Every C corporation in the United States has one federal income tax return that anchors its annual compliance: Form 1120. Miss the Form 1120 deadline, or assume that an extension also delays the tax payment, and the corporation can face penalties and interest that were entirely avoidable. Fiscal-year corporations, especially those with a June 30 year-end, have additional timing rules that change for tax years beginning in 2026.

This guide explains who files Form 1120, how to work out the due date, how a Form 7004 extension works, what an extension does not cover, which documents to gather and the mistakes that most often cause problems.

What Is Form 1120 and Who Must File It?

According to the IRS Instructions for Form 1120, unless exempt under section 501, all domestic corporations, including corporations in bankruptcy, must file Form 1120 whether or not they have taxable income, unless they are required to file a special return instead.

That covers:

• Domestic corporations taxed as C corporations
• Eligible entities, such as LLCs, that have elected to be classified as corporations and have not elected S corporation status
• Corporations with losses or no income, since having no taxable income does not by itself remove the filing requirement
• Newly formed corporations filing a first, possibly short-year, return
• Dissolved corporations filing a final return

Other entities use different returns. Partnerships, including LLCs taxed as partnerships, generally file Form 1065. Corporations with an effective S election file Form 1120-S. Foreign corporations generally file Form 1120-F, and special entities such as REITs, regulated investment companies and insurance companies have their own versions. An LLC’s legal form alone does not decide which return it files; its federal tax classification does.

Form 1120 Filing Deadline

Calendar-Year C Corporations

A corporation generally must file Form 1120 by the 15th day of the 4th month after its tax year ends. For a calendar-year corporation, that is April 15. If the date falls on a Saturday, Sunday or legal holiday, the return is due on the next business day.

• Tax year 2025: due Wednesday, April 15, 2026; extended deadline Thursday, October 15, 2026.
• Tax year 2026: due Thursday, April 15, 2027; extended deadline Friday, October 15, 2027.

Always confirm dates against the current IRS Publication 509 tax calendar, since holidays and disaster relief can shift them.

Fiscal-Year C Corporations

A corporation with a fiscal year follows the same 15th-day-of-the-4th-month rule, counted from its own year-end. For example, a corporation with a September 30 year-end generally files by January 15. The exception is a June 30 year-end.

Special Rule for June 30 Year-End Corporations

When Congress moved C corporation due dates to the 4th month starting in 2016, it delayed the change for corporations with a June 30 year-end. As a result:

• Tax years beginning before January 1, 2026: the return is due by the 15th day of the 3rd month after year-end, which is September 15, and the automatic extension is 7 months. For the year ending June 30, 2026, that means a September 15, 2026 due date and an extended date of April 15, 2027.
• Tax years beginning in 2026 or later: the general 4th-month rule applies, so a year running July 1, 2026 to June 30, 2027 is due October 15, 2027, and the automatic extension is 6 months.

The Form 1120 instructions also treat a short tax year that ends anytime in June as ending on June 30 for this purpose.

Short Tax Years and Final Returns

A short tax year arises when a corporation starts operations partway through a year, changes its tax year, or dissolves. The return is generally due by the 15th day of the 4th month after the short year ends, applying the June rule where relevant. For a dissolved corporation, the IRS instructions say the return is generally due by the 15th day of the 4th month after the date it dissolved. Check the “Final return” box on the form, and review the current instructions for any special situations.

Form 1120 Deadline at a Glance

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How to Request a Form 1120 Extension

What Is Form 7004?

Form 7004 is the application for an automatic extension of time to file certain business income tax, information and other returns. One Form 7004 covers one return, identified by the form code entered on line 1 and the tax period. It must generally be filed on or before the original due date of the return.

According to the Form 7004 instructions, the IRS no longer sends a notice approving the extension; it only contacts you if the request is disallowed. That makes your own proof of filing important. Form 7004 can be filed electronically for most returns, including Form 1120.

Step-by-Step Form 7004 Extension Process

1. Confirm the return. Make sure the corporation files Form 1120 and identify the correct tax year.
2. Calculate the original due date, including the June 30 rule if it applies.
3. Estimate the tax liability for the year and subtract estimated payments and credits already made.
4. Complete Form 7004 using the current instructions and the correct form code for Form 1120.
5. File Form 7004 electronically where available, or by the approved paper method, by the original due date.
6. Pay the estimated unpaid tax by the original due date. Corporations must make federal tax deposits by electronic funds transfer, for example through EFTPS.
7. Keep the filing acknowledgment and payment confirmation.
8. File the completed Form 1120 by the extended deadline.

An extension gives more time to prepare the return. It does not remove the obligation to file it.

How Long Is the Extension?

The automatic extension for Form 1120 is generally 6 months. The exception is a C corporation with a June 30 year-end whose tax year began before January 1, 2026, which receives 7 months; for tax years beginning in 2026, the extension is 6 months. For calendar-year corporations, the statute originally allowed 5 months for tax years beginning before 2026, but the IRS has granted 6 months under its general authority, so the extended due date is October 15.

Does a Form 1120 Extension Extend the Tax Payment Deadline?

No. The Form 7004 instructions state plainly that Form 7004 does not extend the time to pay any tax due. The corporation should:

• Estimate its total tax before the original due date
• Account for estimated payments, overpayments credited from the prior year and applicable credits
• Pay any expected balance by the original due date

Tax not paid by the original due date accrues interest from that date until it is paid, even if an extension was filed. A late-payment penalty may also apply. The IRS can grant relief in some circumstances, but an extension by itself does not eliminate penalties or interest on unpaid tax.

