An S corporation or a partnership that took a six month extension in March has until 15 September to file. Miss it and the penalty is charged per owner, per month, for up to twelve months, and it applies even when the entity made nothing and owes no tax at all.
That per owner multiplier is why a small late return produces a bill nobody expects.
How the penalty is actually calculated
Three things multiplied together:
- The base rate, per owner per month
- The number of partners or shareholders at any time during the tax year
- The number of months, or partial months, the failure continues, capped at twelve
For a return with an original due date after 31 December 2025, which covers a calendar year 2025 entity return due in March 2026 and extended to September 2026, the base rate is 255 dollars.
Note that the rate is fixed by the return’s original due date, not by when you eventually file. It is also indexed, and it has risen every year recently: 210 dollars for 2021 and 2022, 220 for 2023, 235 for 2024, 245 for 2025, 255 after that. Guidance quoting an older figure is not slightly out of date, it is understating the exposure.
A partial month counts as a month. Filing on the first of the month and filing on the twenty eighth cost the same.
Why it multiplies into real money
Take a three owner S corporation that files six months late. Three owners, six months, at 255 dollars, is 4,590 dollars. Not for unpaid tax. For a missing return.
Take four owners and the full twelve months and it is 12,240 dollars.
The entity may have made no profit at all that year. The penalty is for the absent return, not for tax, so a loss making entity is charged exactly the same as a profitable one. This is the part that people find hardest to believe and it is the reason the number gets so large before anyone takes it seriously.
The consequence for the owners, which is worse than the penalty
Entity returns are due before individual ones for a reason: owners cannot complete their own returns without their K-1s.
So a missed entity deadline does not produce one problem. It produces one for the entity and one for every owner, each of whom is now either filing without information they should have had or extending their own return behind the entity’s. One missed filing becomes several late ones, and the owners usually find out about it late.
What to do with a K-1 that arrives late, and why extending beats filing an estimate, is covered in reading your K-1.
The extension was never an extension to pay
Form 7004 gives an automatic six month extension to file Form 1120-S or Form 1065. Automatic means it is granted on filing the form, not assessed.
It extends the filing. It has never extended the time to pay anything owed. For a pass through entity that often matters less at entity level, since the tax generally sits with the owners, but the principle holds wherever there is a liability, and the owners’ own payment obligations were never extended either.
An extension filed on time and not needed costs nothing. There is no downside to having one in place.
If 15 September has already passed
Two things are worth doing immediately and in this order.
File. The penalty accrues per month up to twelve. Every month of delay is another multiple of the base rate times the number of owners. Nothing about the situation improves with time and the meter is running.
Then look at relief. Relief from this penalty exists and is worth pursuing, but it is a separate exercise and it is not a reason to delay filing. A return filed late is a smaller problem than a return still outstanding.
What to put in place for next year
Diarise the original March deadline rather than the September one, and file the extension then as a matter of routine whether or not you expect to need it.
Then diarise September as the real deadline it is, with enough time before it for the bookkeeping to be finished rather than started.
The fees for entity returns, and for the bookkeeping that has to happen before one can begin, are set out on the pricing page.