Wisconsin splits its calendar the way the IRS does. For the 2025 tax year, individual and C-corporation returns are due April 15, 2026, and S-corporations and partnerships are due March 16, 2026. Wisconsin honors the federal extension automatically. The detail worth planning around is the entity-level tax election, which Wisconsin offers at a flat 7.9% and which has to be chosen on the return itself.

What are Wisconsin's 2026 tax filing deadlines?

For the 2025 tax year, the Wisconsin individual return (Form 1) and C-corporation return (Form 4, or Form 6 for combined groups) are due April 15, 2026. S-corporations (Form 5S) and partnerships (Form 3) are due March 16, 2026, since the statutory March 15 date falls on a Sunday. Wisconsin uses the 15th day of the third month for pass-throughs, matching the federal schedule.

How does Wisconsin's extension work?

Wisconsin automatically honors any federal extension; you attach a copy of the federal Form 4868 to the Wisconsin return rather than pre-filing a state request. The extended individual deadline is October 15, 2026. The extension covers filing only. Wisconsin charges 1% interest per month on tax unpaid after April 15, 2026, so paying by the original date is the only way to avoid it.

What is Wisconsin's pass-through entity tax election?

Wisconsin lets a tax-option S-corporation or a partnership elect to be taxed at the entity level at 7.9%, which moves the state tax deduction above the federal SALT cap. An S-corporation makes the election by checking box 7 in Part A of Form 5S, with consent of shareholders holding more than 50% of shares. A partnership checks box "I" on Form 3, with consent of partners holding more than 50% of capital and profits. The election is annual. Because it is made on the return, it is a filing-season decision that depends on year-end planning, so flag candidates before the March deadline.

How does Wisconsin tax nonresident pass-through owners?

The entity withholds. Wisconsin requires pass-through withholding on a nonresident owner's Wisconsin-source income, remitted on Form PW-1, unless that owner's Wisconsin share is under $2,000. The entity can also file a composite return for nonresident owners (Form 1CNS for S-corp shareholders, Form 1CNP for partners), but filing composite does not remove the entity's PW-1 obligation.

Who owes Wisconsin estimated tax?

Individuals expecting to owe $500 or more after withholding pay quarterly on April 15, June 15, September 15, 2026, and January 15, 2027 (Form 1-ES). Corporations expecting to owe $500 or more in net tax plus surcharge follow the same April, June, September, and December schedule.

How does a firm handle Wisconsin at scale?

Wisconsin's dates are conventional, so the work is in the elections and the withholding. The 7.9% entity election lives on the return and needs owner consent, which is a conversation to have before March, not during it. Nonresident withholding on PW-1 is easy to miss when the owner's share is small but still over the $2,000 line. Track which clients elected the entity tax and which owners cleared the withholding threshold, and the Wisconsin returns stop generating notices. See how we think about scaling tax prep workflows.

Where do these dates come from?

All deadlines and thresholds here trace to the Wisconsin Department of Revenue. Verify against the source before filing.