Key figures
Idaho accounting
2026, with sources
A flat rate that keeps falling
Idaho taxes corporations at a flat 5.3% of Idaho taxable income for tax years beginning on or after January 1, 2025, down from 5.695% in 2024 and 6% in 2022. Every corporation owes at least the $20 minimum, and most also pay the $10 Permanent Building Fund tax. The return is Form 41, due on the 15th day of the fourth month after year end.
Idaho conforms to the Internal Revenue Code as of a date it resets each year. HB 559 moved it to January 1, 2026, retroactively to 2025, bringing in most of the 2025 federal changes. The exceptions show up in the state provision: Idaho stays decoupled from bonus depreciation under §168(k) and from the new §168(n) expensing, and it does not allow the accelerated deduction of 2022–2024 research costs the federal transition rules permit.
Apportionment and combined reporting
Most corporations apportion business income to Idaho by a single sales factor: Idaho sales over sales everywhere. A unitary group files on a combined report, worldwide by default with a water’s-edge option, so a Delaware parent with an Idaho operating subsidiary is taxed on the group’s apportioned income, not the subsidiary’s separate books.
Idaho computes its own net operating loss and requires the federal NOL to be added back; the Idaho NOL is then deducted on Form 56. Track the Idaho NOL separately from the federal one in the provision.
What it means for a small company
Example: a pre-revenue Delaware startup hires one engineer who works from Boise. That employee is enough to be doing business in Idaho; the company files Form 41 each year, pays the $20 minimum and $10 PBF tax in loss years, and makes no estimated payments in its first Idaho year. Once profitable, Idaho income is the federal base, adjusted for bonus depreciation and the Idaho NOL, times its Idaho sales share, at 5.3%.
Deadlines
- 15th day of the 4th month after year end — Apr 15Idaho corporation income tax returnForm 41
- Quarterly, on the federal scheduleEstimated payments, from the second year in Idaho, if federal estimates are required and Idaho tax is $500 or moreForm 41ES
The federal baseline
This page covers what Idaho adds. The federal rules underneath it apply here too.
Common questions
Does Idaho follow the 2025 federal R&D expensing rules?
Mostly. HB 559, signed February 10, 2026, conforms Idaho to the Internal Revenue Code as of January 1, 2026, retroactive to 2025, so domestic research costs incurred from 2025 can be expensed for Idaho as they are federally. Idaho did not adopt the federal transition that lets 2022–2024 costs be deducted early: those keep amortizing for Idaho on their original five-year schedule.
What does a loss-making corporation owe Idaho?
A $20 minimum tax, plus the $10 Permanent Building Fund tax most corporations pay. Idaho also doesn’t require estimated payments in a corporation’s first year in the state; after that they are due if the company makes federal estimates and expects $500 or more of Idaho tax.
Accounting next door
- MTMontana accountingMontana taxes corporations at 6.75% (7% water’s edge) with a $50 minimum, apportions by receipts alone from 2025, and still allows a three-year NOL carryback.
- NVNevada accountingNevada has no corporate income tax. It taxes Nevada gross revenue above $4 million a year through the Commerce Tax, and payroll through the Modified Business Tax.
- OROregon accountingOregon taxes corporations twice: a 6.6%/7.6% excise tax on income with a sales-based minimum, and a 0.57% Corporate Activity Tax on Oregon receipts over $1 million.
- UTUtah accountingUtah’s corporate franchise and income tax is 4.45% of net income for 2026, with a $100 minimum, apportioned on sales for most companies.
- WAWashington accountingWashington has no corporate income tax; it taxes gross receipts through the B&O tax, at 0.471% for retail sales (including SaaS) and 1.5%–2.1% for other services since October 2025.
- WYWyoming accountingWyoming has no corporate, gross receipts or individual income tax; the only state-level levy on a corporation is the annual report license tax — $60 or $0.0002 per dollar of Wyoming assets.
Founder guides on this
- R&D taxThe R&D tax credit and Section 174A for startupsEngineering salaries can earn a federal R&D credit against payroll tax, and Section 174A lets you deduct US research again. What qualifies, what to track.
- US GAAPUS GAAP for early-stage startups, in plain EnglishAccrual accounting, revenue recognition (ASC 606), leases (ASC 842), stock compensation and capitalized software: what each means for a small company.
More of the Idaho guide
- ReceivablesReceivables and sales tax in IdahoSales tax registration, economic nexus, whether SaaS is taxable, and collecting from customers
- PayablesPayables and contractors in IdahoPaying vendors and contractors: 1099 filing, use tax, contractor tests and withholding
- PayrollPayroll taxes and employer rules in IdahoWithholding, unemployment insurance, paid leave, minimum wage and the rules on paydays
- ComplianceCompliance and filings in IdahoRegistering to do business, annual reports, franchise tax and the filing calendar
- AdvisoryAdvisory: credits and decisions in IdahoCredits, incentives and the decisions that are cheaper to get right the first time
Sources
Checked against each agency’s own page on . Rates and thresholds change, usually on January 1.
General information for US companies, not tax, legal or accounting advice for yours. Rules and thresholds change; confirm the specifics with a CPA or the agency before you act on them.