IR-2018-181, Sept. 7, 2018 WASHINGTON ― The Internal Revenue Service today announced that interest rates will remain the same for the calendar quarter beginning Oct. 1, 2018. The rates will be: five (5) percent for overpayments [four (4) percent in the case of a corporation]; 2 and one-half (2.5) percent for the portion of a corporate overpayment exceeding $10,000; five (5) percent for underpayments; and seven (7) percent for large corporate underpayments. Under the Internal Revenue Code, the rate of interest is determined on a quarterly basis. For taxpayers other than corporations, the overpayment and underpayment rate is the federal short-term rate plus 3 percentage points. Generally, in the case of a corporation, the underpayment rate is the federal short-term rate plus 3 percentage points and the overpayment rate is the federal short-term rate plus 2 percentage points. The rate for large corporate underpayments is the federal short-term rate plus 5 percentage points. The rate on the portion of a corporate overpayment of tax exceeding $10,000 for a taxable period is the federal short-term rate plus one-half (0.5) of a percentage point. The interest rates announced today are computed from the federal short-term rate determined during July 2018 to take effect Aug. 1, 2018, based on daily compounding. Revenue Ruling 2018-25PDF, announcing the rates of interest, is attached and will appear in Internal Revenue Bulletin 2018-39, dated Sept. 24, 2018.