If there are monies owed on unpaid taxes, the IRS charges interest from the due date of the return, i.e., typically April 15th, until the date of payment. The interest rate is the federal short term rate plus three percent, and the interest rate is calculated on a quarterly basis. Also, if you file a return and fail to pay the entire amount due, you will be hit with a late payment penalty. The IRS late payment penalty is equal to one-half of one percent of the tax owed for each month, or part of a month, that the tax remains unpaid from the due date, until the tax is paid in full or the 25% maximum penalty is reached. So, for example, if you owe the IRS $10,000, you will be hit with a $50 (i.e., .005 * $10,000) late payment penalty for every month the taxes remained unpaid.
The one-half of one percent rate increases to one percent if the tax remains unpaid 10 days after the IRS issues a notice of intent to levy. For individuals who file by the return due date, the one-half of one percent rate decreases to one-quarter of one percent for any month in which an installment agreement is in effect. So, it is important to meet with qualified tax resolution counsel to work out an installment agreement, or alternatively, some other tax resolution such as an offer in compromise.