Hot Interest: The Large Corporate Underpayment Rate Under IRC 6621(c)

Corporations already pay the standard underpayment rate on unpaid tax. When the underpayment is large and the IRS has formally proposed it, the rate goes up. Practitioners call it hot interest. The Code calls it the rate on a large corporate underpayment, and it has precise triggers you can calendar.

The rate

The standard underpayment rate under IRC 6621(a)(2) is the federal short-term rate plus 3 percentage points. IRC 6621(c)(1) says that for interest on a large corporate underpayment for periods after the applicable date, substitute 5 percentage points for 3. Two points more, on the entire underpayment, from that date forward.

Test 1: is it a large corporate underpayment?

IRC 6621(c)(3)(A) defines a large corporate underpayment as any underpayment of tax by a C corporation for a taxable period that exceeds $100,000. For income tax, the taxable period is the taxable year. S corporations, partnerships, and individuals are outside the rule.

Note what the threshold measures: the underpayment for one taxable period. Two years with $80,000 underpayments each do not combine.

Test 2: what is the applicable date?

Under IRC 6621(c)(2)(A), the applicable date is the 30th day after the earlier of:

1. the date the first letter of proposed deficiency that allows the taxpayer an opportunity for administrative review in the IRS Independent Office of Appeals is sent (commonly called the 30-day letter), or
2. the date the statutory notice of deficiency under IRC 6212 is sent.

For taxes not subject to deficiency procedures, IRC 6621(c)(2)(B)(i) looks to any letter or notice that tells the taxpayer of the assessment or proposed assessment. Treas. Reg. 301.6621-3 fills in the details.

Three ways a letter is ignored

• Withdrawn. A letter or notice the IRS withdraws does not count.
• Paid within 30 days. Under 6621(c)(2)(B)(ii), a letter is disregarded if, during the 30-day period beginning on the day it was sent, the corporation pays the amount shown as due.
• Small amount. Under 6621(c)(2)(B)(iii), a letter is disregarded if the deficiency or proposed deficiency in it, not counting interest and penalties, is $100,000 or less.

Worked example

A calendar-year C corporation's 2023 return was due April 15, 2024. An exam proposes a $400,000 deficiency in a 30-day letter sent May 4, 2026, offering Appeals review. The corporation does not pay within 30 days. The applicable date is June 3, 2026. The corporation pays the tax and all interest on October 30, 2026.

About $4,000 extra for five months. On a bigger underpayment, or a case that sits in Appeals for two years, that becomes real money.

Strategy the math suggests

• Pay inside the 30-day window if you agree with the adjustment. The statute disregards a letter if the amount shown is paid within 30 days of the letter date. No applicable date, no hot interest from that letter.
• Consider a deposit if you disagree. IRC 6603 lets a taxpayer make a deposit to stop the running of underpayment interest on the amount deposited, while preserving the right to contest. Whether a deposit fits depends on the case.
• Track every letter. Note the date sent, the amount, and whether it exceeds $100,000 before interest and penalties. The earliest qualifying letter controls.

Hot interest and overpayments

There is a mirror image on the refund side. Corporations earn a reduced overpayment rate: the federal short-term rate plus 2, dropping to plus 0.5 on the portion of an overpayment above $10,000 (IRC 6621(a)(1)). For the fourth quarter of 2026 those rates are 6 percent and 4.5 percent. When a corporation has both an underpayment at 9 percent and an overpayment earning 4.5 percent for overlapping periods, interest netting under IRC 6621(d) can bring the net rate on the overlap to zero. See interest netting.

What hot interest does not change

• It does not change when interest starts. Interest on the underlying tax still runs from the original due date under IRC 6601(a).
• It does not change penalty computations.
• It does not apply to S corporations or individuals, whatever the size of the underpayment.

For the quarterly rate table and how each rate is set, see IRS interest rates by quarter. If the adjustment is still in dispute, the procedural side is covered in IRS Appeals: How to Challenge a Decision.

Worked example: the small-amount rule

Under IRC 6621(c)(2)(B)(iii), a letter or notice is disregarded if the deficiency or proposed deficiency in it, not counting interest and penalties, is $100,000 or less. That rule can delay the applicable date by months.

Say a C corporation receives a 30-day letter on February 2, 2026 proposing a $90,000 deficiency. That letter is disregarded because the amount is not greater than $100,000. Exam continues, the adjustment grows, and a statutory notice of deficiency for $250,000 is sent September 1, 2026. That is the first qualifying letter or notice, so the applicable date is the 30th day after it, October 1, 2026. Hot interest applies only for periods after that date. If the February letter had proposed $110,000, the applicable date would have been March 4, 2026, and seven more months would have carried the higher rate.

Worked example: the 30-day payment exception

Under IRC 6621(c)(2)(B)(ii), a letter is also disregarded if, during the 30-day period beginning on the day it was sent, the corporation pays the amount shown as due. Take the $400,000 example with the 30-day letter of May 4, 2026. If the corporation agreed with the adjustment and paid the amount shown as due within that 30-day period, the letter would be disregarded, no applicable date would arise from it, and the $3,934.52 of extra hot interest in the example would never accrue. The corporation would still owe standard-rate interest from the original due date to the payment date.

Taxes outside deficiency procedures

Some taxes are assessed without a notice of deficiency. For those, IRC 6621(c)(2)(B)(i) looks to any letter or notice that tells the taxpayer of the assessment or proposed assessment. The same $100,000 and 30-day payment exceptions apply. The practical lesson is the same for every tax type: the first IRS letter that proposes more than $100,000 of tax is the one to calendar, because 30 days later the rate on the whole underpayment goes up two points.

Netting can help on the back end. Rev. Proc. 2000-26, quoting the conference report on IRC 6621(d), says the net interest rate of zero applies even when the large corporate underpayment rate is in effect. A corporation paying 9 percent on an underpayment while earning 4.5 percent on an overlapping overpayment has a 4.5-point spread to recover; see the interest netting guide.

A practical calendar for a corporation under exam: log every letter's date sent and proposed amount, mark whether it exceeds $100,000 before interest and penalties, add 30 days, and decide before that date whether to pay, deposit under IRC 6603, or accept the higher rate while contesting. Treas. Reg. 301.6621-3 contains the detailed rules for applying the applicable date, including how letters and notices are identified.

For S corporations and individuals, none of this applies, no matter how large the adjustment. The plus-5 rate is reserved for C corporations.

The bottom line

Hot interest is the regular rate plus two points, on the whole underpayment, for a C corporation that owes more than $100,000 for a year, starting 30 days after the first qualifying letter. Find the letter, add 30 days, and price the difference before deciding whether to pay, deposit, or fight. That decision deserves a professional who has seen the file.