Wisconsin’s filing dates track the federal calendar, and it was an early adopter of the elective pass-through entity tax. Its corporate extension has a distinctive twist, and an economic development surcharge applies to larger entities on top of the income tax.

Wisconsin Tax Deadlines at a Glance (2026)

Wisconsin S corporations (Form 5S) and partnerships (Form 3) are due March 16, 2026 — the 15th day of the third month, rolled from Sunday, March 15 — extending to September 15. Individuals and C corporations file by April 15. Wisconsin honors the federal extension automatically (attach a copy; no separate state form), but note the corporate extension runs to the later of the federal extended date plus 30 days or a seven-month automatic extension — roughly November 15 for calendar-year C corps.

Rates and the Elective PTE Tax

Wisconsin’s individual income tax is graduated across four brackets: 3.50%, 4.40%, 5.30%, and a 7.65% top rate. The corporate income and franchise tax rate is 7.9%. Wisconsin was one of the earliest states to offer an elective pass-through entity tax — S corporations since 2018 and partnerships since 2019 — also at 7.9%, with the election made on the entity return for SALT-cap relief.

Economic Development Surcharge

On top of income tax, entities with $4 million or more in gross receipts owe Wisconsin’s economic development surcharge — 0.2% of Wisconsin net income, a minimum of $25 and a maximum of $9,800. CPAs should factor this into the total Wisconsin liability for larger corporate and pass-through clients.

Estimates, Extensions, and Penalties

Individual estimated payments are due April 15, June 15, September 15, 2026, and January 15, 2027. Because Wisconsin follows the federal extension automatically, no separate state extension form is needed — but interest still accrues during the extension (an extension of time to file is not time to pay). The corporate extension-period interest is 1% per month; the late-filing penalty is 5% per month (max 25%), and delinquent interest runs at 18% annually once assessed.

CPA Action Steps

• File Form 5S and Form 3 by March 16, 2026 (weekend roll from March 15)

• Apply the 7.9% corporate/PTE rate and the four-bracket individual schedule

• Add the 0.2% economic development surcharge for clients over $4M in gross receipts

• Attach the federal extension — Wisconsin needs no separate form

Verify current rates and dates against the Wisconsin Department of Revenue (revenue.wi.gov) before filing.