Idaho — Personal Income Tax

Idaho resident individuals must file a state income tax return if they are required to file a federal return under Internal Revenue Code section 6012(a)(1). This means the Idaho filing obligation is directly tied to the federal filing requirement—if a resident must file with the IRS, they must also file with Idaho.

The Idaho State Tax Commission specifies income thresholds for residents based on filing status and age. For the 2025 tax year, residents under age 65 must generally file if gross income is $15,000 or more (single) or $30,000 or more (married filing jointly). However, the statutory test remains the federal filing requirement.

One important exception: if a taxpayer files a federal return solely to pay self-employment tax and is not otherwise required to file federally, no Idaho return is required.

Source: Idaho Code § 63-3030(a)(1) Source: Idaho State Tax Commission – Individual Income Tax Basics

Idaho imposes a flat personal income tax rate of 5.3% on taxable income above a zero-rate threshold. The statutory base threshold is $2,500 for single filers and $5,000 for taxpayers filing joint returns. Surviving spouses and heads of household are treated as joint filers for purposes of the threshold amount.

Annual inflation adjustment

Idaho Code § 63-3024(3) requires the Idaho State Tax Commission to adjust these thresholds annually for inflation beginning in taxable year 2000. The statute directs the Commission to prescribe a factor based on the consumer price index for all U.S. urban consumers (published by the U.S. Department of Labor) so that inflation will not result in a tax increase. The adjustment multiplies the last threshold amount by the percentage increase in the consumer price index for the calendar year immediately preceding the calendar year to which the adjusted thresholds will apply.

The Idaho State Tax Commission has adjusted the income tax thresholds each year since 1998 to account for inflation. For tax year 2025, the inflation-adjusted thresholds are $4,811 for single filers and $9,622 for married filing jointly, head of household, and qualifying surviving spouse filers. These inflation-adjusted amounts change annually; practitioners should consult the Idaho State Tax Commission's current-year tax forms (Form 40 or Form 43) or the Commission's rate schedule page to confirm the thresholds for the year at issue.

Source: Idaho Code § 63-3024 Source: Idaho State Tax Commission – Search Category: Income Tax

Idaho defines an individual as a resident for income tax purposes under two alternative tests. First, an individual domiciled in Idaho for the entire taxable year is a resident. Second, an individual who maintains a place of abode in Idaho for the entire taxable year and spends more than 270 days in the state during that year is a resident. For the day-count test, any part of a calendar day in Idaho counts as a full day unless the individual can show presence was for a temporary or transitory purpose.

Source: Idaho Code § 63-3013

Idaho allows individual taxpayers to claim the standard deduction as defined in section 63 of the Internal Revenue Code. Taxpayers may choose either the standard deduction or itemized deductions at their option. By referencing the federal definition, Idaho's standard deduction amounts track the federal standard deduction framework, though the effective conformity date may affect which version of the IRC applies.

Source: Idaho Code § 63-3022(j)(1)

Idaho individual income tax returns are due on or before April 15 following the close of the calendar year. This deadline aligns with the federal income tax return due date. Idaho law allows an automatic six-month extension to October 15 for taxpayers who have paid enough of their tax liability by April 15. Payment is due by April 15 even if an extension to file is obtained.

Source: Idaho Code § 63-3032 Source: Idaho State Tax Commission – Individual Income Tax Filing and Paying

A nonresident individual must file an Idaho income tax return if gross income from Idaho sources for the taxable year exceeds $2,500. This threshold applies regardless of whether the nonresident has any physical presence in Idaho beyond the income-generating activity itself. The return is filed on Form 43.

What constitutes Idaho-source income

For nonresident individuals, Idaho taxable income includes only income derived from or related to sources within Idaho, computed under Idaho Code § 63-3026A(1). The statute specifies several categories of Idaho-source income:

Compensation for personal services performed in Idaho. If a nonresident performs personal services both within and outside Idaho (as an employee, agent, independent contractor, partner, or otherwise), Idaho-source compensation is determined by multiplying total compensation by the "Idaho compensation percentage." Under Idaho Admin. Code r. 35.01.01.270, this percentage equals Idaho workdays divided by total workdays. An "Idaho workday" is any day on which the taxpayer performs personal services in Idaho for a particular employer or principal. If services are performed both inside and outside Idaho on the same day, that day counts as an Idaho workday unless the taxpayer establishes that less than 50% of the services were performed in Idaho.

