Administrative Rules of Montana
Department 42 - REVENUE
Chapter 42.17 - WITHHOLDING AND ESTIMATED TAX PAYMENTS
Subchapter 42.17.3 - Estimated Tax Payments
Rule 42.17.310 - DETERMINATION OF TAX LIABILITY - STATUS CHANGE FROM JOINT TO SEPARATE

(a) 100% of the tax liability of the joint return for the preceding tax year; or

(b) a prorated portion of the preceding year's joint tax liability. This prorated amount is determined by calculating the tax liability of the taxpayers as if they filed separately and totaling their individual taxes. Each spouse's prior year estimated tax liability is found by dividing their separate liability by the total of the two separate liabilities and multiplying this figure by the preceding year's joint tax liability.

Example: Husband and wife file a joint return for the calendar year one showing taxable income of $50,000 and a tax of $3,556. Of the taxable income, $30,000 was attributable to the husband and $20,000 to the wife. Husband and wife filed separate returns on separate forms for calendar year two. The tax shown on the return for the preceding taxable year, for purposes of determining if interest is owed on an underpayment, is determined as follows:

Taxable income of husband for year one: $30,000

Tax on $30,000 (on basis of separate return): $ 1,712

Taxable income of wife for year one: $20,000

Tax on $20,000 (on basis of separate return): $ 934

Aggregate tax of husband and wife (on basis of separate returns): $ 2,646

Portion of year one tax shown on joint return attributable to husband ($1,712/$2,646 X $3,556): $ 2,301

Portion of year one tax shown on joint return attributable to wife ($934/$2,646 X $3,556): $ 1,255