United States / Guides / When Form 1120 Is Due (and What an Extension Really Does)
United States · guideWhen Form 1120 Is Due (and What an Extension Really Does)
The short answer
A C corporation’s Form 1120 is due on the 15th day of the 4th month after the close of its tax year: April 15 for a calendar-year corporation. Form 7004 gives an automatic 6-month filing extension — to October 15 for calendar-year filers — but it only moves the paperwork deadline. Any tax owed is still due on the original date, and interest plus a failure-to-pay penalty start accruing the day after that date even if the extension was filed correctly. The corporation’s CPA files the return and the extension; the run-up work is having final books and a tax-basis trial balance ready before either deadline.
Key facts — verified dates on each
The rule: 15th day of the 4th month
A domestic C corporation files Form 1120 by the 15th day of the 4th month following the end of its tax year. For a calendar-year corporation — tax year ending December 31 — that is April 15 of the following year. A corporation on a fiscal year, say a year ending September 30, files by January 15.
If the 15th falls on a Saturday, Sunday, or legal holiday, the deadline moves to the next business day. This is the same mechanism that pushes the individual Form 1040 deadline around most years, and it applies to Form 1120 the same way.
A new corporation’s first return follows the same math from whatever date its tax year actually starts and ends — a short first year still gets a due date of the 15th day of the 4th month after that short year closes, not a full 12 months later.
The June 30 fiscal-year-end exception is winding down
For years, corporations with a June 30 fiscal year-end filed on a different clock: 15th day of the 3rd month (September 15) instead of the 4th, under a transition rule that dates to 2015 legislation. That transition period runs out for tax years beginning on or after January 1, 2026 — once a June 30 fiscal year corporation’s tax year starts in 2026 or later, it moves onto the standard 4th-month rule like every other C corp.
For a corporation with a June 30 (or other non-calendar) year-end, which rule applies to a specific tax year is a question for its CPA — the transition dates matter more than most filers expect, and getting them wrong misses a deadline by a full month.
What Form 7004 actually extends
Form 7004 requests an automatic extension of time to FILE Form 1120 — 6 months for a calendar-year corporation, moving the filing deadline from April 15 to October 15. It must be filed by the original due date, and it is automatic: the IRS does not need to approve it, only receive it on time and with a reasonable estimate of tax liability.
What it does not do is extend the time to PAY. Any tax the corporation owes is still due on the original April 15 (or fiscal-year-equivalent) deadline. If the return shows a balance due when it is eventually filed in September or October, the corporation owes that tax as of the original due date, plus interest and a failure-to-pay penalty running from the day after that date — the extension only stopped the filing clock, never the payment clock.
Because of that split, a corporation requesting an extension still needs a credible estimate of what it owes at the original deadline, which means the books need to be closed enough to produce that estimate — not final, but far enough along that the estimate is not a guess.
What happens when a return is late
Two separate penalties can apply, and they stack. The failure-to-file penalty is 5% of the unpaid tax for each month or partial month the return is late, capped at 25% of the unpaid tax. It applies when the return itself is late — filing Form 7004 on time avoids this penalty entirely, even if the return isn’t finished until October.
The failure-to-pay penalty is separate: 0.5% of the unpaid tax for each month or partial month the tax remains unpaid, also capped at 25%. This one runs from the original due date regardless of whether an extension was filed, because the extension never touched the payment deadline.
A return filed more than 60 days after its due date (counting any extension) triggers a minimum failure-to-file penalty — the smaller of the tax owed or a fixed dollar floor that IRS adjusts periodically. Both penalties can be reduced or waived for reasonable cause, which the CPA raises with the IRS directly; it is not something the corporation self-certifies on the return.
