When Is Corporate Tax Due? C Corp, S Corp, and Extension Deadlines

For calendar-year corporations, federal income tax returns are due on two different dates: C corporations file Form 1120 by April 15, and S corporations file Form 1120-S by March 15. In 2026, March 15 falls on a Sunday, so the S corporation deadline shifts to Monday, March 16, while the C corporation deadline stays on Wednesday, April 15. Any tax owed must be paid by those same dates, even if the corporation requests more time to file.

C Corporation Deadline

A C corporation reports its annual income, deductions, and credits on Form 1120.¹ Federal law sets the deadline at the 15th day of the fourth month after the corporation’s tax year ends.² For a calendar-year corporation, that means April 15. When the date lands on a Saturday, Sunday, or legal holiday, the return is due the next business day.

One historical wrinkle is ending. C corporations with a June 30 fiscal year previously filed on the 15th day of the third month after year-end, which put their return on October 15. That exception applied only to tax years beginning before January 1, 2026.³ Starting with tax years that begin in 2026, June 30 filers follow the standard fourth-month rule and file by November 15, and their automatic extension shortens from seven months to six.⁴

S Corporation Deadline

S corporations don’t pay income tax at the entity level. They still file Form 1120-S to report income, deductions, and credits.⁵ The filing deadline is the 15th day of the third month after the tax year closes.² For calendar-year S corporations, that’s March 15. Because March 15, 2026, is a Sunday, the 2026 deadline moves to March 16.⁶

The earlier date exists so shareholders can receive Schedule K-1 documents in time to prepare their personal returns. An S corporation must furnish Schedule K-1 to every shareholder by the same date its Form 1120-S is due.⁶ Late K-1s carry their own penalty, assessed per shareholder, per form. For 2026:⁷

Up to 30 days late: $60 per K-1
31 days late through August 1: $130 per K-1
After August 1 or never filed: $340 per K-1
Intentional disregard: $680 per K-1

For a company with many shareholders, the total climbs quickly.

Filing Extensions

A corporation that needs more time can request an automatic six-month extension by filing Form 7004 on or before the original due date.⁴ No justification is required. For a calendar-year C corporation, the extension pushes the filing deadline from April 15 to October 15. For a calendar-year S corporation, it moves from March 15 to September 15.

An extension buys time to file, not time to pay. Any tax the corporation expects to owe must still be paid by the original deadline, and interest starts accruing from that date on any balance left unpaid.⁴ The failure-to-pay penalty applies to late payments regardless of whether an extension was granted.

Estimated Tax Payments

Corporations don’t wait until the annual return to settle up. Any corporation expecting to owe $500 or more in total tax for the year must make quarterly estimated payments throughout the year.⁸ For calendar-year filers, installments are due April 15, June 15, September 15, and December 15. Fiscal-year corporations pay on the 15th day of the fourth, sixth, ninth, and twelfth months of their tax year.

A corporation that underpays an installment owes an addition to tax calculated using the IRS underpayment interest rate, running from the installment date until payment. This isn’t a flat penalty; it behaves like interest that compounds daily. Corporations can avoid the charge by paying at least 25% of the lesser of the current year’s tax or the prior year’s tax with each installment, though large corporations (generally those with $1 million or more in taxable income in any of the three preceding years) can only use the prior-year figure for their first installment.⁹

Penalties for Filing or Paying Late

The IRS assesses separate penalties for filing late and paying late, and both can run at the same time.

Failure to File

A corporation that misses the filing deadline (including any extension) owes 5% of the unpaid tax for each month or partial month the return is late, capped at 25%.¹⁰ If the return is more than 60 days late, the minimum penalty for 2026 is the lesser of the tax due or $525.¹¹ The IRS can waive the penalty for reasonable cause.

Failure to Pay

A separate 0.5% monthly penalty applies to any tax left unpaid after the original due date, also capped at 25%.¹² When both penalties run in the same month, the failure-to-file penalty is reduced by the failure-to-pay amount, so the combined charge is 5% per month rather than 5.5%.¹⁰ After five months, the failure-to-file cap is reached, but failure-to-pay keeps accruing until the balance is cleared or hits its own 25% ceiling.

Interest

Interest runs on top of penalties, compounding daily from the original due date until the tax is paid. The rate resets quarterly. For the first quarter of 2026, the standard corporate underpayment rate is 7%; for the second quarter (April through June 2026), it drops to 6%.¹³ C corporations with underpayments exceeding $100,000 pay a higher rate: the federal short-term rate plus five percentage points, which was 8% for the second quarter of 2026.¹⁴

Fiscal Year Filers

Not every corporation runs on the calendar. A fiscal year is any 12-month period that ends on the last day of a month other than December.¹⁵ The deadline rule is the same, measured from the corporation’s own year-end: fourth month for Form 1120, third month for Form 1120-S.¹⁶ A C corporation with a March 31 year-end files by July 15; an S corporation with the same year-end files by June 15.

Dissolving or Liquidating Corporations

A corporation that shuts down has an extra filing on top of its normal return. Within 30 days of adopting a plan of dissolution or liquidation, it must file Form 966 to notify the IRS, and within 30 days of any amendment to that plan, another Form 966 is due.¹⁷ The final Form 1120 or 1120-S follows the ordinary deadline rules based on the short tax year, with the “Final return” box checked to signal no future filings.¹⁸

1
Internal Revenue Service. About Form 1120, U.S. Corporation Income Tax Return
2
Office of the Law Revision Counsel. 26 USC 6072 – Time for Filing Income Tax Returns
3
eCFR. 26 CFR 1.6072-2 – Time for Filing Returns of Corporations
4
Internal Revenue Service. Instructions for Form 7004
5
Internal Revenue Service. About Form 1120-S, U.S. Income Tax Return for an S Corporation
6
Internal Revenue Service. Instructions for Form 1120-S
7
Internal Revenue Service. Information Return Penalties
8
Internal Revenue Service. Estimated Taxes
9
Office of the Law Revision Counsel. 26 USC 6655 – Failure by Corporation to Pay Estimated Income Tax
10
Internal Revenue Service. Failure to File Penalty
11
Office of the Law Revision Counsel. 26 USC 6651 – Failure to File Tax Return or to Pay Tax
12
Internal Revenue Service. Failure to Pay Penalty
13
Internal Revenue Service. Quarterly Interest Rates
14
Internal Revenue Service. Internal Revenue Bulletin 2026-08
15
Office of the Law Revision Counsel. 26 USC 441 – Period for Computation of Taxable Income
16
Internal Revenue Service. Publication 509 (2026), Tax Calendars
17
eCFR. 26 CFR 1.6043-1 – Return Regarding Corporate Dissolution or Liquidation
18
Internal Revenue Service. Starting or Ending a Business