IRS interest rates are unchanged for the fourth quarter of 2014 (October 1, 2014, through December 31, 2014). These are the rates changed on underpayments and overpayments:
The 3% rate applies to underpayments of estimated taxes by individuals. All of these rates have held steady since October 1, 2011.
Source: Rev. Rul. 2014-23
For an individual, his or her Social Security number; for businesses, fiduciaries, and other non-individual taxpayers, the employer identification number.