Idaho · calendar and forms

Idaho calendar and forms: April 15, the 80% test, and the forms that matter

The dates on an Idaho household’s 2026 tax calendar, including the property-tax relief windows that fall in the same spring; the test that makes the October extension automatic; the forms a resident, a part-year resident and a senior actually use, with links; free filing, where to check a refund, and how to reach the Idaho State Tax Commission.

By Charles Trautman, EA, editor

Need to Know Tax · written from Idaho State Tax Commission sources, November 3, 2026; updated November 12, 2026 after the election · not yet reviewed by an Idaho practitioner

The Idaho return is due April 15, 2027, the same day as the federal one, and that is nearly the whole calendar, because Idaho asks individuals for no estimated payments during the year. The extension to October 15, 2027 is automatic and needs no form, but only if, by April 15, your withholding and voluntary payments add up to 80% of your 2026 Idaho tax or 100% of your 2025 Idaho tax. Interest at 6% a year (the 2026 rate; the 2027 rate is to come) runs on anything unpaid after April 15 whether or not you extend. Two other spring deadlines matter to many Idaho households and are not on the income tax return at all: the Property Tax Reduction and disabled veterans’ applications at the county assessor, January 1 to April 15, 2027. Full-year residents file Form 40 with Form 39R; part-year residents and nonresidents file Form 43 with Form 39NR; a senior who owes nothing and wants the food tax credit files Form 24.

The calendar

Idaho dates for the 2026 tax year
Date	What
December 31, 2026	Last day for IDeal contributions deductible on the 2026 return. Last day of the circuit breaker’s income year: 2026 income sets eligibility for the 2027 program.
January 1, 2027	County assessors open applications for the 2027 Property Tax Reduction, Property Tax Deferral and disabled veterans’ benefit.
Late January 2027; the IRS announces the date in January	W-2s and 1099s due to you under federal rules; IDeal, medical savings and first-time home buyer accounts send year-end statements.
Mid-January to mid-March 2027	Parental Choice Tax Credit application window for the 2026 credit, if the Department follows the 2026 pattern (January 15 to March 15); the 2027 dates had not been announced when this was written.
April 15, 2027	Form 40 or Form 43 and payment due. Form 24 due for seniors claiming only the food tax credit. Last day to have paid in 80% of 2026 tax or 100% of 2025 tax for the automatic extension. Interest begins on any unpaid balance. Last day to apply for the 2027 Property Tax Reduction and disabled veterans’ benefit at the county assessor. Last day to claim a refund on a 2023 Idaho return.
September 7, 2027	Last day to apply for the 2027 Property Tax Deferral.
October 15, 2027	Extended due date for Form 40 or 43. Late-payment penalties start after this date for those who extended.
December 2027	County property tax bills arrive with any circuit breaker or veterans’ credit shown.
December 31, 2027	Deadline to reconcile a 2026 Parental Choice advance payment with receipts in Taxpayer Access Point.
120 or 180 days after a final federal determination	Deadline to report an IRS change to Idaho with an amended return. The 2025 instructions say 120 days; the Department’s September 2026 publication says 180. Use 120 to be safe.

Idaho has no estimated-payment due dates for individuals. Form 51 payments can be made any time.

The extension test

Idaho’s extension is unusual in two ways: nothing is filed to get it, and it is not unconditional. You have until October 15, 2027 to file without a late-filing penalty if, by April 15, 2027, the total of your Idaho withholding, any Form 51 or Quick Pay payments and credits is at least 80% of the tax that will be on your 2026 return, or at least 100% of the tax on your 2025 return, whichever is less. The Form 51 worksheet walks through it; if the shortfall is $50 or less, you need not pay it to qualify. The extension covers filing only. Interest at 6% a year runs from April 15 on whatever is unpaid, and the Department cannot waive it. Pay after October 15 and the penalty is 0.5% a month, up to 50% of the tax. Someone who has paid in enough and owes nothing more does not need to send Form 51 at all; the extension is already theirs.

