The Internal Revenue Service said Thursday that interest rates will decrease in the fourth quarter for tax underpayments and overpayments by a full percentage point.

For the calendar quarter beginning Oct. 1, 2011, the IRS is lowering the interest rate to 3 percent for overpayments, or 2 percent in the case of a corporation. That’s a percentage point lower than the rates in the third quarter for corporations and other taxpayers.

For underpayments, the interest rate will be 3 percent in the fourth quarter, down from 4 percent in the third quarter.

For large corporate underpayments, the interest rate will be 5 percent in the fourth quarter, down from 6 percent in the third quarter.

For the portion of a corporate overpayment exceeding $10,000, the interest rate in the fourth quarter will be 0.5 percent, down from 1.5 percent in the third quarter.

Under the Tax 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 half a percentage point. The interest rates are computed from the federal short-term rate during July 2011 to take effect Aug. 1, 2011, based on daily compounding.

The IRS describes the interest rates in more detail in Revenue Ruling 2011-18.