The IRS views any unpaid taxes as a loan. Therefore, the IRS will add interest to your tax liability based on the applicable federal short-term interest rate. The federal rate is set each quarter, but is typically about 3-4% per year. The abatement of interest is very rare occurance and only accepted if it was incorrectly calculated by the IRS or due to any unnecessary delay by the IRS. A delay by the IRS does not include their vacation time, but unfortunate events such as a lost file. Also, the taxpayer should not assume the interest is calculated correctly. Even though it is computerized, the interest calculation has been known to be incorrect. However, the The Taxperts have many tools at their disposable to determine the correct interest to lower your tax liability.