66: Fiduciary Income Tax Return and Instructions 2025 (2025)
Idaho · Idaho Tax Commission · Used for tax years 2025
Idaho Form 66 is the 2025 Fiduciary Income Tax Return for estates and trusts. It reports income, deductions, beneficiary distributions, taxes, credits, and payments to calculate a refund or amount due.
What you'll need
Include these with Form 66:
- A complete copy of federal Form 1041, including all federal Schedule K-1s.
- All Idaho Forms ID K-1. The trust or estate must also give each beneficiary an Idaho Form ID K-1.
Depending on your return, you may also need:
- Form PTE-12: Required if you answer “Yes” to having nonresident beneficiaries. Include it if the trust or estate files as a pass-through entity.
- ESBT worksheets: For an Electing Small Business Trust return, include the federal Form 1041 ESBT tax calculation worksheet, its appropriate schedules, and the Idaho ESBT tax calculation worksheet.
- Withholding forms: State copies of Forms W-2 or 1099 if you report Idaho income tax withheld.
- Payment details: A schedule of EINs, amounts, and rollforwards if estimated payments were made under other EINs.
- Supporting schedules: Include the schedules requested for income, additions, subtractions, depreciation adjustments, and other applicable calculations. For an Idaho net operating loss, include Form 56 or a schedule showing how the loss applies.
- Credit and tax attachments: Include applicable Forms 44, 75, or 49ER and the certificate for a tax reimbursement incentive credit.
- Other-state returns: If claiming credit for income tax paid to another state, include that state's return and a separate Schedule B for each state.
How to file
Use the 2025 return for calendar year 2025 or a fiscal year beginning in 2025. For a fiscal year, enter the beginning and ending dates at the top of the form.
Enter the estate or trust name, fiduciary's name and title, mailing address, and EIN. Complete the applicable questions, lines, and schedules. Sign the return and include the required attachments, including a complete copy of your federal return. Don't staple.
Mail a return with payment to:
Idaho State Tax Commission
PO Box 83784
Boise, ID 83707-3784
Mail a return without payment to:
Idaho State Tax Commission
PO Box 56
Boise, ID 83756-0056
Common mistakes
- Leaving required questions unanswered: Incomplete returns delay processing. Complete questions 1a through 1d for an estate; otherwise, start with question 2. Answer questions 2 and 3.
- Using cents instead of whole dollars: Round down amounts under 50 cents and round up amounts of 50 cents or more.
- Including a dash in the EIN: Enter the EIN without the dash.
- Including state income taxes in Schedule A deductions: Don't include state income taxes deducted on federal Form 1041, line 11, in Schedule A, line 16.
- Claiming another-state tax credit as a nonresident: Nonresident trusts and estates can't claim this credit.
- Using one beneficiary's unused credits for another beneficiary: Credits not used to offset that beneficiary's tax can't offset tax computed for other beneficiaries.
- Treating a scholarship donation as reversible: A donation to the Idaho Opportunity Scholarship Program reduces your refund or increases tax due. The choice can't be changed, and you can't get the donation refunded later.
Questions people ask
Can I use Form 66 to amend a return?
Yes. Check the amended return box and enter the reason number: 1 for a federal audit, 2 for a net operating loss carryback, 3 for a federal amended return, or 4 for another reason. For reason 2, include Form 56 or a schedule showing how the loss applies. For reason 4, include an explanation. Complete the entire form using corrected amounts.
What information must beneficiaries receive?
Give each beneficiary an Idaho Form ID K-1. It shows the beneficiary's share of Idaho additions, subtractions, and credit information, along with gross income information used to help determine individual Idaho filing requirements. Include all Idaho Forms ID K-1 with Form 66.
Does every trust or estate owe the permanent building fund tax?
No. A trust or estate that meets the Idaho filing requirement and doesn't distribute all its Idaho adjusted income must pay the $10 tax. If all Idaho adjusted income is distributed to beneficiaries who file Idaho income tax returns, the trust or estate doesn't owe it. A trust or estate must also pay the tax for each nonresident individual beneficiary included in a composite return. A Qualified Funeral Trust enters zero.
Can I apply an overpayment to 2026 instead of receiving a refund?
Yes. On an original return, enter the amount you want refunded on line 35. The difference between the overpayment on line 34 and your requested refund is applied to your 2026 tax and won't be refunded. Refunds of less than $1 aren't issued.