To file taxes for an S corporation, you prepare Form 1120-S reporting the company’s income and deductions, issue a Schedule K-1 to every shareholder showing their share of those items, and each shareholder then reports the K-1 amounts on their personal Form 1040. The corporation itself does not pay federal income tax; the income passes through to the owners. For calendar-year filers, Form 1120-S and the K-1s are due March 16, 2026, and the penalty for a late return is $255 per shareholder for each month it is overdue.1Internal Revenue Service. About Form 1120-S, U.S. Income Tax Return for an S Corporation
Confirm the S Election Is in Place
Before filing anything, verify the IRS actually accepted the company’s S election. When a corporation files Form 2553, the IRS sends back a confirmation letter stating whether the election was approved and the effective date. If you never received that letter or can’t locate it, call the IRS Business and Specialty Tax Line at 800-829-4933 to confirm status.2Internal Revenue Service. Filing Requirements for Filing Status Change Filing Form 1120-S without a valid election in place creates problems that are expensive to unwind.
Pull the Records You Need Before You Start
The return is built on year-end financials. You need a finalized income statement and a balance sheet as of December 31, with every account reconciled, because the balance sheet is reported directly on the return.
You also need current shareholder records: full legal name, address, Social Security number or EIN, and the exact ownership percentage held throughout the year. If someone bought or sold shares mid-year, note the precise dates so income and deductions can be prorated by the number of days each person held stock.
Each shareholder should be tracking stock basis, which is the running total of their investment. Basis starts with cash or property contributed for stock, increases with income and additional contributions, and decreases with losses, deductions, and distributions. Basis matters because it caps how much loss the shareholder can deduct and determines whether distributions are taxable. Tracking is ultimately the shareholder’s responsibility, but the corporation has to supply the underlying numbers.
Pay Reasonable Compensation to Owner-Employees
Any shareholder who works in the business must receive a salary that reflects what a comparable employee would earn for the same work. The IRS treats corporate officers as employees, so their pay is subject to income tax withholding plus Social Security and Medicare, and it is reported on a W-2, not on a K-1 or 1099.3Internal Revenue Service. S Corporation Employees, Shareholders, and Corporate Officers
Skipping a reasonable salary or setting it artificially low to reduce payroll taxes is one of the fastest ways to invite an audit. When the IRS reclassifies distributions as wages, the corporation owes back payroll taxes plus penalties and interest.4Internal Revenue Service. Wage Compensation for S Corporation Officers Document how you arrived at the salary using factors like job duties, hours, comparable positions in your industry, and company revenue. Those W-2 wages are deductible on the corporate return, which reduces the ordinary business income flowing through to shareholders.
Health Insurance for More-Than-2% Shareholders
If the S corporation pays health or accident insurance premiums for a shareholder who owns more than 2% of the stock, those premiums must be included as wages in Box 1 of the shareholder’s W-2. They are not subject to Social Security or Medicare tax, so they do not appear in Boxes 3 and 5.5Internal Revenue Service. S Corporation Compensation and Medical Insurance Issues The shareholder then claims the self-employed health insurance deduction on their personal return. Getting the W-2 reporting wrong here is a common mistake that can disqualify the personal deduction.
Complete Form 1120-S
Form 1120-S is informational. It reports the company’s total financial activity and separates out the items that flow through to shareholders.1Internal Revenue Service. About Form 1120-S, U.S. Income Tax Return for an S Corporation
Page 1: Income and Deductions
Fill in the identification section first: name, address, EIN, date of incorporation, and accounting method. The income section calculates gross profit by subtracting cost of goods sold from total gross receipts. Below that, list ordinary business deductions: salaries and wages (including owner-employee compensation not already inside cost of goods sold), rent, repairs, taxes, interest, depreciation from Form 4562, and other standard business expenses. Total deductions come off gross income to produce ordinary business income or loss on Line 21.
Schedule K: The Flow-Through Summary
Line 21 is the biggest number flowing through to shareholders, but it isn’t the only one. Schedule K collects every item that has to be reported separately because it could affect each shareholder’s personal return differently. Portfolio income (interest, dividends, royalties) keeps its character on the way through. Rental real estate income or loss is broken out because passive activity rules may cap the deduction. Section 179 expense, charitable contributions, and tax credits are each stated separately so shareholders can apply their own personal limits. The Schedule K totals feed directly into each shareholder’s K-1.
Balance Sheet and Reconciliation Schedules
Schedule L reports the beginning and ending balance sheets. Schedule M-1 reconciles any differences between book income and the income reported on the tax return, covering items like non-deductible expenses or tax-exempt income. Schedule M-2 tracks the Accumulated Adjustments Account (AAA), which is the running total of income already taxed at the shareholder level but not yet distributed. Distributions are tax-free up to the positive AAA balance, so keeping this schedule accurate matters.
Prepare a Schedule K-1 for Each Shareholder
Prepare a separate Schedule K-1 for every person or entity that held stock at any point during the year. Each K-1 allocates that shareholder’s share of every Schedule K item based on ownership percentage, prorated by days held if ownership changed.
Box 1 reports the shareholder’s share of ordinary business income or loss. The remaining boxes carry the separately stated items: portfolio income, rental income, Section 179 expense, charitable contributions, credits, and distributions. Each retains its tax character on the shareholder’s personal return. A completed K-1 must be furnished to each shareholder by the corporate return’s due date so they can file on time.
How Shareholders Report the K-1
Ordinary business income or loss from K-1 Box 1 goes on Schedule E of the shareholder’s Form 1040.6Internal Revenue Service. About Schedule E (Form 1040), Supplemental Income and Loss If someone holds interests in more than one S corporation, each gets its own column. Separately stated items go to the appropriate schedules: dividends to the dividends section of Form 1040, capital gains and losses to Schedule D,7Internal Revenue Service. Schedules for Form 1040 and Form 1040-SR and charitable contributions to Schedule A if the shareholder itemizes.
