Federal short-term rate plus 3 percent
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 For taxpayers other than corporations, the overpayment and underpayment rate is the federal short-term rate plus 3 percentage points. This rate is determined by the IRS on a quarterly basis. Get the federal short-term interest rate from the IRS website. Add an additional 3 percentage points to determine the total interest. As an example, if the current short-term interest rate is 0.30 percent, add 3 percentage points to get 3.30 percent total interest.