Idaho keeps its calendar simple and its extension rule specific. For the 2025 tax year, individual, C-corporation, S-corporation, and partnership returns are all due April 15, 2026. Idaho uses the 15th day of the fourth month for every return type, so pass-throughs do not file on the federal March date. The extension is automatic but conditioned on how much you have already paid.

What are Idaho's 2026 tax filing deadlines?

For the 2025 tax year, the Idaho individual return (Form 40), C-corporation return (Form 41), S-corporation return (Form 41S), and partnership return (Form 65) are all due April 15, 2026. Idaho applies the fourth-month due date to all of them, so its pass-through returns are a month later than the federal March schedule.

How does Idaho's extension work?

Idaho grants an automatic six-month extension to October 15, 2026 with no form, but the extension is only valid if you have prepaid enough by April 15, 2026: either 80% of the current-year tax or 100% of the prior-year tax. The extension is to file only, and interest still runs on any tax unpaid after the original date. Falling short of the 80% threshold can void the extension.

What minimum taxes does Idaho charge?

Idaho corporations owe a $20 minimum income tax regardless of profit. On top of that, most businesses, including C-corporations, S-corporations, and partnerships, pay a $10 permanent building fund tax on the return. These are small but apply every year, so build the $10 line into every Idaho business return and the $20 floor into corporate ones.

What is Idaho's ABE (pass-through entity) election?

Idaho lets a partnership or S-corporation make an Affected Business Entity election to pay Idaho income tax at the entity level, which moves the deduction above the federal SALT cap. The election is available for 2025, made by checking the box on a timely-filed return and filing Form ABE, and must be renewed each year. The entity-level rate is 5.3%, matching the flat individual rate.

How does Idaho tax nonresident pass-through owners?

For each nonresident owner with at least $1,000 of Idaho distributable income, the entity uses one of three methods: the owner files a Form PTE-NROA agreement, the entity includes the owner in a composite filing at 5.3%, or the entity withholds at the highest individual rate. All three require Form PTE-12 and an Idaho K-1 for each owner.

How does a firm handle Idaho at scale?

Idaho's dates are uniform, so the attention goes to the prepayment rule and the small fixed taxes. The 80% safe harbor is what makes an extension valid, so track each client's prepayment against it rather than assuming the extension is free. Add the $10 building fund tax to every business return and the ABE election to the year-end checklist for SALT-cap clients. See how we think about scaling tax prep workflows.

Where do these dates come from?

All deadlines and thresholds here trace to the Idaho State Tax Commission. Verify against the source before filing.