Late 1120-S filing penalty: 5% per month cap

Quick answer: For a 2025 tax year Form 1120-S, if the return is filed late, the general corporate late-filing penalty framework of 5% per month (up to 25%) applies, but the documents provided don't contain the special per-shareholder, per-month penalty that actually governs late-filed S corporation returns — so I can only give you the general corporate rule below and flag the gap. If the return still isn't filed after two years, the penalty accumulates only for the first 5 months (reaching its cap), but interest and other consequences (like loss of S status risk or accuracy penalties) can keep building indefinitely.

What it depends on

The documents I have cover the general corporate late-filing penalty under IRC § 6651(a)(1) and IRS Publication 542, not the S-corporation-specific penalty (which is actually found in IRC § 6699, a section not included in these documents). I'll explain what the provided sources say, but you should treat the S-corp-specific penalty as unconfirmed here.

What the general rule says

IRS Publication 542 explains that a corporation that does not file its tax return by the due date, including extensions, may be penalized 5% of the unpaid tax for each month or part of a month the return is late, up to a maximum of 25% of the unpaid tax.1 If a late-payment penalty applies for the same period, the penalty for late filing is reduced by the amount of the penalty for late payment.2 Publication 542 also notes that a minimum penalty applies for a return that is over 60 days late, and that minimum penalty amount may be adjusted for inflation — see the Instructions for Form 1120 (or the instructions for your applicable return) for the current-year figure.3

The underlying statute, IRC § 6651(a)(1), confirms this mechanic: there shall be added to the amount required to be shown as tax on such return 5 percent of the amount of such tax if the failure is for not more than 1 month, with an additional 5 percent for each additional month or fraction thereof during which such failure continues, not exceeding 25 percent in the aggregate.4 The Code also sets a floor for very late returns: in the case of a failure to file a return of tax imposed by chapter 1 within 60 days of the date prescribed for filing, unless due to reasonable cause, the addition to tax shall not be less than the lesser of $435 or 100 percent of the amount required to be shown as tax on such return.5 That $435 figure is inflation-adjusted for returns filed in later years, per IRC § 6651(j).

Important caveat — most S corps owe little or no tax

This 5%-of-unpaid-tax penalty is calculated on tax owed with the return. Because S corporations are generally pass-through entities with little or no entity-level tax, this dollar-based penalty is often small or zero — which is exactly why Congress created a separate, flat per-shareholder-per-month penalty for late S corp returns (IRC § 6699). That section is not in the documents provided, so I can't give you its current dollar amount; you'll want to check the current Form 1120-S instructions or confirm the figure with a CPA.

If it's not filed for 2 years

Under the framework here, the percentage-based failure-to-file penalty stops growing once it hits its 25% cap — not exceeding 25 percent in the aggregate4 — which happens after about 5 months of nonfiling. Beyond that point, no additional failure-to-file percentage accrues under this section, but reasonable cause is not automatically lost, and other consequences (interest on unpaid tax, potential accuracy-related penalties, and administrative risks to the S election) can still apply for as long as the return remains unfiled.

Current § 6699 penalty amounts (Form 1120‑S) For a late or incomplete Form 1120‑S:

Base penalty per month = applicable rate × number of persons who were shareholders at any time during the tax year.

Applies for up to 12 months (not 5).

Rates by return due date (per shareholder, per month):

Returns due in 2024: $235

Returns due in 2025: $245

Returns due after 12/31/2025 (e.g., 2025 tax year filed in 2026): $255

If tax is due, the penalty is:

§ 6699 amount (above) plus

The usual § 6651 failure‑to‑file penalty of 5% per month of unpaid tax (up to 25%).

Minimum penalty for a return required to be filed in 2026 that is more than 60 days late: the smaller of tax due or $525.

So for a typical 3‑shareholder S corp with a 2025 tax year return filed late in 2026 and no tax due, the penalty could be:

$255 × 3 shareholders × (months late, up to 12) $255×3 shareholders×(months late, up to 12) If 12 months late: $9,180, even with $0 tax due.

What happens if it isn’t filed for 2 years? Under § 6699:

The per‑shareholder, per‑month penalty accrues for up to 12 months, then stops.

After 12 months, no additional § 6699 penalty accrues, but:

Interest on any unpaid penalties and tax continues indefinitely.

The IRS can still pursue information‑return penalties, accuracy‑related penalties, and potentially administrative actions (e.g., jeopardizing S status if filings are chronically missing or inconsistent with the S election).

The § 6651 percentage penalty (5% per month of unpaid tax) still caps at 25% after ~5 months, as your answer said.

So “2 years late” doesn’t mean 24 months of § 6699 penalty; it means up to 12 months of § 6699, then ongoing interest and other risks.

Reasonable cause

Both the statute and Publication 542 note this penalty can be avoided: § 6651(a)(1) doesn't apply if it is shown that such failure is due to reasonable cause and not due to willful neglect4, and Publication 542 advises that if you get an IRS notice about the penalty, you should send the IRS an explanation and they will determine if the corporation meets reasonable-cause criteria — do not attach an explanation when the return is filed; see the instructions for your income tax return instead.6

Given that the specific S-corp penalty (§ 6699) and the current inflation-adjusted dollar figures aren't in the documents provided, it's worth having a CPA calculate the exact exposure for your specific late 1120-S filings and evaluate a reasonable-cause or first-time-abatement request.

Citations (6) IRS Publication 542 — Corporations: “A corporation that does not file its tax return by the due date, including extensions, may be penalized 5% of the unpaid tax for each month or part of a month the return is late, up to a maximum of 25…” IRS Publication 542 — Corporations: “If the corporation is charged a penalty for late payment of tax (discussed next) for the same period of time, the penalty for late filing is reduced by the amount of the penalty for late payment.…” IRS Publication 542 — Corporations: “A minimum penalty applies for a return that is over 60 days late. The minimum penalty amount may be adjusted for inflation. See the Instructions for Form 1120 (or the instructions for your applicable …” 26 U.S.C. § 6651 — Failure to file tax return or to pay tax: “In case of failure— (1) to file any return required under authority of subchapter A of chapter 61 (other than part III thereof), subchapter A of chapter 51 (relating to distilled spirits, wines, and…” 26 U.S.C. § 6651 — Failure to file tax return or to pay tax: “In the case of a failure to file a return of tax imposed by chapter 1 within 60 days of the date prescribed for filing of such return (determined with regard to any extensions of time for filing), unl…” IRS Publication 542 — Corporations: “Generally, if the corporation receives a notice about interest and penalties after it files its return, send the IRS an explanation and we will determine if the corporation meets reasonable -cause cri…”