Because there are no required estimates and no underpayment penalty, a self-employed person or a retiree with no withholding can wait until April 15 to pay the whole year. The cost of waiting is zero until April 15 and 6% a year afterward. The cost of forgetting the 80% test is the late-filing penalty on top.

The forms

Idaho individual income tax forms (2025 versions; 2026 versions to come)
Form	Who uses it
Form 40, Individual Income Tax Return	Full-year residents. Line 7 federal AGI, lines 13–18 deductions, line 19 taxable income, line 31 the $10 permanent building fund tax, line 42 Parental Choice credit, line 43 food tax credit.
Form 39R, Resident Supplemental Schedule	Attached to Form 40 for additions (Part A), subtractions such as Social Security, the retirement benefits deduction and IDeal (Part B), the credit for taxes paid to another state (Part C) and charitable credits (Part D).
Form 43, Part-Year Resident and Nonresident Income Tax Return	Anyone who moved in or out during the year, or lives elsewhere and has more than $2,500 of Idaho-source income. Two columns, an Idaho percentage, and prorated deductions.
Form 39NR, Part-Year Resident and Nonresident Supplemental Schedule	Attached to Form 43; the counterpart of Form 39R. On the forms page.
Idaho Worksheet 1-A	Form 43 filers who took the federal tips, overtime, senior or car-loan deductions; apportions them for line 40.
Form 24, Food Tax Credit Refund	Residents 65 or older, below the filing threshold, with no dependents, not filing separately and not on SNAP during the year. Claims $155 per person. Due April 15.
Form CG, Capital Gains Deduction	Anyone claiming the 60% deduction on qualifying Idaho property; see the capital gains page. Carries to Form 39R, Part B, line 10.
Form 51, Estimated Payment of Individual Income Tax	Voluntary prepayments and the extension worksheet. Mail with a check to PO Box 83784, Boise ID 83707-3784, or pay online and skip the form.
Form ID-VP, Income Tax Voucher Payment	Mailing a payment without a return. On the forms page.
Form ID W-4, Employee’s Withholding Allowance Certificate	Sets Idaho withholding with an employer or, on request, a pension payer. Anyone whose Idaho W-4 claimed children should file a new one; the credit it was built around ended.
Form ID-MS1, Military Spouse Withholding Exemption Certificate	A qualifying military spouse who elects another state’s residency; filed with the employer each year. On the withholding forms page.
Amended return	A Form 40 or 43 with the amended box checked and an explanation attached; include Form 1040-X if the federal return was amended.

All current forms are at tax.idaho.gov/IITforms. The 2026 versions, with the indexed zero bracket and pension maximums, are expected in early 2027.

Filing free

The Department lists free e-file offers each season at tax.idaho.gov/freefile. For 2025 returns the offers came from Free File Alliance members (FreeTaxUSA, TaxAct, OLT.com, TaxSlayer, 1040.com, 1040NOW) with income and age limits that differed by company, most at $89,000 of AGI or less, plus GetYourRefund for AGI of $89,000 or less. Reach them through the links on that page; the same product reached from the company’s own site may charge. If you e-file both federal and Idaho returns, the Department requires the same software for both. In-person help comes from IRS VITA sites (AGI of $69,000 or less, simple returns) and AARP Foundation Tax-Aide for seniors. Taxpayer Access Point (TAP) takes payments and shows account history but cannot file an income tax return. The 2027 list is to come; Idaho was an IRS Direct File state for the 2025 season, Direct File was not offered for 2026, and the IRS has since ended the program.

Paying

Quick Pay at tax.idaho.gov/quickpay takes a one-time bank payment free, no account needed; cards carry a fee. TAP takes bank or card payments for those with an account. Checks go with the return to PO Box 83784, Boise ID 83707-3784 (returns with payment), or with Form ID-VP if sent alone. Returns without a payment go to PO Box 56, Boise ID 83756-0056. Cash is accepted in person at the Department’s offices. If you cannot pay in full, file anyway and ask for a payment plan; the late-filing penalty is the expensive part.

