How to Complete and File Wisconsin Form 5S: Deadlines, 5K-1, Penalties

Wisconsin Form 5S is the annual franchise or income tax return every tax-option (S) corporation files with the Wisconsin Department of Revenue. If your business has a valid federal S election and does business in Wisconsin, you file Form 5S to report the corporation’s income, deductions, credits, and each shareholder’s allocated share. Calendar-year filers owe the return by March 15, and the state requires most S corporations to file it electronically.

Who Has to File

Two groups of corporations file Form 5S. The first is any corporation organized under Wisconsin law that is treated as an S corporation under Subchapter S of the Internal Revenue Code and has not elected out of tax-option status under sec. 71.365(4)(a) of the Wisconsin Statutes. The second is any foreign corporation (one not organized under Wisconsin law) that does business in Wisconsin or otherwise has nexus with the state through employees, property, or significant sales activity.¹

You file even if the corporation owes no tax. Wisconsin uses the return to track income flowing to resident and nonresident shareholders. A corporation that has elected out of tax-option status files Form 4 instead, and once it elects out it cannot re-elect for at least four taxable years.²

What to Gather Before You Start

Most of Form 5S pulls directly from the federal return, so start there:

A completed federal Form 1120-S. Attach a full copy to the Wisconsin return, including when the corporation did no business in Wisconsin during the year.¹
The corporation’s Federal Employer Identification Number.
The 15-digit Wisconsin tax account number issued by the Department of Revenue. If the business doesn’t have one, register through My Tax Account before filing.³
Full names, addresses, and Social Security or Taxpayer Identification Numbers for every person or entity that held shares at any point during the year.
Total company assets from federal Form 1120-S, Item F.
Wisconsin sales figures if the corporation operates in more than one state, so you can compute the apportionment percentage.
Supporting schedules for any credits claimed, such as the manufacturing and agriculture credit or the research credit.

When Form 5S Is Due

Form 5S is due by the 15th day of the third month after the close of the corporation’s taxable year. That falls on March 15 for calendar-year filers.¹

Two extension paths are available, and neither requires you to file anything with the Department of Revenue by the original due date:

If the IRS has granted a federal extension, your Wisconsin due date is automatically extended to 30 days after the federal extended due date. Attach a copy of the federal extension to the Wisconsin return when you file.¹
If there is no federal extension, Wisconsin provides an automatic 7-month extension, or until the original due date of the corresponding federal return, whichever is later.⁴

An extension gives you more time to file, not more time to pay. Interest on any unpaid tax runs from the original due date. To stop the clock, pay the estimated amount owed by the unextended deadline using Form Corp-ES.¹

How to Submit the Return

Wisconsin requires corporations to file through the Federal/State E-Filing Program using approved third-party tax software that supports Wisconsin corporate returns. For most S corporations, electronic filing is not optional.¹

Two narrow exceptions let you file on paper without asking: the corporation must have 10 or fewer shareholders, and gross income reported on Schedule 5K, line 20, column (d) must be $20,000 or less. If the corporation exceeds either threshold but electronic filing creates a genuine hardship, request a waiver using Form EFT-102, Electronic Filing or Electronic Payment Waiver Request.¹

Paper returns filed under the small-filer exception or an approved waiver go to:

Wisconsin Department of Revenue
PO Box 8965
Madison, WI 53708-8965⁵

Whichever way you file, include every supporting document: a Schedule 5K-1 for each shareholder, any credit schedules, and a copy of the federal extension if you filed under one. After submission, watch your My Tax Account portal for correspondence and respond quickly to any adjustment notices.

Apportioning Income for Multi-State Operations

If the corporation earns income both inside and outside Wisconsin, apportion it. Wisconsin uses a single-sales-factor formula: only the ratio of Wisconsin sales to total sales determines the apportionment percentage.⁶ That percentage flows into Part II of Form 5S and reduces total income to the Wisconsin-taxable share. The same percentage carries through to every shareholder’s Schedule 5K-1, so misallocating sales between states creates problems in more than one place.

