EST-PTI: Interest on Underpayment of Estimated Tax for Composite and Pass-Through Entity Tax (Form EST-PTI) (Revision)

Who files this form

Form EST-PTI helps pass-through entities determine whether they owe interest for underpaying estimated pass-through entity tax (PTET) or composite tax.

Most taxpayers do not need to include this form with Form PTE because the department can calculate the interest. If you use the annualized income installment method, you must include Part III with Form PTE.

Entities that elect PTET or composite tax generally must make estimated payments. The 2025 instructions list these exceptions:

• It is your first year electing composite tax.
• It is your first year electing PTET.
• Your 2024 tax period covered 12 months and your Montana PTET and/or composite tax liability was zero.
• Your 2025 income tax liability after PTET credits and/or pass-through withholding is less than $500. Use Part I, lines 1 through 4, to check this.
• You were not required to file a 2024 Montana income tax return.
• At least two-thirds of your gross income comes from farming or ranching operations.

Estimated payments of pass-through withholding are not required.

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 March 16, 2026, whichever is earlier. This is an interest-calculation cutoff, not a separate estimated payment due date.

What you'll need

Have these forms and amounts available for the 2025 calculation:

• Your pass-through entity name and FEIN.
• 2025 Form PTE: PTET and/or composite tax liability from lines 25 and 28, and flow-through payments applied to those taxes from lines 26 and 29.
• 2024 Form PTE: PTET and/or composite tax liability from lines 25 and 28, and any overpayment from line 46 applied to those taxes.
• Estimated payment amounts and payment dates.

If you use the annualized method, you also need federal income and deductions and Montana additions and subtractions for each period. Complete Form PTE, Schedule I, for each period to determine the apportionment factor. If you elected composite tax, complete Form PTE, Schedule IV, for each period.

How to file

You may calculate the underpayment interest yourself, or let the department calculate it. Most taxpayers do not need to attach Form EST-PTI to Form PTE.

If you calculate it yourself:

1. Complete Part I first to determine the required annual payment. If line 4 is less than $500, stop. You do not owe underpayment interest.
2. If using the annualized method, complete Part III before Part II.
3. Complete Part II to calculate interest. Enter the total from Part II, line 15, on Form PTE, line 31.
4. If using the annualized method, include Part III with Form PTE. The form also requires this attachment if you marked the box on Form PTE, line 21.

The supplied text does not provide a mailing address or e-filing directions.

Common mistakes

• Using the discontinued Short Method. Use Part II to calculate interest, or let the department calculate it.
• Including payments made for pass-through withholding. Report only PTET and composite tax amounts in the underpayment calculation.
• Using amended-return amounts. If you file an amended return, use the amounts on your original return to figure the underpayment.
• Completing sections out of order. Complete Part I first. If annualizing, complete Part III before Part II.
• Using annualization for only some installments. If you use Part III for one payment due date, you must use it for all payment due dates.
• Omitting the required attachment. Include Part III with Form PTE when using the annualized method.
• Applying a payment only to its designated period. Payments first cover any underpayment on an earlier required installment.

Questions people ask

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

Yes. Interest is calculated separately for each installment due date. Paying enough later does not necessarily remove interest for an earlier underpayment, even if your return shows a refund.

How is the required annual payment calculated?

For 2025, Part I, line 6, uses the lesser of 90% of the 2025 PTET and/or composite tax liability on line 1 or the 2024 liability on line 5.

Can seasonal or uneven income reduce my required installments?

Possibly. The annualized income installment method may lower or eliminate one or more required installments if your income varied during the year. Use Part III for all payment due dates and include it with Form PTE.

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

You do not owe interest on your underpayment.

Where can I ask questions?

Call (406) 444-6900. Montana Relay is available at 711 for people with hearing impairments.

Official sources

• https://revenuefiles.mt.gov/files/Forms/Interest-on-Underpayment-of-Estimated-Tax-for-Composite-Tax-and-Pass-Through-Entity-Tax.pdf