The IRS has announced the interest rates for tax overpayments and underpayments for the calendar quarter beginning October 1, 2008.
For noncorporate taxpayers, the rate for both underpayments and overpayments will be 6%.
For corporations, the overpayment rate will be 5%. Corporations will receive 3.5% for overpayments exceeding $10,000. The underpayment rate for corporations will be 6%, but will be 8% for large corporate underpayments.
Rev. Rul. 2008-47