EST-I: Underpayment of Estimated Tax by Individuals, Estates, and Trusts (Form EST-I) (2025)

Who files this form

Individuals, estates, and trusts can use Form EST-I to check whether they owe interest for underpaying 2025 estimated tax and to calculate that interest. The department generally calculates the interest for you. The instructions say not to include Form EST-I with your tax return.

You are not required to make estimated payments or pay underpayment interest if any of these exceptions applies:

• Your 2024 tax period covered 12 months and your Montana tax liability was zero.
• Your 2025 income tax liability after credits and withholding is less than $500. Complete Part I, lines 1 through 4, to check this.
• You are a nonresident or part-year resident who was not required to file a 2024 Montana income tax return.
• You retired and were at least age 62. This exception applies only for the retirement year and one additional year.
• You became disabled in 2024 or 2025.
• At least two-thirds of your gross income comes from farming or ranching operations.

When it's due

For tax year 2025, the estimated payment due dates for calendar-year taxpayers are:

• April 15, 2025
• June 16, 2025
• September 15, 2025
• January 15, 2026

Fiscal-year filers must adjust these dates accordingly.

For the 2025 interest calculation, Part II, line 12, uses the date you paid the underpayment or April 15, 2026, whichever is earlier.

The 2025 instructions also say that if you file your return and pay the full amount due by January 31, no underpayment interest is calculated on the fourth installment. The instructions do not print a year next to that January 31 date.

What you'll need

Have these returns and records available to complete the form:

• Your 2025 Montana Form 2 or Form FID-3, including tax liability, withholding, and refundable credit amounts.
• Your 2024 Montana return, showing tax liability and any overpayment applied to 2025.
• Records of estimated tax payments, including amounts and payment dates.
• Your name and Social Security number, spouse's information if applicable, or the estate or trust name and FEIN.

If you use the annualized income installment method, you also need income, deduction, Montana addition and subtraction, and credit amounts for each period shown in Part III. Section 2 also calls for capital gain information and, when applicable, Montana source income information.

How to file

The department generally figures underpayment interest for you. Do not include Form EST-I when you file your tax return. No mailing address or e-filing directions are provided in this text.

If you calculate the interest yourself:

1. Complete Part I first to determine your required annual payment. Stop if the form says you do not owe interest.
2. If using the annualized income installment method, complete Part III before Part II.
3. In Part II, complete lines 1 through 4 across all columns. Then complete lines 5 through 11 in one column before moving to the next.
4. Complete lines 12 through 15 for columns with an underpayment on line 10.
5. Enter the total interest from Part II, line 15, on Form 2, Schedule IV, line 2, or Form FID-3, page 2, line 24.

Common mistakes

• Using the discontinued Short Method. For 2025, use Part II or let the department calculate the interest.
• Using amended return amounts. If you file an amended return, the instructions say to use the amounts on your original return to figure the underpayment.
• Assuming a later payment erases earlier interest. Interest is figured separately for each installment, even if you later catch up or receive a refund.
• Assigning payments only to the period you intended. Payments first cover any underpayment from an earlier period.
• Using the annualized method for only some due dates. If you use it for any payment due date, you must use it for all payment due dates.
• Including excluded deductions in the annualized calculation. Do not include a state income tax deduction. Estates and trusts must also exclude the federal Qualified Business Income Deduction.

Questions people ask

Can I owe underpayment interest even if I get a refund?

Yes. Interest is calculated separately for each installment due date, so an earlier underpayment can result in interest even if your return shows a refund.

What if my income varied during the year?

The annualized income installment method may lower or eliminate one or more required installments. The instructions give seasonal business income and a large capital gain late in the year as examples.

What if Part II, line 10, is zero for every payment period?

You do not owe interest on your underpayment.

Where can I get help?

Call (406) 444-6900, or use Montana Relay at 711 for the hearing impaired.

Official sources

• https://revenuefiles.mt.gov/files/Forms/EST-I/EST-I-2025.pdf