Back taxes refer to any federal (or state) tax liability from a prior year that remains unpaid. This can happen for several reasons: you filed a return but couldn’t afford to pay the balance, you failed to file altogether and the IRS assessed a liability on your behalf through a Substitute for Return (SFR) under IRC §6020(b), or you underreported income and the IRS corrected your return after an audit or automated matching.
When a tax balance goes unpaid, the IRS applies two primary additions:
- Failure-to-Pay Penalty— 0.5% of the unpaid balance per month, up to a maximum of 25% (IRC §6651(a)(2)).
- Interest— Compounded daily based on the federal short-term rate plus 3% (IRC §6621). As of recent years, this rate has hovered between 7% and 8% annually, meaning your balance grows significantly each year.
If you also failed to file, the Failure-to-File Penalty of 5% per month (up to 25%) stacks on top of the failure-to-pay penalty for the first five months (IRC §6651(a)(1)). When both penalties apply in the same month, the IRS reduces the failure-to-file penalty by the failure-to-pay amount, but the net effect is still punishing: combined penalties can reach 47.5% of the unpaid tax (after about four years and two months), before interest is even factored in.
The IRS has 10 years from the date of assessment to collect a tax liability — this is called the Collection Statute Expiration Date (CSED) under IRC §6502. After that period, the balance expires. However, certain actions (such as submitting an Offer in Compromise or filing for bankruptcy) can toll (pause) the statute, extending the collection window.