A calendar-year S corporation has to file its federal income tax return, Form 1120-S, by March 15 each year. When that date lands on a weekend or legal holiday, the deadline moves to the next business day, so for the 2025 tax year the filing deadline is Monday, March 16, 2026, because March 15 falls on a Sunday.1Internal Revenue Service. Instructions for Form 1120-S (2025) That single date is the one most S corp owners are looking for, but it isn’t the only deadline on the calendar. Estimated tax installments, payroll tax filings, contractor 1099s, and shareholder K-1s all have their own due dates, and missing any of them can trigger penalties that grow month by month.
The Form 1120-S Filing Deadline
S corporations report their income, deductions, and credits on Form 1120-S, even though the corporation itself usually owes no income tax.2Internal Revenue Service. About Form 1120-S, U.S. Income Tax Return for an S Corporation Under federal law, the return is due by the 15th day of the third month after the tax year ends.3Office of the Law Revision Counsel. 26 USC 6072 – Time for Filing Income Tax Returns Most S corps run on a calendar year ending December 31, which puts the standard deadline at March 15.
When March 15 falls on a Saturday, Sunday, or legal holiday, the deadline shifts to the next business day.4Office of the Law Revision Counsel. 26 USC 7503 – Time for Performance of Acts Where Last Day Falls on Saturday, Sunday, or Legal Holiday That is what puts the 2026 deadline at March 16.
Fiscal-Year S Corporations
Federal law generally requires an S corporation to use a calendar year. A different fiscal year is allowed only when the corporation can show the IRS a legitimate business purpose or qualifies under a special election that limits the deferral period.5Office of the Law Revision Counsel. 26 USC 1378 – Taxable Year of S Corporation When a fiscal year is approved, the same formula applies: the return is due on the 15th day of the third month after the year ends.3Office of the Law Revision Counsel. 26 USC 6072 – Time for Filing Income Tax Returns A June 30 fiscal year end means a September 15 deadline; September 30 means December 15. The weekend-and-holiday adjustment applies the same way.
Schedule K-1 to Shareholders
The corporation also has to prepare a Schedule K-1 for every person who was a shareholder at any point during the year. Each K-1 shows that shareholder’s individual share of income, deductions, and credits for use on their personal return.2Internal Revenue Service. About Form 1120-S, U.S. Income Tax Return for an S Corporation K-1s are due to shareholders on the same date as the Form 1120-S, so shareholders should have their 2025 K-1s in hand by March 16, 2026, or by the extended deadline if the corporation files for more time.1Internal Revenue Service. Instructions for Form 1120-S (2025)
A missed or incorrect K-1 carries its own penalty: $340 per K-1 for each failure, rising to $680 (or 10 percent of the total amounts required to be reported, if greater) when the error is intentional.1Internal Revenue Service. Instructions for Form 1120-S (2025)
Getting More Time to File
An S corporation that needs more time can request an automatic six-month extension by filing Form 7004 on or before the original due date.6Internal Revenue Service. Instructions for Form 7004 (Rev. December 2025) For a calendar-year filer in 2026, that pushes the deadline from March 16 to September 15, 2026. Fiscal-year filers get the same six months from their original due date.7Internal Revenue Service. About Form 7004, Application for Automatic Extension of Time to File Certain Business Income Tax, Information, and Other Returns
The extension only pushes back the filing date. It does not extend the time to pay any tax the corporation itself owes on built-in gains, excess passive income, or investment credit recapture. Those payments are still due on the original deadline, and interest accrues on unpaid amounts from that date forward. As of early 2026, the IRS charges 7 percent on standard corporate underpayments, compounded daily.8Internal Revenue Service. Quarterly Interest Rates
Form 7004 can be submitted through the IRS e-file system or mailed to the processing center for the corporation’s location. It has to correctly identify the corporation’s tax year end and Employer Identification Number. A Form 7004 filed late, even by a day, is typically denied, and the corporation faces immediate late-filing penalties. A federal extension does not automatically extend a state deadline; some states accept the federal form, others want their own, so check with the state tax agency before assuming Form 7004 covers you there too.
