Whether you’re filing on time or need an extension, making estimated payments or catching up on a missed deadline, this guide covers every deadline, penalty, and strategy you need to stay compliant.
Key Takeaways
- C-Corp tax returns (Form 1120) are due the 15th day of the 4th month after fiscal year-end
- Calendar year C-Corps: April 15 filing deadline
- 6-month automatic extension available via Form 7004 (extends to October 15 for calendar year)
- Extension is to file, NOT to pay—you must still pay estimated tax by the original deadline
- Estimated tax payments due quarterly: April 15, June 15, September 15, December 15
- Late filing penalty: 5% of unpaid tax per month (maximum 25%)
- Late payment penalty: 0.5% of unpaid tax per month (maximum 25%)
- Estimated tax penalty if you owe $500+ and didn’t meet safe harbor requirements
- First-year C-Corps have special rules for estimated taxes
Table of Contents
C-Corp Tax Return Due Dates
Calendar Year C-Corporations
If your C-Corp uses a calendar year (January 1 – December 31), your Form 1120 is due:
Original deadline: April 15 of the following year
Extended deadline: October 15 (if Form 7004 filed by April 15)
For 2025 tax year (calendar year C-Corp):
- Original deadline: April 15, 2026
- Extended deadline: October 15, 2026
Fiscal Year C-Corporations
If your C-Corp uses a fiscal year, Form 1120 is due on the 15th day of the 4th month after your fiscal year ends.
Examples:
| Fiscal Year-End | Original Due Date | Extended Due Date |
|---|---|---|
| March 31 | July 15 | January 15 |
| June 30 | October 15 | April 15 |
| September 30 | January 15 | July 15 |
| December 31 | April 15 | October 15 |
Weekend and Holiday Adjustments
If the due date falls on a Saturday, Sunday, or legal holiday, the deadline moves to the next business day.
For example, if April 15 falls on a Saturday:
- Deadline moves to Monday, April 17
- Unless Monday is a holiday (like Emancipation Day in DC), then Tuesday, April 18
Always verify the exact due date for your filing year using the IRS tax calendar.
How to File for an Extension
C-Corps can request an automatic 6-month extension by filing Form 7004 by the original due date.
Form 7004 Requirements
What Form 7004 does:
- Grants an automatic 6-month extension to file
- Does NOT extend the time to pay
- Must be filed by the original due date
What Form 7004 requires:
- Corporation name, address, EIN
- Tax form being extended (Form 1120)
- Estimated tax liability for the year
- Payment of any expected tax due
Filing Form 7004
Electronic filing: Most tax software supports e-filing Form 7004. This is the fastest and most reliable method.
Paper filing: Mail Form 7004 to the IRS address for your state (found in Form 7004 instructions). Allow adequate mailing time—the form must be received or postmarked by the deadline.
Payment with extension: If you owe tax, pay with Form 7004 via:
- Electronic Federal Tax Payment System (EFTPS)
- Direct Pay on IRS.gov
- Check or money order with the paper form
Critical Point: Extension to File ≠ Extension to Pay
This is where many businesses get tripped up. Filing Form 7004 extends your time to file the return, not your time to pay the tax.
If you owe tax, you must:
- Estimate your tax liability
- Pay that estimated amount by the original deadline
- File your return by the extended deadline
Failure to pay by the original deadline results in late payment penalties and interest—even if you filed a valid extension.
What “Valid Extension” Means
Your extension is valid only if:
- Form 7004 is filed by the original due date
- The form is properly completed
- You made a reasonable estimate of tax liability
- You paid at least 90% of your actual tax due
If the IRS determines you significantly underpaid with your extension, they may invalidate the extension and assess late filing penalties.
Estimated Tax Payments
C-Corps with expected tax liability of $500 or more must make quarterly estimated tax payments.
Estimated Tax Due Dates
For calendar year C-Corps:
| Quarter | Period Covered | Payment Due |
|---|---|---|
| Q1 | January 1 – March 31 | April 15 |
| Q2 | April 1 – May 31 | June 15 |
| Q3 | June 1 – August 31 | September 15 |
| Q4 | September 1 – December 31 | December 15 |
Note: Q2 covers only 2 months; Q3 covers 3 months. This is an IRS quirk, not a typo.
For fiscal year C-Corps, estimated payments are due on the 15th day of the 4th, 6th, 9th, and 12th months of your fiscal year.
How Much to Pay: Safe Harbor Rules
To avoid estimated tax penalties, C-Corps must pay the lesser of:
Option 1: 100% of the current year’s tax liability (paid in equal quarterly installments)
Option 2: 100% of the prior year’s tax liability (paid in equal quarterly installments)
Example:
- 2024 actual tax: $80,000
- 2025 expected tax: $100,000
To avoid penalty, pay either:
- $25,000 per quarter (100% of current year estimate), OR
- $20,000 per quarter (100% of prior year)
Large Corporation Rules
If your C-Corp had taxable income of $1 million or more in any of the preceding 3 years, special rules apply:
- You can use the prior year safe harbor for Q1 only
- Q2-Q4 must be based on current year tax liability
- Any shortfall from using prior year in Q1 must be made up in Q2
This means large corporations effectively must estimate current year liability accurately—the prior year safe harbor provides limited protection.
