S Corp Tax Filing Deadline: Form 1120-S, Extensions, K-1s and What Happens If You File Late

Updated September 7, 2026. Dates are from the IRS 2026 tax calendar (Publication 509) and the 2025 Form 1120-S instructions. If you need the return prepared, see S corp tax preparation; this article is about the calendar.

The short answer: a calendar-year S corporation files Form 1120-S by the 15th day of the third month after year end — March 15, or the next business day. The tax-year 2025 return was due March 16, 2026 (March 15 fell on a Sunday), extended to September 15, 2026. The tax-year 2026 return is due March 15, 2027, extended to September 15, 2027. Form 7004 gives an automatic six-month extension with no signature or explanation, but it must be filed by the original due date, and it does not extend the time to pay any tax the corporation itself owes. Shareholders get their K-1s when the return is filed, so an extended corporation usually means extended shareholders. File late and the penalty is $255 per shareholder for every month or part of a month, up to 12 months, whether or not any tax is due.

The 2026 and 2027 dates

Fiscal-year S corporations follow the same rule — the 15th day of the third month after the year ends — so a June 30 year end files by September 15.

How the Form 7004 extension works

Form 7004 is an automatic six-month extension. You do not sign it or give a reason; you file it, electronically or on paper, on or before the original due date, and the filing deadline moves to September 15. Three things it does not do:

• It does not extend the time to pay. Most S corporations owe no federal income tax, but those with built-in gains tax, excess net passive income tax or investment credit recapture must estimate and pay it by the original due date.
• It does not extend the shareholders’ personal returns. A shareholder whose K-1 will not arrive until September files Form 4868 for their own return by April 15 and pays their estimated personal tax by then.
• It does not help if filed late. A Form 7004 filed after the due date is not valid, and the late-filing penalty runs from the original date.

Extending is not a red flag. It is the normal course when the books are not reconciled by March, and a correct return in August beats a rushed one in March that has to be amended.

K-1 timing: why the corporation’s deadline is really the shareholders’ deadline

The corporation must give each shareholder a Schedule K-1 by the date it files the return. Each shareholder then reports the K-1 on their Form 1040. In practice this means: if the 1120-S is filed by March 16, shareholders can file personal returns by April 15; if the corporation extends, shareholders should extend too and pay in an estimate of what the K-1 will show. Failing to furnish a correct K-1 on time carries its own penalty — $340 per K-1, and more if the failure is intentional — separate from the late-return penalty.

What happens if you file the 1120-S late

• Late-filing penalty: for returns due after December 31, 2025, $255 per shareholder for each month or part of a month the return is late, for up to 12 months. Two shareholders, five months late: $2,550. This applies even when the corporation owes no tax.
• If the corporation does owe tax: an additional 5% of the unpaid tax per month, up to 25%, with a minimum of $525 if the return is more than 60 days late, plus the failure-to-pay penalty and interest.
• Shareholders are affected too: without K-1s they cannot file accurately, so their returns go late or get amended.
• Penalty relief: an S corporation with a clean three-year history can request First-Time Abate for the late-filing penalty, and from 2027 the IRS applies that relief automatically at processing under its new Automatic Exemption from Penalty. Reasonable cause is the alternative when the history is not clean. See our guide to first-time penalty abatement.
• Several unfiled years: the per-shareholder penalty accrues on each one. Start with unfiled tax returns.

Other S corp dates during the year

• Payroll: shareholder-employees are paid through payroll, so Forms 941 are due quarterly (April 30, July 31, October 31, January 31) and W-2s are due to employees and the SSA by the end of January — February 2, 2026 for 2025 wages.
• Shareholder estimated taxes: K-1 income is not subject to withholding, so shareholders typically make quarterly estimates: April 15, June 15, September 15 and January 15 for 2026.
• Corporate estimated tax: only if the corporation expects $500 or more of built-in gains, excess passive income or recapture tax.
• State: California Form 100S is due March 15 (March 16 in 2026) with the 1.5% tax and the $800 minimum franchise tax; the FTB grants a six-month filing extension automatically, but the tax must be paid by the original date.

Electing S status: the deadline that trips up new companies

Form 2553 is due no more than 2 months and 15 days after the beginning of the tax year the election is to take effect — March 15 for a calendar-year company — or at any time during the preceding year. Miss it and the company stays a C corporation (or an LLC stays on its default classification) for the year, unless late-election relief under Revenue Procedure 2013-30 applies; that relief is available for up to three years and 75 days when there is reasonable cause and the company has been acting as an S corporation. The IRS confirms acceptance with notice CP261.

A simple deadline plan

1. January: close the books, run payroll year-end, issue W-2s and 1099s.
2. February: reconcile every account through December 31, gather the checklist items, review officer compensation and distributions.
3. Early March: prepare and review the 1120-S and K-1s. If it will not be ready, file Form 7004 now, not on the 15th.
4. March 15: file the return or the extension; pay any entity-level tax; pay California’s tax.
5. April 15: shareholders file or extend their own returns and pay first-quarter estimates.
6. September 15: extended 1120-S filed, K-1s delivered, shareholders finish by October 15.

Clients on a monthly bookkeeping plan arrive at January with the books already reconciled, which is what makes a March filing realistic. If the books are behind, catch-up bookkeeping comes first and the extension buys the time to do it properly.

Common questions

When is the S corp tax return due in 2027?

March 15, 2027 for calendar-year corporations, extended to September 15, 2027 with Form 7004.

Does Form 7004 need a reason or signature?

No. It is automatic if filed by the original due date.

Do shareholders have to extend if the corporation extends?

Not automatically, but usually yes in practice: without the K-1 they cannot complete their return. They file Form 4868 and pay an estimate by April 15.

Is there a penalty for filing an S corp return late if no tax is owed?

Yes: $255 per shareholder per month, up to 12 months, for returns due after December 31, 2025.

Can I file the 1120-S on paper?

Only if the corporation files fewer than 10 returns of any type during the year; otherwise e-filing is required. We e-file every return.

Need the return or the extension handled? Get started and we will confirm scope and price before the deadline, not after.

Sources

• IRS, Instructions for Form 1120-S (2025) — irs.gov
• IRS, Publication 509, Tax Calendars (2026) — irs.gov
• IRS, About Form 7004 and instructions — irs.gov
• IRS, Instructions for Form 2553 — irs.gov
• IRS, Failure to file penalty (partnership and S corporation amounts) — irs.gov
• IRS, Administrative penalty relief — irs.gov
• California FTB, S corporations; due dates and extensions — ftb.ca.gov, ftb.ca.gov