Corporation taxes are due on the 15th day of the third or fourth month after the tax year ends, depending on the entity type. For a calendar-year C corporation, the federal return (Form 1120) is due April 15. For a calendar-year S corporation, Form 1120-S is due March 15. Corporations expecting to owe $500 or more also owe quarterly estimated tax payments on April 15, June 15, September 15, and December 15. Missing any of these dates starts penalties and interest running immediately.
C Corporation Filing Deadline
A C corporation’s return is due on the 15th day of the fourth month after the close of its tax year.1Internal Revenue Service. Publication 509 (2026), Tax Calendars Most C corporations use a calendar year, which puts the deadline on April 15. A fiscal-year C corporation counts four months forward from its own year-end: a March 31 fiscal year produces a July 15 deadline.2Internal Revenue Service. Starting or Ending a Business 3
When the due date falls on a Saturday, Sunday, or a legal holiday in the District of Columbia, filing rolls to the next business day.1Internal Revenue Service. Publication 509 (2026), Tax Calendars Check the calendar each year, because a weekend shift can move your date without warning.
One long-standing exception is being phased out. C corporations with a fiscal year ending June 30 have historically filed by September 15 rather than under the standard fourth-month rule. That treatment still applies for tax years that began before January 1, 2026, and end on June 30, 2026. For tax years beginning in 2026 or later, these corporations move to the standard rule, making their deadline October 15.1Internal Revenue Service. Publication 509 (2026), Tax Calendars
S Corporation Filing Deadline
An S corporation files Form 1120-S by the 15th day of the third month after its tax year ends. For a calendar-year S corporation, that is normally March 15. In 2026, March 15 lands on a Sunday, so the deadline shifts to March 16.3Internal Revenue Service. Instructions for Form 1120-S, U.S. Income Tax Return for an S Corporation
The earlier date exists because S corporations pass income through to shareholders, who need Schedule K-1s in hand to file their personal returns by April.
Quarterly Estimated Tax Payments
A corporation that expects to owe $500 or more in tax for the year has to pay in four installments during the year rather than settling up at filing time.4Internal Revenue Service. Underpayment of Estimated Tax by Corporations Penalty For a calendar-year corporation, the installments are due:
- April 15 (first installment)
- June 15 (second installment)
- September 15 (third installment)
- December 15 (fourth installment)
Those dates correspond to the 15th day of the 4th, 6th, 9th, and 12th months of the tax year, so fiscal-year corporations follow the same pattern counting from the start of their own tax year. Weekend and holiday shifts apply here too.4Internal Revenue Service. Underpayment of Estimated Tax by Corporations Penalty
You can avoid an underpayment penalty by paying installments that add up to at least 100% of the tax shown on the prior year’s return, even if the current year turns out bigger. Large corporations, defined as those with taxable income of $1 million or more in any of the three preceding tax years, cannot use that safe harbor and must base payments on the current year’s expected liability.5Office of the Law Revision Counsel. 26 U.S. Code 6655 – Failure by Corporation to Pay Estimated Income Tax
What Happens If You File or Pay Late
Filing late and paying late are separate penalties, and they can run at the same time. Interest compounds on top.
C Corporations
The failure-to-file penalty is 5% of the unpaid tax for each month or partial month the return is late, capped at 25%. If the return is more than 60 days late, the minimum penalty is $525 or 100% of the unpaid tax, whichever is less.6Internal Revenue Service. Instructions for Form 1120 (2025) The failure-to-pay penalty is a separate 0.5% of the unpaid tax per month, also capped at 25%.7Internal Revenue Service. Topic No. 653, IRS Notices and Bills, Penalties and Interest
When both penalties apply in the same month, the failure-to-file penalty is reduced by the failure-to-pay amount, so the combined rate for the first five months works out to 5% per month rather than 5.5%.8Internal Revenue Service. Failure to File Penalty After five months the filing penalty maxes out, but the payment penalty keeps running until the balance is paid or it reaches its own 25% cap.
S Corporations
S corporations face a flat-dollar penalty rather than a percentage. For returns required to be filed in 2026, it is $255 per shareholder for each month or partial month late, up to 12 months. A five-shareholder S corporation that files three months late owes $3,825, even if no entity-level tax is due. If tax is owed, an additional 5% per month of the unpaid tax applies on top of the per-shareholder penalty.9Internal Revenue Service. Instructions for Form 1120-S (2025)
Interest
Interest runs on any unpaid balance from the original due date until paid in full. The rate resets quarterly and equals the federal short-term rate plus three percentage points. The corporate underpayment rate is 7% for the first quarter of 2026 and 6% for the second quarter. Large corporate underpayments over $100,000 carry a higher rate: the federal short-term rate plus five percentage points.10Internal Revenue Service. Quarterly Interest Rates
Getting More Time With Form 7004
Filing Form 7004 before the original deadline automatically gives your corporation a six-month extension to file. The IRS does not require an explanation, and it only contacts you if the extension is denied. You will not get a confirmation that it was approved.11Internal Revenue Service. Instructions for Form 7004
The form can be filed electronically through the IRS Modernized e-File platform, which produces an electronic acknowledgment.12Internal Revenue Service. E-Filing Form 7004 If you file on paper, the IRS treats the postmark date as the filing date, and that rule extends to IRS-designated private delivery services in addition to the Postal Service.13Office of the Law Revision Counsel. 26 USC 7502 – Timely Mailing Treated as Timely Filing and Paying Certified mail or a tracked service gives you proof of the date if the IRS later questions it.
One narrow variation: C corporations with a fiscal year ending June 30 that began before January 1, 2026, qualify for a seven-month extension rather than six. Starting with tax years beginning in 2026, that reverts to the standard six months.11Internal Revenue Service. Instructions for Form 7004
An Extension to File Is Not an Extension to Pay
This is the trap. A valid Form 7004 gives you six more months to send in the return, but the IRS still expects full payment of any estimated tax balance by the original due date.11Internal Revenue Service. Instructions for Form 7004 If you don’t pay by that date, the 0.5% per month failure-to-pay penalty and interest start accruing right away. The extension does nothing to stop them.7Internal Revenue Service. Topic No. 653, IRS Notices and Bills, Penalties and Interest
If you cannot pin down the exact liability by the deadline, estimate and pay. Overpayments come back as a refund when the return is filed; underpayments generate penalty and interest only on the shortfall, so a partial payment still reduces the exposure.
State Deadlines Are Separate
Most states with a corporate income tax set their own filing deadlines, and they don’t always match the federal ones. Some states accept a federal extension automatically; others require their own extension form. State due dates can fall anywhere from about a month before the federal deadline to several months after it. Check with your state tax agency early, because state penalties run independently of anything happening at the federal level.