The full 2026 deadline calendar
All dates below are for calendar-year S corporations. Fiscal-year corporations count from their own year end (return due the 15th day of the 3rd month after close).
What happens if you miss the deadline
$255 per shareholder, per month. That's the federal late-filing penalty for Form 1120-S returns due in 2026 — charged even when the S corp owes zero tax, for up to 12 months. A two-shareholder S corp filing three months late owes $1,530. California adds its own per-shareholder late penalty on Form 100S, plus interest on any unpaid franchise tax.
The extension is free and automatic — filing Form 7004 by March 16 takes minutes and eliminates the late-filing penalty entirely (as long as you file by September 15 and pay anything owed on time). There is no good reason to miss the March deadline unextended.
Frequently asked questions
When is the S corp tax return due in 2026?
March 16, 2026, for calendar-year S corporations. The statutory deadline is the 15th day of the third month after year end (March 15), but that's a Sunday in 2026, so it moves to Monday the 16th. Schedule K-1s are due to shareholders the same day.
Can I get an extension for Form 1120-S?
Yes — file Form 7004 by March 16, 2026 for an automatic 6-month extension to September 15, 2026. It extends filing only; any tax owed is still due March 16.
What's the penalty for filing late?
$255 per shareholder per month (or part of a month) for returns due in 2026, up to 12 months — even with no tax owed. It's one of the most expensive small-business penalties relative to the effort of avoiding it.
When is California's $800 franchise tax due?
With your first Form 100-ES installment on April 15, 2026. It's owed every year your corporation exists — active, inactive, or operating at a loss (waived only for a qualifying first year). Any remaining 1.5% franchise tax balance from 2025 is due with Form 100S on March 16.
When is CA Form 100S due?
March 16, 2026 for calendar-year filers. California grants an automatic paperless extension to file by September 15, 2026 — no form required — but full payment is still due March 16.
Why is there no California personal estimate in September?
California front-loads individual estimated taxes at 30% (April), 40% (June), 0% (September), and 30% (January). Corporate estimates on Form 100-ES follow the regular four-installment schedule instead, with the last one in December.
Are these the same dates as an LLC or C corp?
No. C corporations and single-member LLCs taxed as sole proprietorships generally file by April 15. The March deadline applies to S corps (and partnerships) because shareholders need their K-1s before their personal April 15 deadline.