26 U.S.C. § 6621: Determination of rate of interest

the Federal short-term rate determined under subsection (b), plus 3 percentage points (2 percentage points in the case of a corporation). the Federal short-term rate determined under subsection (b), plus 3 percentage points. The Secretary shall determine the Federal short-term rate for the first month in each calendar quarter. Except as provided in subparagraph (B), the Federal short-term rate determined under paragraph (1) for any month shall apply during the first calendar quarter beginning after such month. In determining the addition to tax under section 6654 for failure to pay estimated tax for any taxable year, the Federal short-term rate which applies during the 3rd month following such taxable year shall also apply during the first 15 days of the 4th month following such taxable year. The Federal short-term rate for any month shall be the Federal short-term rate determined during such month by the Secretary in accordance with section 1274(d). Any such rate shall be rounded to the nearest full percent (or, if a multiple of ½ of 1 percent, such rate shall be increased to the next highest full percent). For purposes of determining the amount of interest payable under section 6601 on any large corporate underpayment for periods after the applicable date, paragraph (2) of subsection (a) shall be applied by substituting “5 percentage points” for “3 percentage points”. the date on which the 1st letter of proposed deficiency which allows the taxpayer an opportunity for administrative review in the Internal Revenue Service Independent Office of Appeals is sent, or the date on which the deficiency notice under section 6212 is sent. In the case of any underpayment of any tax imposed by this title to which the deficiency procedures do not apply, subparagraph (A) shall be applied by taking into account any letter or notice provided by the Secretary which notifies the taxpayer of the assessment or proposed assessment of the tax. For purposes of subparagraph (A), a letter or notice shall be disregarded if, during the 30-day period beginning on the day on which it was sent, the taxpayer makes a payment equal to the amount shown as due in such letter or notice, as the case may be. For purposes of this paragraph, any letter or notice shall be disregarded if the amount of the deficiency or proposed deficiency (or the assessment or proposed assessment) set forth in such letter or notice is not greater than $100,000 (determined by not taking into account any interest, penalties, or additions to tax). The term “large corporate underpayment” means any underpayment of a tax by a C corporation for any taxable period if the amount of such underpayment for such period exceeds $100,000. in the case of any tax imposed by subtitle A, the taxable year, or in the case of any other tax, the period to which the underpayment relates. To the extent that, for any period, interest is payable under subchapter A and allowable under subchapter B on equivalent underpayments and overpayments by the same taxpayer of tax imposed by this title, the net rate of interest under this section on such amounts shall be zero for such period. The amendments made by subsections (a) and (b) [amending this section] shall apply to adjustments made after the date of the enactment of this Act [ Aug. 13, 1981 ]. The amendment made by subsection (c) [amending this section] shall apply to adjustments made for periods after 1982.”