Interest Netting Under IRC 6621(d): Zero Net Rate Math

Corporations pay interest on underpayments at one rate and earn interest on overpayments at a lower one. If both exist at the same time, the taxpayer is paying the IRS a spread on its own money. Congress closed that gap for overlapping periods in 1998. The fix is called net rate netting, and the math is cleaner than the name.

The statute

IRC 6621(d) says that to the extent interest is payable on an underpayment and allowable on an equivalent overpayment by the same taxpayer, for the same period, the net rate of interest on those amounts is zero for that period.

IRM 20.2.14.6 explains what that means in practice: the interest rates themselves are not zero. Interest on the overpayment and interest on the underpayment for the overlap are equalized, so the rate differential nets to zero.

Why it mostly matters for corporations

For individuals, the overpayment rate and underpayment rate are the same: the federal short-term rate plus 3 under IRC 6621(a). There is no spread to eliminate. Corporations are different. In the fourth quarter of 2026, under Rev. Rul. 2026-15:

A corporation owing tax at 7 percent while waiting on a refund earning 4.5 percent loses 2.5 points on the overlapping dollars. If hot interest applies, the spread is 4.5 points. See hot interest.

The four steps

IRM 20.2.14.3 and 20.2.14.6.4 lay out the computation:

1. Find the overlapping period. The period when interest was running on both the underpayment and the overpayment.
2. Find the equivalent amount. The lesser of the two balances at the start of the overlap. IRM 20.2.14.6.4.2 says either the underpayment is charged the overpayment rate on an amount equal to the overpayment, or the overpayment is allowed the underpayment rate on an amount equal to the underpayment.
3. Find the rates for the overlap. The rates actually charged and allowed, quarter by quarter.
4. Compute the adjustment. The difference between interest at the two rates on the equivalent amount for the overlap.

Worked example

A calendar-year C corporation has a $200,000 underpayment for 2023 (interest from April 15, 2024), paid September 30, 2026. It also has a $150,000 overpayment for 2024, with interest allowed from April 15, 2025, refunded June 30, 2026. No hot interest applies.

Without netting, the corporation paid $4,651.51 more in interest on those $150,000 than it earned on its own $150,000 at the same time. Netting returns that, usually as a reduction in underpayment interest or an increase in overpayment interest. The remaining $50,000 of underpayment, and every day outside the overlap, are charged at normal rates.

How to request it

Under section 4 of Rev. Proc. 2000-26, which IRM 20.2.14.6.3 follows, the request is made on Form 843, Claim for Refund and Request for Abatement. If the periods are already before an IRS function, such as Examination or Appeals, a letter or written statement to that function can take the place of Form 843.

Two time limits matter, per IRM 20.2.14.6.2. A request to reduce underpayment interest is a refund claim under IRC 6511: the later of three years from filing the return or two years from paying the tax and interest. A request for more overpayment interest is governed by the six-year suit period in 28 U.S.C. 2401 and 2501, starting from the overpayment's schedule date.

Within one period: a separate rule

Netting across different tax periods is the 6621(d) problem. Netting within a single tax period, where an account swings between owing and being owed, is handled under Rev. Proc. 94-60 and generally computed by the IRS systems automatically, per IRM 20.2.14.4. If you only have one year in play, you probably do not need a Form 843 for this.

Related tools that reduce the spread

• Credit elections. Applying an overpayment to an existing balance under IRC 6402 stops interest on both sides as of the relevant dates; see overpayment interest.
• IRC 6601(f). When tax is satisfied by credit of an overpayment, no underpayment interest runs on that portion for any period the overpayment would have earned interest, except where 6621(d) applies.
• Deposits under IRC 6603. A deposit stops underpayment interest on the deposited amount while a dispute continues.

Checklist

1. List every period with underpayment interest and every period with overpayment interest, with start and end dates.
2. Mark the overlaps. Each overlap is a candidate.
3. For each, compute the equivalent amount at the start of the overlap.
4. Compute the interest at both rate paths on that amount and take the difference.
5. File Form 843 (or a statement to the IRS function handling the case) inside the applicable time limit.

For the daily compounding method used in each leg, see how IRS interest compounds daily. For the rate history, see IRS interest rates by quarter.

What Rev. Proc. 2000-26 says

Rev. Proc. 2000-26, 2000-24 I.R.B. 1257, is the IRS's guidance on applying IRC 6621(d) to interest accruing on or after October 1, 1998. It explains that 6621(d) generally applies to interest for periods beginning after July 22, 1998, the date the provision was enacted, and points to Rev. Proc. 99-43 for earlier periods. IRM 20.2.14, revised in 2025, still applies Rev. Proc. 2000-26, and the IRS's guidance index shows nothing superseding it.

Three points from the revenue procedure matter for the math. First, quoting the conference report, it says the zero net rate applies without regard to whether the overpayment or underpayment is currently outstanding. A refund already paid or a balance already settled can still be netted. Second, each overpayment or underpayment is considered only once in determining whether equivalent amounts overlap. Third, the net rate of zero applies even when the large corporate underpayment rate under 6621(c) or the reduced rate on large corporate overpayments applies.

On procedure, section 4 of the revenue procedure says requests should be made on Form 843, labeled at the top "Request for Net Interest Rate of Zero Under Rev. Proc. 2000-26," and filed with the service center where the taxpayer filed its most recent income tax return. Line 5 identifies the overpaid and underpaid periods and when tax was paid or refunded. A separate Form 843 is not required for each period. Under its special procedure, no Form 843 is needed when the returns are under consideration by an IRS function such as Examination or Appeals; a written statement to that function is used instead.

Worked example: hot interest on one side

Now make the underpayment a large corporate underpayment carrying hot interest at 9 percent, overlapping for one full year with a $150,000 overpayment that earns 6 percent on the first $10,000 and 4.5 percent on the rest. Using those rates, compounded daily for 365 days, interest on $150,000 at 9 percent is $14,124.32, and interest on the overpayment is $7,061.81. The spread for one year is $7,062.51, roughly half the underpayment interest on the overlapping amount.

Because Rev. Proc. 2000-26 confirms the zero net rate applies even with hot interest, that entire spread is recoverable for the overlapping amount and period. On a multi-year corporate account with recurring exam adjustments, the netting computation can be worth more than the audit adjustment that triggered it.

The bottom line

Interest netting erases the rate spread when the same taxpayer owes and is owed at the same time. For corporations, the spread is real money and the IRS will not net across years unless asked. Map the overlaps, price them, and file the request before the time limits run. A multi-year corporate account is worth a professional review for exactly this reason.