Do S Corps File Tax Returns? Form 1120-S, K-1, and Deadlines

Yes. Every S corporation must file a federal tax return each year, even when the business owes no federal income tax itself. The return is Form 1120-S, and for calendar-year S corporations reporting tax year 2025, it is due March 16, 2026.¹

Why the Return Is Required Even When No Tax Is Owed

Federal law under 26 U.S.C. § 6037 requires every S corporation to file a return for each tax year, reporting its gross income, deductions, shareholder details, distributions, and each owner’s share of every financial item.² Form 1120-S is an information return. The company usually writes no check with it. Instead, the return tells the IRS how the corporation’s profits and losses were divided among the shareholders, who then report those amounts on their personal Form 1040s and pay tax at their individual rates.

That pass-through structure is why the filing obligation is absolute. Under 26 U.S.C. § 1366, each shareholder’s pro-rata share of the corporation’s items passes through to their individual return.³ Without the 1120-S, the IRS has no way to verify that shareholders are reporting the right numbers on their own returns.

What Goes on Form 1120-S

The return pulls together the company’s core financial data for the year. The main items include:

Gross receipts and cost of goods sold, which establish gross profit.¹
Operating expenses such as officer compensation, rent, taxes and licenses, depreciation, and other ordinary business costs.
Ordinary business income or loss, the net figure after deductions, reported on Schedule K.
The corporation’s Employer Identification Number along with identifying information for every shareholder.
Names, addresses, and ownership percentages for each person who held stock at any point during the year.

Every figure needs backup documentation. The IRS recommends keeping records that support income, deductions, and credits for at least three years from the filing date, with longer retention required in certain situations such as underreporting income by more than 25 percent.⁴

S corporations that file 10 or more returns of any type during the calendar year (counting employment tax and information returns) must e-file Form 1120-S. The IRS can grant a waiver from that requirement.¹

Schedule K-1 for Each Shareholder

Filing Form 1120-S is not the end of the corporation’s paperwork. Once the numbers are final, the company must allocate income, losses, deductions, and credits to each shareholder on Schedule K-1, based on percentage of stock ownership. The K-1 goes to the shareholder by the same deadline as the 1120-S itself.⁵

The K-1 breaks income into categories: ordinary business earnings, rental income, interest, dividends, capital gains, and various credits. A 40 percent owner sees exactly 40 percent of each line. Those numbers then flow onto specific lines of the shareholder’s personal return.

When the S Corp Itself Owes Federal Tax

The general rule is no entity-level federal income tax, but two exceptions catch owners off guard.

Built-In Gains Tax

When a C corporation converts to S corporation status, appreciation that existed in its assets at the time of conversion can trigger a corporate-level tax if those assets are sold within five years. The tax applies at the flat 21 percent corporate rate on the net recognized built-in gain.⁶⁷ The same rule reaches assets acquired from a C corporation in certain tax-free transactions. After the five-year recognition period ends, built-in gains are no longer an issue.

Excess Passive Investment Income Tax

An S corporation carrying accumulated earnings and profits from prior C corporation years faces a separate tax when more than 25 percent of its gross receipts come from passive sources like interest, dividends, rents, royalties, and annuities. The tax hits the excess passive income at the 21 percent corporate rate.⁸ If the condition persists for three consecutive years, the S election automatically terminates and the company reverts to C corporation status.⁹ Distributing the old C corporation earnings and profits down to zero is the standard fix.

Deadline and Extension

Form 1120-S is due by the 15th day of the third month after the end of the tax year. For calendar-year corporations that normally means March 15. Because March 15, 2026 falls on a Sunday, the deadline for the 2025 tax year moves to March 16, 2026.¹ Schedule K-1s must reach shareholders by the same date.

Need more time? Filing Form 7004 before the deadline grants an automatic six-month extension, moving the due date to September 15 for calendar-year filers.¹⁰ The extension only delays the filing. If the S corporation owes any entity-level tax on built-in gains or excess passive income, an estimated payment is still due by the original deadline. Shareholders waiting on K-1s to finish their personal returns may need to file their own extensions.

Penalties for Late or Missing Returns

The penalty for a late or incomplete Form 1120-S is $245 per shareholder for each month or partial month the return is overdue, up to a maximum of 12 months.¹¹ The amount is indexed for inflation. For a company with four shareholders, a return filed six months late generates a $5,880 penalty. The same penalty applies to a return filed on time but missing required information.¹²

The IRS can waive the penalty if the corporation shows reasonable cause. First-time filers with straightforward facts have better odds of abatement; “I didn’t know I had to file” rarely qualifies. The penalty is charged to the S corporation itself, not to individual shareholders.¹³

Amending a Filed Return

If the S corporation discovers errors after filing, it can submit an amended Form 1120-S with the “Amended Return” checkbox selected. The amended return must include the complete corrected Form 1120-S, any schedules or attachments that changed, and a supporting document identifying each amended line item with the old amount, new amount, and an explanation.¹⁴ The corporation must also issue corrected K-1s to every affected shareholder, who may then need to amend their own personal returns.

State Returns Are Separate

Federal filing is only part of the picture. Most states require S corporations to file a state-level return as well, and the rules vary significantly. Some states fully recognize the federal S election and treat the entity as a pass-through for state purposes. Others impose their own entity-level tax on S corporations regardless of the federal election. A handful charge an annual franchise tax or minimum fee for doing business there, separate from any income tax. Shareholders can also owe income tax in every state where the S corporation operates, not just their home state.

1
Internal Revenue Service. 2025 Instructions for Form 1120-S
2
Office of the Law Revision Counsel. 26 USC 6037 – Return of S Corporation
3
Office of the Law Revision Counsel. 26 USC 1366 – Pass-Thru of Items to Shareholders
4
Internal Revenue Service. How Long Should I Keep Records?
5
Internal Revenue Service. 2025 Instructions for Form 1120-S – Section: Schedule K-1 General Information
6
Office of the Law Revision Counsel. 26 USC 1374 – Tax Imposed on Certain Built-In Gains
7
Office of the Law Revision Counsel. 26 USC 11 – Tax Imposed
8
Office of the Law Revision Counsel. 26 USC 1375 – Tax Imposed When Passive Investment Income of Corporation Having Accumulated Earnings and Profits Exceeds 25 Percent of Gross Receipts
9
Office of the Law Revision Counsel. 26 USC 1362 – Election; Revocation; Termination
10
Internal Revenue Service. Instructions for Form 7004 (Rev. December 2025)
11
Internal Revenue Service. Information About Your Notice, Penalty and Interest
12
Office of the Law Revision Counsel. 26 USC 6699 – Failure to File S Corporation Return
13
Internal Revenue Service. Failure to File Penalty
14
Internal Revenue Service. Amended and Superseding Corporate Returns