- Failure-to-file: 5% of unpaid tax per month late, capped at 25%
- Failure-to-pay: 0.5% of unpaid tax per month unpaid, capped at 25%
- Interest accrues on unpaid tax and on unpaid penalties, separately from both
- A minimum penalty applies past 60 days late — confirm the current dollar floor on the IRS penalty page before relying on a number
Where the books fit into the deadline
Neither the April deadline nor the extended October deadline is really a books problem — it is a books-readiness problem. A CPA can prepare an accurate Form 1120, or a defensible extension estimate, only from a general ledger that is reconciled through the tax year-end: bank and credit card accounts tied out, fixed assets and depreciation schedules current, intercompany and shareholder loan balances confirmed, and a trial balance that maps cleanly to the return’s line items.
CapEasy’s bookkeeping work closes those books to that standard and hands the CPA a tax-basis trial balance rather than a raw export — it does not prepare, review, or file Form 1120 or Form 7004, and it does not estimate the corporation’s tax liability. Those stay with the corporation’s CPA, who is positioned to sign what they file.
The figures, and when we checked them
These numbers change by year or by notification. Each one shows the date we last verified it against the source — if that date looks old, check the source before relying on it.
Questions on this
When is Form 1120 due for a calendar-year C corporation?
The 15th day of the 4th month after the tax year ends — April 15 of the following year for a corporation on a calendar year. If April 15 falls on a weekend or legal holiday, the deadline moves to the next business day.
Does Form 7004 extend the deadline to pay taxes owed?
No. Form 7004 only extends the deadline to file the return — 6 months for a calendar-year C corporation. Any tax owed is still due on the original due date; interest and a failure-to-pay penalty accrue from the day after that date regardless of the extension.
How do I know how much to pay if the return isn’t finished by the original deadline?
Form 7004 requires a reasonable estimate of the tax liability, calculated with the corporation’s CPA from books that are closed enough to support that estimate. Underpaying the estimate can still trigger a failure-to-pay penalty on the shortfall even with a timely-filed extension.
What is the due date for a corporation with a fiscal year instead of a calendar year?
The same rule applies on the corporation’s own calendar: 15th day of the 4th month after its fiscal year-end. A September 30 fiscal year-end means a January 15 due date.
Is the June 30 fiscal-year-end rule different?
It has been, historically — corporations with a June 30 year-end filed by the 15th day of the 3rd month under a transition rule. That transition period ends for tax years beginning on or after January 1, 2026, after which June 30 fiscal-year corporations move onto the standard 4th-month rule. Confirm which rule applies to the specific tax year with a CPA.
What happens if Form 1120 is filed late without an extension?
A failure-to-file penalty applies: 5% of the unpaid tax per month or partial month late, capped at 25%. Filing Form 7004 on time before the original due date avoids this penalty even if the actual return isn’t filed until the extended deadline.
What happens if the tax is paid late even with a filed extension?
A separate failure-to-pay penalty applies — 0.5% of the unpaid tax per month or partial month unpaid, capped at 25% — running from the original due date. This penalty is not affected by having filed Form 7004, since the extension never moved the payment deadline.
Can both the failure-to-file and failure-to-pay penalties apply to the same return?
Yes, they can stack when a return is both filed late and paid late. The IRS applies a combined-penalty offset in some cases; the CPA calculates the actual exposure from the specific facts of the return.
Who actually files Form 1120 and Form 7004?
The corporation’s CPA or tax preparer prepares and files both. CapEasy’s bookkeeping work supports that filing by closing the books and delivering a tax-basis trial balance; it does not prepare, review, sign, or file either form, and does not calculate the corporation’s tax liability.
Does a new corporation with a short first tax year get extra time to file?
No. A short first tax year still follows the 15th-day-of-the-4th-month rule measured from whenever that short year actually ends, not from the incorporation date and not extended to a full 12 months.
Primary sources
- IRS — Instructions for Form 1120
- IRS — Instructions for Form 7004
- IRS — Failure to File Penalty
- IRS — Failure to Pay Penalty
Last reviewed 2026-08-14. Statutes and schedules change — the sources above are authoritative, this page is orientation.
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