Annualized Income Installment Method
If your income is uneven throughout the year (seasonal business, large Q4, etc.), you may be able to reduce early-quarter payments using the annualized income installment method.
This method bases each quarterly payment on actual income through that period, annualized. It requires completing Schedule A of Form 2220 and keeping detailed records of when income was earned.
Penalties for Late Filing and Payment
The IRS assesses separate penalties for late filing and late payment, and they can stack.
Late Filing Penalty
Rate: 5% of unpaid tax for each month (or part of a month) the return is late
Maximum: 25% of unpaid tax
Example:
- Tax due: $50,000
- Return filed 3 months late
- Penalty: $50,000 × 5% × 3 = $7,500
If no tax is due: The minimum late filing penalty is $485 (for returns due in 2024) or the tax due, whichever is less.
Late Payment Penalty
Rate: 0.5% of unpaid tax for each month (or part of a month) the tax remains unpaid
Maximum: 25% of unpaid tax
Interaction with late filing: If both penalties apply in the same month, the late filing penalty is reduced by the late payment penalty. So the combined maximum is 5% per month (not 5.5%), up to a combined maximum of 47.5% (25% late filing + 22.5% late payment, since late filing maxes out at 5 months).
Estimated Tax Penalty
When it applies: If you owe $500+ at filing and didn’t meet safe harbor requirements
Calculation: Based on the underpayment amount and the period of underpayment, using IRS interest rates
Form: Calculated on Form 2220 (Corporation Underpayment of Estimated Tax)
Exceptions: No penalty if:
- Tax liability is less than $500
- You met one of the safe harbor tests
- The IRS waives the penalty for reasonable cause
Interest on Underpayments
In addition to penalties, interest accrues on unpaid tax from the original due date.
Rate: Federal short-term rate plus 3 percentage points (changes quarterly)
Compounding: Daily
As of 2024, the underpayment rate has been around 8%. This compounds daily on any unpaid balance.
Failure to Pay Penalty Increase
If tax remains unpaid after IRS notice and demand for payment, the late payment penalty increases from 0.5% to 1% per month.
First-Year C-Corp Filing Considerations
New C-Corps face unique challenges in their first tax year.
Short Tax Year
If your C-Corp was formed mid-year, you’ll have a short tax year for your first return.
Example: Corporation formed July 1, 2025
- First tax year: July 1, 2025 – December 31, 2025 (6 months)
- Form 1120 due: April 15, 2026 (4th month after December year-end)
Short tax years can complicate depreciation calculations, NOL carryforwards, and various elections.
First-Year Estimated Taxes
First-year corporations have no prior year to use for safe harbor calculations.
Options:
- Pay 100% of estimated current year tax in quarterly installments
- Use annualized income method to match payments to actual income
- Accept that some penalty may apply if estimates are wrong
Practical approach: Make reasonable estimates and document your methodology. First-year penalties are often waived for reasonable cause if you made good-faith estimates.
Selecting Your Tax Year
New C-Corps can choose any fiscal year-end without IRS approval (unlike S-Corps, which generally must use calendar year).
Consider:
- Natural business cycle (retail: January year-end; schools: June year-end)
- Owner’s personal tax planning
- Availability of accounting staff
- Industry norms
Once selected, changing fiscal year requires IRS approval in most cases.
For help with first-year compliance requirements, consider our C-Corporation tax services.
Form 1120: What’s Required
Basic Filing Requirements
Every C-Corp must file Form 1120 for each tax year, even if:
- The corporation had no income
- The corporation had no business activity
- The corporation is inactive
The only exception: corporations that have formally dissolved under state law and have no assets may file a final return and cease filing.
Key Schedules
Schedule K: Shareholders and ownership
- Reports shareholders owning 1%+ of stock
- Required for all C-Corps
Schedule L: Balance sheet
- Assets, liabilities, equity at year-end
- Required if total assets are $250,000+
Schedule M-1: Book-to-tax reconciliation
- Reconciles book income to taxable income
- Required if total assets are $250,000+
Schedule M-2: Retained earnings analysis
- Beginning balance, additions, subtractions, ending balance
- Required if total assets are $250,000+
Schedule M-3: Detailed book-to-tax reconciliation
- Required for corporations with total assets of $10 million or more
- More detailed than Schedule M-1
Common Issues That Delay Filing
Missing K-1s: If your C-Corp owns interests in partnerships or S-Corps, you need their K-1s to complete your return. Those aren’t due until March 15 (partnerships/S-Corps).
Complex calculations: Depreciation, NOL carryforwards, and credits can require significant computation.
State returns: Multi-state operations require separate state returns, often with different due dates.
Audit-ready documentation: Ensure support for major deductions before filing.
For complex business tax preparation, working with a CPA experienced in corporate returns can prevent errors and ensure all deductions are captured.
