Why S corps file in March, not April
An S corporation doesn't pay federal income tax itself — profits and losses pass through to shareholders on Schedule K-1, and shareholders report them on their personal returns. Those personal returns are due April 15. If the S corp filed in April too, no shareholder would have their K-1 in time. So the IRS puts the S corp deadline a month earlier: the 15th day of the third month after year end.
When that date lands on a weekend or federal holiday, the deadline moves to the next business day — which is why 2026's deadline is March 16 rather than March 15.
How the extension works
File Form 7004 by the original deadline and you get an automatic 6-month extension — no explanation needed, no IRS approval to wait for. For calendar-year 2026 filers that moves the deadline to September 15, 2026, and there are no further extensions past that.
Two things the extension does not do: it doesn't extend any tax payment the S corp owes (built-in gains tax, excess passive income tax — both rare but real), and it doesn't extend your personal April 15 deadline. If your K-1 will arrive late because the corp extended, you'll likely need to extend your 1040 too.
What filing late costs
$255 per shareholder, per month or part of a month — up to 12 months — even if no tax is owed. One shareholder, filed 2 months late: $510. Two shareholders, 6 months late: $3,060. Two shareholders, forgotten for a year: $6,120. The penalty accrues whether the S corp made money or not.
Compare that to the fix: Form 7004 is a single page that takes minutes to file. If you're not ready in March, extend — the penalty math never favors waiting.
Fiscal-year S corps and new elections
Fiscal year: the same rule counts from your own year end. A June 30 fiscal year means a September 15 due date; a September 30 year end means December 15.
Electing S status: an existing corporation or LLC that wants S corp treatment for calendar 2026 must file Form 2553 within 2 months and 15 days of the year's start — March 16, 2026. Miss it and the election generally takes effect the following year, though late-election relief is often available under Rev. Proc. 2013-30.
California note: CA S corps file Form 100S on the same March schedule, plus franchise tax obligations the IRS doesn't have — see our California S corp deadlines guide.
Frequently asked questions
When is Form 1120-S due in 2026?
March 16, 2026 for calendar-year S corps — March 15 falls on a Sunday, so the deadline moves to Monday.
What's the extended deadline?
September 15, 2026, by filing Form 7004 before the March deadline. No further extensions exist.
When do shareholders get their K-1s?
K-1s are due when the return is due — March 16, or September 15 if extended. Extended K-1s usually mean extending your personal 1040 as well.
What's the late-filing penalty?
$255 per shareholder per month (or part of a month), up to 12 months, for returns due in 2026 — regardless of whether any tax is owed.
What if my S corp uses a fiscal year?
The return is due the 15th day of the third month after your fiscal year ends, with the same weekend/holiday shift rule.
When is the Form 2553 election deadline for 2026?
March 16, 2026 — within 2 months and 15 days of the start of the tax year. Late elections may qualify for relief under Rev. Proc. 2013-30.