Schedule 5K-1 for Each Shareholder

Every tax-option corporation must provide each shareholder with a completed Schedule 5K-1. The schedule translates the corporation’s income, gains, losses, deductions, and credits into each shareholder’s allocated share, which the shareholder reports on their personal Wisconsin return.⁷

Schedule 5K-1 is still required when the corporation has elected to pay tax at the entity level. In that case, box 3 in Part B is checked, shareholders do not report the pass-through income on their own Wisconsin returns, and the schedule still gets prepared so the Department of Revenue can reconcile the figures.⁷

The Entity-Level Tax Election

Wisconsin lets an S corporation elect to pay franchise or income tax at the entity level at a flat 7.9 percent of net income reportable to the state.⁸ Holders of more than 50 percent of the corporation’s shares must consent. Make the election on or before the due date (or extended due date) of the Form 5S return for that taxable year. The tax is computed on Schedule 5S-ET, which you attach to the return.²

A corporation that makes this election cannot also file a composite return on behalf of nonresident shareholders for the same tax year.⁹ Estimated tax payments still apply, and the interest rate on underpayments can run as high as 12 or 18 percent depending on the circumstances. An extension of time to file does not extend the time to pay.¹⁰

Nonresident Shareholders

Pass-Through Withholding

An S corporation with nonresident shareholders who earn Wisconsin income must withhold state income tax on each nonresident’s share when that share is $2,000 or more. The withholding rate is the highest individual income tax rate for the year. The corporation files an annual withholding return and notifies each affected shareholder of the amount withheld.¹¹

Withholding is not required if the nonresident’s Wisconsin share is under $2,000, if the shareholder files an affidavit on Form PW-2 agreeing to file a Wisconsin return, or if the corporation has elected the entity-level tax.¹¹

Composite Returns

Rather than have each nonresident file an individual Wisconsin return, the corporation can file a composite return on Form 1CNS, bundling eligible nonresidents and paying the tax on their behalf. A shareholder qualifies only if their sole Wisconsin income comes from the S corporation and they are nonresidents for the full year. Shareholders who want to claim credits or itemized deductions, who file on a fiscal year, or who are entities or electing small business trusts cannot be included.⁹

Built-In Gains Tax

Wisconsin imposes an additional tax on S corporations that have a net recognized built-in gain during the federal recognition period. This mainly reaches corporations that converted from C corporation status to S status while holding appreciated assets. Wisconsin calculates the tax along the same lines as the federal built-in gains tax under IRC section 1374, but applies its own corporate tax rate, the Wisconsin basis of the assets, and the Wisconsin apportionment percentage. Corporations whose S election predates January 1, 1987, and that have not elected out and back in since then, are not subject to it.¹²

Penalties for Late or Incorrect Returns

A return filed after the extension period expires triggers a flat $150 late-filing fee, separate from interest on unpaid tax, which accrues from the original due date.¹

Errors and omissions cost more. Civil penalties can reach 100 percent of the tax not reported. Filing a false return is a criminal offense carrying fines up to $10,000 and possible imprisonment. If the corporation participates in reportable transactions and fails to disclose them, an additional penalty of up to $30,000 applies for listed transactions under sec. 71.81 of the Wisconsin Statutes.¹

The best way to stay out of trouble is straightforward: file electronically, pay any estimated tax by the original due date even when using an extension, and make sure the figures on each Schedule 5K-1 reconcile with the amounts on the return. Mismatches between the entity return and what shareholders report on their personal returns are the most common trigger for a closer look from the department.

1
Wisconsin Department of Revenue. 2025 Wisconsin Form 5S Instructions
2
Wisconsin State Legislature. Wisconsin Statutes 71.365 – General Provisions
3
Wisconsin Department of Revenue. DOR Additional Detail Information of WT-6 Batch Payment Schema
4
Cornell Law Institute. Wisconsin Administrative Code Tax 2.96 – Extensions of Time to File Corporation Franchise or Income Tax Returns
5
Wisconsin Department of Revenue. Tax Return Mailing Addresses
6
Wisconsin State Legislature. Wisconsin Statutes 71.25(6)
7
Wisconsin Department of Revenue. Tax-Option (S) Corporation Filing Schedule 5K-1 Questions
8
Wisconsin Department of Revenue. Pass-Through Entity-Level Tax
9
Wisconsin Department of Revenue. DOR Composite Returns
10
Wisconsin Department of Revenue. Pass-Through Entity-Level Tax
11
Wisconsin State Legislature. Wisconsin Statutes 71.775(4)(em)
12
Wisconsin State Legislature. Wisconsin Statutes 71.35 – Imposition of Additional Tax on Tax-Option Corporations