Basis Limits and Form 7203
A shareholder cannot deduct more in flow-through losses than their total basis, which is stock basis plus debt basis from loans the shareholder personally made to the corporation. Guaranteeing a bank loan the corporation took out does not create debt basis. Losses beyond total basis are suspended and carried forward until the shareholder restores basis. Distributions in excess of stock basis become taxable gain.
Shareholders who claim a loss deduction, receive a distribution, sell stock, or receive a loan repayment from the corporation should file Form 7203 with their personal return.8Internal Revenue Service. About Form 7203, S Corporation Shareholder Stock and Debt Basis Limitations The form walks through the basis calculation and is how the IRS verifies the loss stayed within legal limits.
Passive Activity and the Net Investment Income Tax
A shareholder who does not materially participate in the business treats flow-through income or loss as passive. Passive losses can only offset passive income, and Form 8582 calculates how much is deductible in the current year.9Internal Revenue Service. About Form 8582, Passive Activity Loss Limitations
A 3.8% net investment income tax under Section 1411 also applies to income from passive activities, including an S corporation interest held by a non-participating shareholder.10Internal Revenue Service. Questions and Answers on the Net Investment Income Tax It kicks in when modified adjusted gross income exceeds $200,000 for single filers or $250,000 for married couples filing jointly.11Office of the Law Revision Counsel. 26 USC 1411 – Imposition of Tax Shareholders who actively run the business and meet material participation tests are generally not subject to the NIIT on their S corporation income.
The Qualified Business Income Deduction
Shareholders may be eligible for a deduction of up to 20% of their qualified business income under Section 199A.12Office of the Law Revision Counsel. 26 USC 199A – Qualified Business Income It is claimed on the personal return and reduces taxable income without reducing adjusted gross income. Qualified business income is the net income flowing through, and it does not include the owner’s W-2 wages, capital gains, dividends, or interest. That creates a real tension in setting owner pay: a higher salary reduces payroll taxes on pass-through income but also shrinks the QBI deduction.
Above certain taxable income thresholds, the deduction cannot exceed the greater of 50% of the S corporation’s W-2 wages, or 25% of those wages plus 2.5% of the cost of qualifying depreciable property the business owns. Certain professional service fields, including health care, law, and financial services, face additional restrictions that phase the deduction out entirely at higher income levels. The thresholds adjust annually for inflation.
Estimated Tax Payments
Because the corporation doesn’t pay federal income tax, shareholders pay tax on flow-through income during the year. If you expect to owe $1,000 or more when you file your personal return, make quarterly estimated tax payments using Form 1040-ES.13Internal Revenue Service. Estimated Taxes The 2026 deadlines are April 15, 2026; June 15, 2026; September 15, 2026; and January 15, 2027.
You avoid the underpayment penalty by paying at least 90% of the current year’s tax liability or 100% of the prior year’s tax, whichever is smaller.14Internal Revenue Service. Topic No. 306, Penalty for Underpayment of Estimated Tax For shareholders whose income fluctuates, the annualized income installment method can match payments to the quarters when income was actually earned. Missing estimated payments is the single most common way S corporation shareholders end up with unexpected penalties at filing time.
Deadline, Extension, and E-Filing
The standard due date for Form 1120-S is March 15. In 2026, March 15 falls on a Sunday, so the deadline shifts to Monday, March 16, 2026. That is also the date for furnishing Schedule K-1 to each shareholder, who then generally needs the K-1 to file their personal return by April 15.
If the corporation needs more time, file Form 7004 on or before the March 16 deadline for an automatic six-month extension, pushing the corporate due date to September 15, 2026.15Internal Revenue Service. About Form 7004, Application for Automatic Extension of Time To File Certain Business Income Tax, Information, and Other Returns Extending the corporate return also delays K-1 delivery, so shareholders may need to extend their personal returns as well. An extension of time to file is not an extension of time to pay; any estimated state tax owed at the entity level must still be paid by the original deadline.
Corporations that file 10 or more information returns in aggregate (W-2s, 1099s, K-1s, and other forms counted together) must file electronically. Most S corporations with even a handful of employees or shareholders hit that threshold easily, so plan on e-filing.
Penalties for Filing Late or Sending K-1s Late
The penalty for filing Form 1120-S late is $255 per shareholder for each month or partial month the return is overdue, up to 12 months.16Internal Revenue Service. Revenue Procedure 2024-40 For a corporation with five shareholders, a return three months late generates a $3,825 penalty. The penalty applies even though no tax is owed at the corporate level.17Office of the Law Revision Counsel. 26 USC 6699 – Failure to File S Corporation Return
Separate penalties apply for failing to furnish correct K-1 statements on time. For statements due in 2026, the per-statement penalty is $60 up to 30 days late, $130 from 31 days late through August 1, and $340 after August 1 or if not filed at all. Intentional disregard raises it to $680 per statement with no maximum cap.18Internal Revenue Service. Information Return Penalties The simplest way to avoid all of this is to file on time or file Form 7004 before the deadline. Even a last-minute extension eliminates the late-filing penalty entirely.
State Filing Is Separate
Most states recognize the federal S election, but state filing requirements vary. Expect a separate state corporate return that mirrors much of Form 1120-S, with deadlines typically aligning with the federal March 15 date. Some states impose an entity-level tax based on gross receipts, net income, or net worth regardless of the pass-through structure, and annual registration or franchise fees are common. If shareholders live in different states, the corporation may need to file composite returns or withhold state income tax on behalf of non-resident owners. Ignoring the multistate rules can create liability for both the corporation and its owners.