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

Form EST-PTI explains how Montana calculates interest on the underpayment of estimated taxes for entities that elect to pay composite tax or pass-through entity tax (PTET). These taxes apply to partnerships and S corporations that choose to pay tax at the entity level on behalf of their owners.

Entities must make timely estimated payments for PTET and/or composite tax throughout the year. If payments are late or insufficient, interest is charged on the underpaid amount. The interest is calculated from the due date of each installment to the date payment is made or the original due date of the return, whichever is earlier.

In most cases, entities do not need to submit Form EST-PTI with their Form PTE. The Montana Department of Revenue generally calculates underpayment interest and will send a bill if necessary.

However, if your entity uses the annualized income installment method to determine estimated tax payments, you must complete Part IV of Form EST-PTI and include it when filing your Form PTE. This method is often used by entities with uneven income distributions throughout the year.

This form is intended for tax professionals and business owners filing on behalf of Montana pass-through entities.

Download Form EST-PTI (PDF, 244 KB)