Tax Used to Compute Underpayment of Estimated Tax

Any interest due for underpaying estimated taxes is computed using the total tax shown on the return. If the return is adjusted in initial processing, the recomputed tax must be used for determining any underpayment interest. Prior to October 6, 2001, subsequent amendments to the tax will not affect the underpayment interest amount unless the amended return is received prior to the statutory due date of the original return. Amended returns filed on or after October 6, 2001 will not affect the underpayment interest amount unless the amended return is received by the statutory due date of the original return or within the extension period granted for the original return.

Example 1: Mary files an Oregon income tax return on a calendar year basis. She filed a return for tax year 2000 on February 15, 2001, showing a tax liability of $1,700. On April 10, 2001, she filed an amended return for tax year 2000 showing a tax liability of $1,450. The return for the taxable year for purposes of computing any interest on underpayment of estimated tax is the amended return filed on April 10, 2001.

Example 2: Using the same facts as given in Example 1 except that Mary's amended return was filed on May 20, 2001. The original return filed on February 15, 2001, is the return for the taxable year for purposes of computing any interest on underpayment of estimated tax.

Example 3: Mark files an Oregon income tax return on a calendar year basis and had an extension to October 15, 2008 in which to file his 2007 return. He filed his 2007 return on May 1, 2008 showing a tax liability of $2,150. On October 15, 2008, he filed an amended return for 2007 showing a tax liability of $1,375. The return for the taxable year for purposes of computing any interest on underpayment of estimated tax is the amended return filed on October 15, 2008.