Form 1120 due date: April 15, or October 15 with Form 7004

A C corporation files Form 1120 by the 15th day of the 4th month after its tax year ends, which is April 15 for a calendar-year startup. Form 7004 extends that by 6 months to October 15, but it does not extend the time to pay: any tax owed is still due in April.

Every Delaware C corporation files, whether it made money or not. The 2025 Instructions for Form 1120 say all domestic corporations, unless tax-exempt, "must file an income tax return whether or not they have taxable income". If your 2025 return is on extension, it is due Thursday, October 15, 2026. This guide covers the dates, the extension, what filing or paying late costs, estimated tax and the forms that catch first-time filers.

When is Form 1120 due

Form 1120 is due on the 15th day of the 4th month after the corporation's tax year ends. For a calendar year, that is April 15. A corporation with a June 30 fiscal year is the exception: it files by the 15th day of the 3rd month, September 15. When the date falls on a weekend or legal holiday, the next business day counts.

Those rules are in the "When To File" section of the Form 1120 instructions, which also cover the first and last years. A new corporation that files a short-period return files by the 15th day of the 4th month after the short period ends, and a dissolved corporation by the 15th day of the 4th month after it dissolved.

The IRS publishes the dates for each year in its Form 7004 due date chart for 2026 tax periods. For a tax year ending December 2026, the return is due April 15, 2027.

How a Form 7004 extension works

Form 7004 gives a C corporation an automatic 6-month extension of time to file. The extension is automatic when you file the form by the return's original due date, and most returns can be extended electronically. It extends the filing date only. Tax not paid by the original due date accrues interest and can draw a late payment penalty.

The Instructions for Form 7004 are blunt about payment: "Form 7004 does not extend the time to pay any tax due." The corporation estimates its tax on line 6 of the form, enters zero if it expects to owe nothing, and pays any balance by the original due date.

There is one exception to the 6 months. A corporation with a tax year ending June 30 that began before January 1, 2026 gets 7 months. For tax years beginning in 2026, the instructions set the extension at 6 months for June year-ends too.

A worked example: a calendar-year startup's federal dates

A Delaware C corporation on a calendar year extended its 2025 return. These are its federal income tax dates through 2027, from the IRS due date chart and the Form 1120 instructions. Estimated tax applies only if it expects to owe $500 or more for 2027, which a startup with losses usually does not.

The 2025 return is also where domestic research costs become deductible again under section 174A, with the accounting method statements attached to that return. A return that is late for that reason has the same deadline as any other.

What happens if you file Form 1120 late

A late return costs 5% of the unpaid tax for each month or part of a month, up to 25%. Late payment costs 0.5% a month, also up to 25%, and in months when both apply the filing penalty is reduced by the payment penalty. A return more than 60 days late has a minimum penalty, which for returns due in 2026 is the smaller of the tax due or $525.

Those rates are in the "Penalties" section of the Form 1120 instructions, and the offset between the two penalties is IRC §6651(c)(1). The penalties are figured on unpaid tax, so a corporation with no tax due owes none of them. Interest runs separately on unpaid tax from the original due date.

Take three corporations whose 2025 returns were due April 15, 2026, with no extension, no tax paid and the return filed July 10, 2026. That is 3 months or part of a month late, and more than 60 days:

The $8,400 row is $40,000 of taxable income at the 21% rate in the instructions. Late filing is 15% of $8,400, or $1,260, and late payment is 1.5%, or $126. The offset takes the filing penalty to $1,134. For $400 of tax, 15% is $60, but the minimum for a return more than 60 days late is the smaller of $400 and $525, so the filing penalty is $400. The late filing penalty is not imposed when the corporation can show reasonable cause for the delay.

Do startups pay estimated tax

A corporation makes quarterly estimated tax payments if it expects its total tax for the year, less credits, to be $500 or more. The installments are due on the 15th day of the 4th, 6th, 9th and 12th months of the tax year: April 15, June 15, September 15 and December 15 for a calendar year.

A startup that loses money all year owes no estimated tax. The year it turns profitable is the one to watch, because the Form 1120 instructions apply an underpayment penalty when tax of $500 or more was not paid on time, figured on Form 2220. Paying at least the smaller of the current year's tax or the prior year's tax avoids it. The same instructions require corporations to make federal tax deposits by electronic funds transfer.

Which other forms go with a startup's first return

Two filings catch founders on the first return. A corporation that files 10 or more returns of any type in a calendar year, counting employment tax and information returns, must file Form 1120 electronically. And a corporation with a foreign owner of 25% or more may owe Form 5472, with a $25,000 penalty for a missed one.

The e-file rule applies to returns filed on or after January 1, 2024, under Regulations section 301.6011-5. A startup with payroll files four quarterly Forms 941 before it counts a single information return, and the Form 1099s that count toward the 10 have their own deadline and a new $2,000 threshold.

Form 5472 is for a corporation with at least one foreign person holding 25% or more of its vote or value at any time in the year, which describes many startups whose founders live outside the US. Under the Instructions for Form 5472, a 25% foreign-owned corporation with a reportable transaction with a related party files it with Form 1120, and the penalty for failing to file is $25,000. The $25,000 applies whether or not the corporation owes any income tax, so a loss year is no protection.

The return is signed by a corporate officer, such as the president, treasurer or chief accounting officer. State income tax returns and the Delaware franchise tax report due March 1 are separate filings with their own dates.

What to do before October 15

If your 2025 return is on extension, or your 2026 year is closing, work through this list:

1. Confirm your tax year end and write down the original and extended due dates for the return.
2. Close the books for the year, so the return starts from a reconciled balance sheet and income statement.
3. Estimate the tax. If it is more than zero, pay it by the original due date, even when you extend.
4. File Form 7004 by the original due date if the return will not be ready.
5. Check whether any foreign person owns 25% or more of the company, and whether Form 5472 is needed.
6. Count your returns for the year and plan to e-file if you reach 10.
7. Put next year's estimated tax dates on the calendar if you expect to owe $500 or more.

A return starts from a year-end close, and a close is faster when the books were kept monthly. LedgerIt keeps a startup's books closed and reconciled every month, so the trial balance your tax preparer asks for in March already exists.