1) Why IRS interest rates matter
IRS interest can affect both balances due and overpayments. Underpayment interest generally applies when tax is not paid by the prescribed payment date. Overpayment interest may be allowed when the government holds an overpayment long enough to fall within the rules of § 6611. These rules are separate from penalties and additions to tax.
2) Interest paid by the taxpayer and interest paid by the IRS
The Code treats the two directions separately:
- Underpayment interest. Section 6601(a) generally requires interest on unpaid tax from the prescribed payment date until payment, using the underpayment rate established under § 6621.
- Overpayment interest. Section 6611(a) allows interest on an overpayment at the rate established under § 6621, subject to the timing rules and exceptions in § 6611, including the 45-day rule in § 6611(e)(1).
Interest is not the same as a penalty. A taxpayer can owe interest, a penalty, or both, depending on the facts and the provision involved.
3) Common situations for small businesses
- Tax paid after the prescribed payment date. Underpayment interest generally runs until the unpaid tax is paid.
- A payment plan. An installment agreement changes how a balance is paid, but interest generally continues while tax remains unpaid.
- Estimated-tax underpayments. Individuals and corporations can face additions calculated under §§ 6654 and 6655. Those provisions use the underpayment rate but are not accurately described as ordinary late-payment interest.
- An examination or notice that increases tax. Interest may be computed from the applicable prescribed payment date, not merely from the date of the later notice.
- An overpayment or delayed refund. Overpayment interest depends on § 6611’s date rules and exceptions; a refund delay does not produce interest in every case.
4) What the published quarterly rate list means
The IRS sets and publishes rates quarterly. The maintained IRS quarterly interest-rate table is the correct place to check the rate for the specific quarter and category. The categories include noncorporate overpayments, corporate overpayments, underpayments, large corporate underpayments, and special treatment for part of a large corporate overpayment.
Section 6621 ties the statutory formulas to the federal short-term rate. Because the rates can change by quarter, a fixed percentage copied into an evergreen article will become stale.
5) Practical ways to reduce uncertainty
- Pay earlier when possible. Reducing the outstanding tax generally reduces the balance on which future underpayment interest is calculated.
- Use extensions correctly. A filing extension generally does not extend the payment date. Estimate the payment using reliable records rather than treating the extension as a payment delay.
- Review estimated-tax requirements. The required installments and exceptions differ for individuals and corporations.
- Open notices promptly. Confirm the tax period, amount, and effective date before deciding how to respond.
- Keep clean records. Payment confirmations, filed returns, notices, amended-return support, and source documents make interest and refund questions easier to trace.
Federal authority and current rate source
- 26 U.S.C., Subtitle F, Chapter 67, Subchapter A, § 6601(a), (b)(1), and (e)(2)(A) — interest on unpaid tax from the prescribed payment date and interest on specified unpaid penalties after notice and demand.
- 26 U.S.C., Subtitle F, Chapter 67, Subchapter B, § 6611(a), (b)(2), and (e)(1) — overpayment interest, refund-period rule, and the 45-day rule.
- 26 U.S.C., Subtitle F, Chapter 67, Subchapter C, § 6621(a)(1)–(2) and (b)(1) — overpayment and underpayment rates tied to the federal short-term rate.
- 26 U.S.C., Subtitle F, Chapter 68, Subchapter A, Part I, § 6654(a), (b)(1)–(2), and (d)(1) — addition for an individual’s underpayment of estimated income tax and required-installment framework.
- 26 U.S.C., Subtitle F, Chapter 68, Subchapter A, Part I, § 6655(a) and (b)(1)–(2) — addition for a corporation’s underpayment of estimated income tax and the underpayment period.
- 26 U.S.C., Subtitle F, Chapter 62, Subchapter A, § 6151(a) — tax shown on a return is generally paid at the time and place fixed for filing, without assessment or notice.
Current administrative source reviewed September 10, 2026: IRS Quarterly Interest Rates. The IRS page publishes the current and prior quarterly categories and rates.
This article is general federal information. Interest and penalty calculations depend on the tax, taxpayer type, period, payment history, notices, and other facts.