Ownership or disposition of Idaho real or tangible personal property. Income from real property located in Idaho, or tangible personal property located in Idaho, is Idaho-source income. This includes rents, royalties, and gains or losses from the sale or other disposition of such property.

Intangible personal property employed in an Idaho business. Income from intangible personal property is Idaho-source income only to the extent the property is employed in a business, trade, profession, or occupation conducted or carried on in Idaho. However, interest income from an installment sale of real or tangible personal property located in Idaho is Idaho-source income under the same rule. Nonresident individuals are not taxable on investment income from a "qualified investment partnership," defined as a partnership deriving at least 90% of its gross income from investments that would not be taxable to a nonresident if held directly.

Pass-through entity income. A nonresident's share of income from a partnership or S corporation doing business in Idaho is sourced to Idaho. For partnerships, guaranteed payments to a partner for services or the use of capital are sourced to Idaho based on where the services are performed or the capital is used. Guaranteed payments in excess of $250,000 (adjusted annually for inflation under Idaho Code § 63-3024) are sourced based on the partnership's Idaho apportionment factor. Gains or losses on the sale of a partnership interest or S corporation stock are sourced to Idaho to the extent of the entity's Idaho apportionment factor in the taxable year immediately preceding the sale.

Gambling and wagering. Income from pari-mutuel wagering, charitable gaming, or other gambling conducted within Idaho is Idaho-source income, except as limited by Idaho Code § 67-7439.

Exclusions from Idaho-source income

Active-duty military pay received by a nonresident for service in the U.S. armed forces is not Idaho-source income under Idaho Code § 63-3026A(3)(d), regardless of where the service is performed.

Source: Idaho Code § 63-3030(a)(2) Source: Idaho Code § 63-3026A Source: Idaho Admin. Code r. 35.01.01.270 (via Idaho State Tax Commission)

Idaho grants an automatic six-month extension to October 15 for individual income tax returns, but this extension is not unconditional. To qualify for the automatic extension, a taxpayer must pay at least 80% of the total tax due by the April 15 unextended due date.

Two payment safe harbors

Idaho Code § 63-3033(a) provides two alternative safe harbors to meet the 80% payment threshold:

1. Current-year safe harbor: Payment of at least 80% of the total tax due on the income tax return when it is filed, or
2. Prior-year safe harbor: Payment of 100% of the total tax due on the prior year's income tax return (if a return was filed for the prior year).

A taxpayer who satisfies either safe harbor qualifies for the automatic six-month extension. The prior-year safe harbor mirrors the federal estimated tax safe harbor in structure—the taxpayer may satisfy the payment requirement by paying 100% of the prior year's actual tax liability, even if the current year's liability is higher.

Small-balance exception

Under Idaho Code § 63-3033(b), if the payment required to meet the 80% threshold (after accounting for withholding credits and estimated payments already made) is $50 or less, no additional payment is required to qualify for the extension. However, interest will accrue on any unpaid balance from the April 15 due date.

Penalty for underpayment

If the taxpayer pays less than 80% of the total tax due and the payment is also less than 100% of the prior year's tax liability—and the small-balance exception does not apply—Idaho Code § 63-3033(f) provides that "a penalty may be applied to the total of the balance due unless reasonable cause can be established." The penalty structure depends on when the balance is ultimately paid:

• If the tax is paid by the extended October 15 due date, the penalty is 2% per month from the April 15 original due date to the payment date.
• If the tax is not paid by the extended October 15 due date, the penalty prescribed by Idaho Code § 63-3046(c) applies from the April 15 original due date. That penalty is 5% of the unpaid tax for each month (or part of a month) the return or payment is late.

Interest accrual

Even when the extension is granted, interest accrues on any unpaid tax from the April 15 original due date to the payment date at the rate specified in Idaho Code § 63-3045. Idaho Code § 63-3033(g) provides that interest runs in all cases of extension except those related to IRC § 7508 (military or terrorist action postponements). The extension grants additional time to file, but it does not extend the time to pay without interest.

