Rev. Rul. 2020-28

Section 6621 of the Internal Revenue Code establishes the interest rates on overpayments and underpayments of tax. Under section 6621(a)(1), the overpayment rate is the sum of the federal short-term rate plus 3 percentage points (2 percentage points in the case of a corporation), except the rate for the portion of a corporate overpayment of tax exceeding $10,000 for a taxable period is the sum of the federal short-term rate plus 0.5 of a percentage point. Under section 6621(a)(2), the underpayment rate is the sum of the federal short-term rate plus 3 percentage points. Section 6621(c) provides that for purposes of interest payable under section 6601 on any large corporate underpayment, the underpayment rate under section 6621(a)(2) is determined by substituting “5 percentage points” for “3 percentage points.” See section 6621(c) and section 301.6621-3 of the Regulations on Procedure and Administration for the definition of a large corporate underpayment and for the rules for determining the applicable date. Section 6621(c) and section 301.6621-3 are generally effective for periods after December 31, 1990.

Section 6621(b)(1) provides that the Secretary will determine the federal shortterm rate for the first month in each calendar quarter. Section 6621(b)(2)(A) provides that the federal short-term rate determined

under section 6621(b)(1) for any month applies during the first calendar quarter beginning after that month. Section 6621(b) (3) provides that the federal short-term rate for any month is the federal short-term rate determined during that month by the Secretary in accordance with section 1274(d), rounded to the nearest full percent (or, if a multiple of 1/2 of 1 percent, the rate is increased to the next highest full percent).