Interest on Underpayment of Estimated Taxes. You are required to pay your income tax liability throughout the year. You can make your payments through employer withholding, installment payments of estimated taxes, or a combination of both. If you did not pay in advance at least 90% of your income tax liability (after applying your credits) or 100% of your prior year income tax liability (after applying your credits), you may have to pay interest on the underpayment of your estimated tax.
If you are required to pay interest on your underpayment, you can use this short method to determine your interest, but you can use this short method only if: You made no estimated tax payments (in other words, your only payments were Montana withholding), or You made four equal estimated payments by the required due dates. If you are not eligible to use this short method to calculate your interest on your underpayment, use Montana Form EST-I, Interest on Underpayment of Estimated Tax Payments. You can get this form by visiting our website at revenue.mt.gov or by calling us toll free at (866) 859-2254 (in Helena, 444-6900).