Practical summary

The Idaho extension is automatic if the taxpayer pays at least 80% of the current year's tax (or 100% of the prior year's tax) by April 15. Taxpayers who underpay the threshold may still file late under the extension if they can establish reasonable cause, but penalties and interest will apply. The extension is not truly "automatic regardless of payment"; payment of at least 80% (or the prior-year amount, or a de minimis balance of $50 or less) is a condition precedent to penalty-free use of the six-month extension period.

Source: Idaho Code § 63-3033 Source: Idaho Code § 63-3046

Idaho conforms to the Internal Revenue Code (IRC) as amended and in effect on a rolling conformity date specified in Idaho Code § 63-3004. The conformity date determines which version of the federal tax code Idaho adopts for state income tax purposes, including provisions governing gross income, deductions, exemptions, and credits.

House Bill 559 (signed February 10, 2026)

According to the Idaho State Tax Commission, Governor Brad Little signed House Bill 559 on February 10, 2026, updating Idaho's IRC conformity to the Internal Revenue Code as in effect on January 1, 2026. The bill was enacted with retroactive application to January 1, 2025, meaning Idaho taxpayers filing 2025 tax year returns would apply the IRC as in effect on January 1, 2026.

The Idaho State Tax Commission announced on February 17, 2026, that "House Bill 559 conforms to most of the provisions in the federal One Big Beautiful Bill Act. This includes the larger standard deduction amounts, the enhanced senior deduction, and the deductions for qualified tips from wages, car loan interest, and overtime compensation."

Impact on 2025 tax year standard deduction amounts

Idaho Code § 63-3022(j)(1) allows taxpayers to claim the standard deduction "as defined in section 63 of the Internal Revenue Code." Because Idaho's standard deduction definition directly references the federal IRC, updating the conformity date to January 1, 2026, automatically adopted the standard deduction amounts in effect under the federal IRC as of that date for Idaho 2025 tax year returns.

On February 20, 2026, the Idaho State Tax Commission announced that it "programmed its systems to automatically give the larger standard deduction amounts to taxpayers who take the standard deduction on their 2025 income taxes. This means that those taxpayers who've already filed their taxes won't need to file amended returns to claim the larger standard deduction."

The Tax Commission issued updated instructions for Idaho Form 40 (Individual Income Tax Return) and Idaho Form 43 (Part-Year Resident and Nonresident Income Tax Return) on March 3, 2026, showing how to claim the enhanced senior deduction and the deductions for tips from wages, car loan interest, and overtime compensation for the 2025 tax year. For tax year 2025, the Tax Commission did not add new lines to the tax forms; instead, taxpayers claim those deductions using existing form lines as described in the updated instructions.

Retroactive conformity mechanism

The Idaho State Tax Commission's conformity guidance page explains that "Once Idaho conforms to the IRC, it follows the federal effective date of any federal changes adopted, including any retroactive dates." This principle allowed the provisions of federal tax legislation effective for the 2025 tax year to apply in Idaho for the 2025 tax year once House Bill 559 was enacted in February 2026, even though many taxpayers had already filed their 2025 Idaho returns before the Idaho conformity bill was signed.

Decoupling from certain federal provisions

According to the Idaho State Tax Commission, House Bill 559 did not conform Idaho to all federal provisions. Idaho continues its decoupling from IRC § 168(k) bonus depreciation. House Bill 559 also does not conform to IRC § 168(n) (qualified production property expensing under the federal One Big Beautiful Bill Act). The bill also maintains separate treatment for domestic research and experimental expenditures incurred between January 1, 2022, and January 1, 2025, under which Idaho taxpayers must continue to apply the IRC as in effect immediately prior to enactment of the federal transition rules for those years.

Source: Idaho Code § 63-3004 Source: Idaho Code § 63-3022(j)(1) Source: Idaho State Tax Commission – Conformity to Federal Internal Revenue Code (IRC) Source: Idaho State Tax Commission – Update on filing 2025 Idaho income taxes now that conformity is law (Feb. 17, 2026) Source: Idaho State Tax Commission – More guidance on conformity deductions and filing 2025 Idaho income taxes (Feb. 20, 2026) Source: Idaho State Tax Commission – File now to get your conformity deductions (March 3, 2026)