Refunds

Check status at tax.idaho.gov/refund, which tracks the return through four stages. The Department’s posted times are about 7 to 8 weeks after acknowledgment for an e-filed return and 10 to 11 weeks for paper, plus about 3 weeks for a first-time Idaho filer and about 6 weeks after you answer a letter. Every return passes fraud screening, and an identity-verification or PIN letter stops the refund until you respond. A refund must be claimed within three years of the original due date. Direct deposit goes on Form 40 line 57; check the routing and account numbers, since a wrong number is the commonest cause of a lost refund.

Reaching the Department

Taxpayer Services: (208) 334-7660 in the Boise area, (800) 972-7660 toll free, Monday through Friday, 8 a.m. to 5 p.m. Mountain time, closed 8 to 9:30 a.m. on the second Wednesday of each month. The contact page has a question form; replies take five to seven business days. Offices with drop boxes are in Boise (11321 W. Chinden Blvd.), Coeur d’Alene, Idaho Falls, Lewiston, Pocatello and Twin Falls; the smaller offices are open Monday through Thursday, 9 a.m. to 4 p.m., with Fridays by doorbell, and Lewiston is staffed on the 20th of each month. Property-tax relief questions go to the county assessor first; the Department’s property tax division is at (208) 334-7736.

Behind the scenes: why there are no estimated payments

The federal system asks for four estimated payments a year and penalizes anyone who falls short. Idaho never adopted that for individuals. The Legislature chose instead to make the filing extension conditional: pay in enough by April 15 and you may file late; pay in too little and you may not. The 80%-of-this-year or 100%-of-last-year test is the federal safe-harbor rule repurposed as the price of an extension rather than the trigger for a penalty. For the wage earner it makes no difference. For the self-employed, the landlord and the retiree with no withholding it means the whole year’s tax can legally wait until April 15, at the cost of a lump sum then, and the interest meter starts only if the lump sum is short.

What to bring

Last year’s Idaho return, for the 100%-of-prior-year extension test and to compare lines
Confirmation numbers for any Quick Pay, TAP or Form 51 payments
Idaho withholding from every W-2 and 1099-R
Bank routing and account numbers for direct deposit
Any letter from the Idaho State Tax Commission, especially an identity-verification or PIN letter
County assessor paperwork, if you intend to apply for the circuit breaker by April 15

Questions people ask

When is the Idaho return due?

April 15, 2027, for the 2026 tax year. The extended date is October 15, 2027.

Do I need to file a form to extend?

No. The extension is automatic if you have paid in 80% of your 2026 Idaho tax or 100% of your 2025 Idaho tax by April 15. Form 51 is only for sending a payment to meet that test.

Does Idaho have quarterly estimated payments?

No, not for individuals, and no underpayment penalty. You can prepay any time with Form 51 or Quick Pay. Interest at 6% runs from April 15 on anything unpaid.

How long will my refund take?

About 7 to 8 weeks after acknowledgment if you e-filed, 10 to 11 weeks on paper, longer if this is your first Idaho return or you receive a letter. Check at tax.idaho.gov/refund.

Can I file my Idaho return for free?

Probably, if your AGI is under about $89,000. The Department lists the offers at tax.idaho.gov/freefile each season; use its links to get the free version.

Which form do I use if I moved to Idaho this year?

Form 43 with Form 39NR. Form 40 is for people who lived in Idaho all year.

I’m retired, owe nothing and want the grocery credit. What do I file?

If you or your spouse is 65 or older and your gross income is below the filing threshold, Form 24, due April 15. If you are under 65, a Form 40 showing the credit.

Pay in enough by April 15 and the rest takes care of itself

Then put the circuit breaker application on the same date, and check the forms page in January for the 2026 versions.

Written by Charles Trautman, EA, editor  ·  Need to Know Tax, Lone Tree, Colorado
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Forms and official pages named in this article

Sources and where to check

Updated November 12, 2026, after the November 3 election · tax year 2026 figures · awaiting review by an Idaho practitioner — use the right article