Idaho keeps its calendar simple in a way that trips up filers coming from the federal schedule: every entity — individual, C-corp, S-corp, and partnership — files by April 15. Idaho does not use the federal March pass-through date. The state also cut its flat rate for 2025.
Idaho Tax Deadlines at a Glance (2026)
Entity Type | Return Due | Extended Due Date |
|---|---|---|
Individuals (Form 40) | April 15, 2026 | October 15, 2026 |
C Corporations (Form 41) | April 15, 2026 | October 15, 2026 |
S Corporations (Form 41S) | April 15, 2026 | October 15, 2026 |
Partnerships (Form 65) | April 15, 2026 | October 15, 2026 |
The correction to note: Idaho’s S-corporation (Form 41S) and partnership (Form 65) returns are due April 15, 2026 — the 15th day of the 4th month — not the federal March 16. Idaho puts all entity types on the same April 15 deadline. The extension is automatic to October 15, but only if you pay 80% of the current-year or 100% of the prior-year tax by April 15.
The Flat Rate Fell to 5.3%
Idaho’s flat income tax rate is 5.3% for the 2025 tax year — applied to both individual and corporate income — down from 5.695% in 2024. Pass-through income is taxed to owners at that rate. Idaho’s flat structure makes projections straightforward, but confirm the rate each year, as it has been cut repeatedly.
The ABE Elective PTE Tax
Idaho’s elective Affected Business Entity (ABE) tax lets partnerships and S corporations pay Idaho tax at the entity level — at the corporate rate, 5.3% for 2025 — with owners receiving a matching credit, a federal SALT-cap workaround. The election is made annually on a timely-filed original return and is irrevocable for the year. (Note: older sources cite a 6.5% ABE rate — that is outdated; the rate now follows the corporate rate.)
Estimates, Extensions, and Penalties
Idaho’s automatic extension requires the 80%/100% payment test above; interest accrues at 6% per year on tax unpaid after April 15. The late-filing penalty is 5% per month (maximum 25%); late payment is 0.5% per month; filing without a valid extension carries a 2% per month charge. An extension covers filing only, not payment.
CPA Action Steps
File Form 41S and Form 65 by April 15 — Idaho does not use the federal March date
Apply the 5.3% flat rate (2025) to individual and corporate returns
Meet the 80%/100% payment test to secure the automatic extension
Use the corporate rate (5.3%), not 6.5%, for the ABE election
Verify current rates and dates against the Idaho State Tax Commission (tax.idaho.gov) before filing.