Estimated Tax Payments
An S corporation itself has to make quarterly estimated tax payments when it expects to owe $500 or more in built-in gains tax, excess net passive income tax, or investment credit recapture tax.9Internal Revenue Service. Estimated Tax The installment dates for a calendar-year corporation are:10Office of the Law Revision Counsel. 26 USC 6655 – Failure by Corporation to Pay Estimated Income Tax
- First quarter (January 1 through March 31): April 15
- Second quarter (April 1 through May 31): June 15
- Third quarter (June 1 through August 31): September 15
- Fourth quarter (September 1 through December 31): January 15 of the following year
Each installment covers 25 percent of the estimated annual liability. A missed payment triggers an underpayment penalty based on the shortfall, the current IRS interest rate, and how long the payment is late.10Office of the Law Revision Counsel. 26 USC 6655 – Failure by Corporation to Pay Estimated Income Tax
Individual shareholders separately owe personal estimated taxes on their share of pass-through income. Those personal payments follow the same April 15, June 15, September 15, and January 15 schedule and are made by each shareholder, not by the corporation.9Internal Revenue Service. Estimated Tax If S corp income is a large share of your total earnings, staying current on those quarterly payments is what keeps a surprise bill and underpayment penalty off your April 15 return.
Payroll Tax Deadlines
An S corporation that pays wages, including wages to shareholder-employees, has to file payroll tax returns on their own schedule.
- Form 941 reports federal income tax withheld from wages plus the employer and employee shares of Social Security and Medicare taxes. It is due by the last day of the month after each quarter ends: April 30, July 31, October 31, and January 31.11IRS.gov. Instructions for Form 941 (Rev. March 2026)
- Form 940, the annual federal unemployment (FUTA) tax return, is due by January 31 following the end of the tax year. If the employer deposited all FUTA tax on time, the deadline extends to February 10.12IRS.gov. 2025 Instructions for Form 940 – Employer’s Annual Federal Unemployment (FUTA) Tax Return
The weekend-and-holiday adjustment applies here too. Payroll tax deposits are separate from the quarterly return itself, and depending on the size of the payroll they are made on either a semiweekly or a monthly schedule.
1099 Filings for Contractors
If the S corporation pays $600 or more to an independent contractor or other non-employee during the year, it has to report those payments on information returns. Key deadlines for the 2026 filing season:13IRS.gov. Publication 1099 General Instructions for Certain Information Returns (For Use in Preparing 2026 Returns)
- Form 1099-NEC (nonemployee compensation): due to both the IRS and the recipient by January 31.
- Form 1099-MISC (other miscellaneous payments): due to the IRS by February 28 on paper, or March 31 if filed electronically. Recipient copies are due by January 31.
These fall well before Form 1120-S, so they need their own preparation timeline.
The S Corporation Election Itself
If the business hasn’t yet elected S status, none of the above deadlines apply until it does. Form 2553 must be filed no more than two months and 15 days after the start of the tax year the election should take effect for, or any time during the preceding tax year.14Internal Revenue Service. Instructions for Form 2553 For calendar-year S status starting in 2026, that puts the election deadline at March 16, 2026. Late-election relief is available in some circumstances under Revenue Procedure 2013-30.15Internal Revenue Service. Late Election Relief
What Filing Late Costs
The penalty for filing Form 1120-S late, or filing without required information, is $255 per shareholder for each month or partial month the return is overdue, capped at 12 months.16Internal Revenue Service. Failure to File Penalty That rate applies to returns due after December 31, 2025.17Office of the Law Revision Counsel. 26 USC 6699 – Failure to File S Corporation Return A five-shareholder S corp that files three months late owes $3,825 in penalties alone: $255 × 5 × 3.
The penalty can be waived if the corporation shows reasonable cause. Common accepted arguments include reliance on a tax professional who failed to file, destruction of records in a natural disaster, or serious illness of the person responsible for the return. Being unaware of the deadline is not reasonable cause.
The K-1 penalties described earlier stack on top of the late-filing penalty, so the total cost of a late or incomplete return grows with each shareholder involved.1Internal Revenue Service. Instructions for Form 1120-S (2025) Filing Form 7004 by the original due date avoids all of it, even if the corporation ends up using the full six extra months.