Multi-State Filing Requirements
C-Corps operating in multiple states may have filing obligations beyond their home state.
Nexus Triggers
You may have filing requirements in a state if you have:
- Physical presence (office, employees, inventory)
- Significant sales into the state
- Remote employees working from that state
- Certain economic thresholds met (varies by state)
State Due Dates
State returns are typically due on or around the federal due date, but not always:
| State | Due Date | Extension |
|---|---|---|
| California | April 15 | 6-month auto (7 months for FY) |
| Texas (franchise tax) | May 15 | August 15 |
| New York | April 15 | 6-month auto |
| Illinois | April 15 | 6-month auto |
Always verify state-specific deadlines, as they change periodically.
State Estimated Taxes
States with corporate income tax generally require estimated payments similar to federal. Due dates may differ from federal dates.
Some states (like Texas with its franchise tax) have completely different filing systems and deadlines.
For businesses with multi-state operations, our business tax services include handling state filing complexity.
What Happens If You Miss a Deadline
Missed Filing Deadline (No Extension)
Immediate steps:
- File as soon as possible—penalties accrue monthly
- Pay any tax due with the return
- Consider requesting penalty abatement
Penalty abatement options:
- First-time penalty abatement (FTA): If you have a clean compliance history for 3 prior years
- Reasonable cause: Death, serious illness, natural disaster, reliance on professional advice
Missed Payment Deadline
Immediate steps:
- Pay immediately—interest accrues daily
- Set up installment agreement if you can’t pay in full
- Consider offers in compromise for severe hardship
IRS payment options:
- Full payment (obviously preferred)
- Short-term payment plan (120 days or less, no setup fee)
- Long-term payment plan (installment agreement, setup fee applies)
- Offer in compromise (settle for less if you qualify)
Missed Extension Deadline
If you missed the April 15 deadline and didn’t file Form 7004:
- File immediately with any tax payment
- The return is late—penalties will apply from April 15
- You cannot retroactively obtain an extension
Missed Estimated Tax Payment
If you missed a quarterly payment:
- Pay as soon as possible
- Increase subsequent payments to catch up
- Calculate whether you’ll meet safe harbor by year-end
- Consider the annualized income method if income was genuinely lower in earlier quarters
Frequently Asked Questions
When is my C-Corp tax return due?
Form 1120 is due on the 15th day of the 4th month after your fiscal year-end. For calendar year C-Corps (most common), that’s April 15. A 6-month extension is available by filing Form 7004 by the original due date.
Can I get an extension for my C-Corp taxes?
Yes. File Form 7004 by the original due date to receive an automatic 6-month extension. However, the extension only extends the time to file—not the time to pay. You must estimate and pay your tax liability by the original deadline.
What are C-Corp estimated tax due dates?
For calendar year C-Corps: April 15, June 15, September 15, and December 15. Estimated payments are required if you expect to owe $500 or more for the year.
What is the penalty for filing C-Corp taxes late?
5% of unpaid tax for each month (or part of a month) the return is late, up to a maximum of 25%. If you also paid late, the late payment penalty adds an additional 0.5% per month.
Do I have to pay C-Corp taxes if I’m filing an extension?
Yes. The extension extends your filing deadline, not your payment deadline. You must estimate your tax liability and pay it by the original due date (April 15 for calendar year) to avoid late payment penalties and interest.
What if my C-Corp has a fiscal year?
Your return is due on the 15th day of the 4th month after your fiscal year-end. For example, a June 30 fiscal year-end has an October 15 original deadline. Estimated payments follow the same pattern, adjusted for your fiscal year.
Can I avoid estimated tax penalties in my first year?
First-year corporations have no prior year safe harbor. Make reasonable estimates based on projected income. First-year penalties are often waived for reasonable cause if you documented good-faith estimates. Consider using the annualized income method if income is uneven.
What happens if I can’t pay my C-Corp taxes?
File the return on time anyway—the late filing penalty is steeper than late payment. Then contact the IRS to set up a payment plan. Options include short-term extensions (up to 120 days), installment agreements, or offers in compromise for severe hardship.
Staying on Top of C-Corp Tax Deadlines
C-Corp tax deadlines require careful calendar management. Miss one, and penalties start immediately. Miss several, and you’re facing significant financial consequences.
Annual compliance checklist:
- [ ] Mark all quarterly estimated tax due dates
- [ ] Set reminders 2 weeks before each deadline
- [ ] File extension by original deadline if needed
- [ ] Pay estimated tax with extension (don’t just extend)
- [ ] Track state deadlines separately if multi-state
- [ ] Reconcile estimated payments before final filing
Common mistakes to avoid:
- Filing an extension without paying estimated tax
- Using prior year safe harbor when you’re a “large corporation”
- Assuming state deadlines match federal
- Waiting until deadline day to file (what if technology fails?)
For C-Corps with complex filing requirements, tight deadlines, or multi-state obligations, professional support ensures nothing falls through the cracks. Schedule a consultation to discuss your C-Corp compliance needs—whether you’re current and want to stay that way, or you’re behind and need to catch up.