What Is the Penalty for IRS Underpayment?

The IRS underpayment penalty is interest at the federal short-term rate plus 3 percentage points, compounded daily, charged on the unpaid amount for each day it remains outstanding. For the third quarter of 2026 (July 1 to September 30, 2026), the underpayment interest rate is 7% (a 4% federal short-term rate plus 3%). The IRS sets this rate every quarter under Internal Revenue Code Section 6621, so the exact cost of underpaying changes throughout the year. It's not a flat percentage or a fixed dollar fine.

How does the IRS calculate the underpayment penalty?

The IRS calculates the penalty as daily-compounding interest on the underpaid amount, using the federal short-term rate plus 3 percentage points, for the specific number of days each installment was late. The formula runs separately for each of the four estimated-tax due dates (April 15, June 15, September 15, January 15), because a payment made late for the June installment accrues its own interest clock independent of the September installment. That's why Form 2210, the form used to compute and report the penalty, asks for the underpayment amount and dates by period rather than a single year-end shortfall figure.

What is the current IRS underpayment rate?

The current IRS underpayment rate is 7% for the quarter running July 1 through September 30, 2026. That's a federal short-term rate of 4% plus the standard 3-percentage-point add-on that applies to both individual and corporate underpayments (large corporate underpayments carry a steeper 5-point add-on, for a 9% rate in the same quarter). The rate applies uniformly to individual taxpayers, partnerships, and S-corp shareholders who underpay their quarterly estimates. It's not a different number depending on entity type, only on whether the underpayment counts as a "large corporate underpayment" under IRC §6621(c).

Does the underpayment rate change during the year?

Yes. The IRS resets underpayment interest rates every calendar quarter, so a client who underpaid across two or three quarters may owe interest at two or three different rates depending on which quarter each unpaid day fell in. For 2026, the rate moved from 7% (Q1, Jan–Mar) to 6% (Q2, Apr–Jun) and back up to 7% (Q3, Jul–Sep). A firm calculating a client's exact penalty for a multi-quarter underpayment needs the rate table for every quarter the underpayment spanned, not just the rate on the day the return is filed.

Can the underpayment penalty be avoided?

Yes. A taxpayer avoids the penalty entirely by meeting one of two safe harbor thresholds: paying in at least 90% of the current year's tax, or 100% of the prior year's tax (110% if prior-year AGI exceeded $150,000), through withholding and timely estimated payments combined. The full mechanics of that safe harbor calculation, including which threshold applies by default, are covered in a companion answer, "Safe Harbor Rule for Estimated Taxes."

Is the underpayment penalty deductible?

No. The underpayment penalty is nondeductible interest and is treated as a penalty for federal tax purposes, not as deductible investment or business interest. Preparers should not include it as a deduction when reconciling a client's return, even though the IRS itself labels the charge using interest-rate mechanics.

Because this rate resets every quarter, the practical risk for a firm is using a stale rate from a prior filing season when estimating a client's exposure. Always pull the current-quarter rate from the IRS before quoting a penalty estimate.

SignalsHQ flags underpayment exposure automatically against the current-quarter IRS rate as part of the 1040 review workflow: see how it fits your firm's estimated-tax process.