A Note on Estimated Tax

Separately from the annual return, a corporation generally must make estimated tax payments if it expects its total tax for the year, after applicable credits, to be $500 or more. Installments are generally due on the 15th day of the 4th, 6th, 9th and 12th months of the tax year. Underpayment is calculated on Form 2220. Special rules apply to large corporations and to annualized or seasonal income methods, so check the current instructions and the IRS estimated taxes guidance for your situation.

Documents Needed to Prepare Form 1120

The exact list depends on the corporation’s operations, but a typical preparation file includes:

• Prior-year federal corporate tax return
• Trial balance and general ledger
• Income statement and balance sheet
• Bank statements and reconciliations
• Revenue and expense schedules
• Accounts receivable and payable reports
• Inventory records, where relevant
• Fixed asset register and depreciation schedules
• Loan agreements and interest records
• Payroll and employee-related records
• Shareholder and ownership information
• Related-party transaction records
• Estimated tax payment confirmations
• Records of credits, carryforwards and prior-year adjustments
• Restructuring or dissolution documents, if relevant

Most of these support preparation and are kept in the corporation’s records rather than attached to the return. What must be filed with Form 1120, such as certain schedules, forms and elections, depends on the corporation’s circumstances. Foreign-owned US corporations, for example, may need to file Form 5472 with the return.

Common Form 1120 Filing Mistakes

Missing the Filing Deadline

Late filing usually starts with incomplete books or poor calendar management. Prevention: track both the original and extended deadlines, and close the books early enough to leave time for tax preparation.

Confusing an Extension to File With an Extension to Pay

Filing Form 7004 and paying nothing leads to interest and possibly penalties. Prevention: manage the payment deadline separately and pay the estimated balance with the extension.

Incorrect Entity Classification

An LLC or corporation that has made, or failed to make, an election may be filing the wrong return. Prevention: confirm classification and keep IRS acceptance letters for any Form 8832 or Form 2553 election.

Incorrect Income or Expense Reporting

Missing revenue, misclassified expenses and book-to-tax differences that are not adjusted cause errors. Prevention: reconcile the books before preparing the return and document each book-tax adjustment. Our guide to bookkeeping cleanup before tax season covers this step.

Depreciation and Fixed Asset Errors

Missing additions, unrecorded disposals and the wrong method or recovery period distort taxable income. Prevention: keep a current fixed asset register and apply the tax depreciation rules for the relevant year.

Missing Estimated Tax Payments

Payments recorded in the books may not match what the IRS credited. Prevention: reconcile estimated payments against EFTPS records or the IRS business tax account before filing.

Overlooking Related-Party Transactions

Loans, management fees and payments to shareholders or affiliates need documentation and correct tax treatment, and may carry reporting requirements. Prevention: keep agreements and support for every related-party transaction.

Ignoring Carryforwards and Prior-Year Adjustments

Net operating losses, credit carryforwards and prior-year adjustments are easy to miss when a preparer changes. Prevention: maintain a carryforward schedule that rolls forward each year.

Assuming a Dormant Corporation Has No Filing Obligation

Because domestic corporations must generally file whether or not they have taxable income, an inactive corporation can still have a filing requirement. Prevention: keep filing until the corporation is formally dissolved and a final return is filed, and confirm the rules for your situation.

Ignoring State Tax Obligations

Filing Form 1120 does not satisfy state income tax, franchise tax or annual report requirements, which have their own forms and due dates. Prevention: list every state where the corporation is registered or has nexus and track its deadlines separately.

What Happens if a Corporation Files Form 1120 Late?

Late filing and late payment are separate problems with separate consequences:

• Late-filing penalty: generally 5% of the unpaid tax for each month or part of a month the return is late, up to 25%. For returns required to be filed in 2026, a return more than 60 days late faces a minimum penalty of the smaller of the tax due or $525 (an amount adjusted for inflation).
• Late-payment penalty: generally ½ of 1% of the unpaid tax for each month or part of a month, up to 25%. When both penalties apply for the same month, the late-filing penalty is reduced by the late-payment penalty for that month.
• Interest: charged on unpaid tax from the original due date until paid, at the rate set under section 6621, regardless of any extension.

Because the late-filing penalty is based on unpaid tax, a corporation that owes nothing may face little or no late-filing penalty under this provision, but other penalties or information-return penalties can still apply. Late returns also generate IRS notices and extra administrative work. Penalties may be abated where the corporation shows reasonable cause and not willful neglect, subject to IRS requirements; relief is never automatic.

Form 1120 vs Form 1120-S: What Is the Difference?

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How a CPA or Corporate Tax Professional Can Help

A corporate tax professional can reconcile the books before filing, confirm entity classification and tax year, track filing and payment deadlines, prepare Form 1120 and its schedules, reconcile estimated payments, review depreciation and carryforwards, file Form 7004 where needed, spot issues before filing and coordinate federal and state compliance. Professional help reduces avoidable errors, but it cannot guarantee a particular tax outcome or rule out an IRS examination.

AAPT & Associates offers Form 1120 tax preparation for C corporations, including foreign-owned US corporations, supported by outsourced bookkeeping to keep year-end records ready.

Frequently Asked Questions

Final Form 1120 Filing Checklist

• Confirm the entity’s federal tax classification
• Identify the correct tax year and original deadline
• Prepare and reconcile financial statements and schedules
• Review income, expenses, depreciation and tax attributes
• Reconcile estimated tax payments and credits
• Decide whether an extension is needed
• File Form 7004 by the original deadline if extending
• Pay the estimated unpaid tax by the original due date
• File Form 1120 by the extended deadline
• Keep filing confirmations and supporting records
• Review state corporate tax obligations separately

This article provides general information and is not individualized tax advice. Due dates, penalty amounts and rules change; confirm current requirements with the IRS or a qualified tax professional.

IRS sources: About Form 1120 · Instructions for Form 1120 · Instructions for Form 7004 · Publication 509 · Estimated taxes