IRC § 6166 Closely-Held Business Estate Tax Installment Calculator

How this calculator works

Section 6166 of the Internal Revenue Code allows the executor of a decedent's estate to defer and pay in installments the portion of the federal estate tax attributable to an interest in a closely-held business, provided that interest exceeds 35% of the Adjusted Gross Estate. The election converts what would otherwise be a single lump-sum payment due nine months after the date of death into a 14-year schedule: four annual interest-only payments under § 6166(f), followed by ten annual installments of principal-plus-interest under § 6166(a)(3). The deferred tax bears interest at the reduced 2% special rate under § 6601(j)(1) on the tax attributable to the first 1.85 million dollars of closely-held value (2025 indexed under § 6166(b)(7)), with the remainder bearing interest at 45% of the IRS underpayment rate under § 6621.

This calculator runs the 35% qualifier under § 6166(a)(1), allocates the deferrable tax across the 2% and regular-rate tranches, builds the year-by-year payment schedule, and compares the present value of the deferred stream against the cash-due-now alternative. The output is a planning view for executors and estate counsel evaluating whether the election is worth making and what the long-term cash-flow obligation looks like.

Why § 6166 exists: the illiquid-business problem

The federal estate tax is due nine months after the date of death. For families whose wealth is concentrated in a closely-held business, that deadline can be brutal. A family operating a regional manufacturing firm with 90% of the estate value tied up in the business and 10% in cash and securities faces a single hard choice without § 6166: sell the business, sell parts of it, or borrow against it at unfavorable terms. The very enterprise the estate tax is taxing becomes the source of its own destruction.

Congress recognized this problem in 1958 when it enacted § 6166A, the predecessor provision permitting a ten-year installment payment. The Tax Reform Act of 1976 replaced § 6166A with the current § 6166, expanding the deferral to fourteen years and introducing the 4% special interest rate (later reduced to 2% in subsequent reforms). The Economic Growth and Tax Relief Reconciliation Act of 2001 raised the indexed-value ceiling under § 6166(b)(7) and clarified the holding-company rules under § 6166(b)(8) and § 6166(b)(9). The Tax Relief Act of 2010 extended and restored the relief during the unsettled estate-tax period of that era. Today the provision is a stable feature of the estate tax landscape and a routine planning consideration for any closely-held business interest above the unified-credit threshold.

The 35% qualifier under § 6166(a)(1)

The election is available only if the value of the decedent's closely-held business interest exceeds 35% of the Adjusted Gross Estate. The AGE under § 6166(b)(6) is defined as the gross estate minus deductions allowable under § 2053 (administration expenses, debts, funeral) and § 2054 (casualty losses). Critically, the AGE is NOT the taxable estate — it does not subtract the § 2056 marital deduction or the § 2055 charitable deduction.

Congress specifically constructed the qualifier this way because the goal is to measure the real asset mix at the date of death, not the post-deduction estate. A surviving-spouse trust funded entirely with marital assets does not lower the AGE; only administration expenses, debts, and casualty losses do. This matters in practice: a large marital bequest can shrink the taxable estate dramatically without shrinking the AGE, potentially making the 35% qualifier easier to satisfy than a naive look at the taxable estate would suggest.

The 35% test is a strict inequality. A ratio of exactly 35.000% does NOT qualify; 35.001% does. In practice, estates close to the boundary should be valued conservatively to give the executor room to defend the qualifier on audit.

The 2% special interest rate and the indexed ceiling

Under IRC § 6601(j)(1), the portion of the deferred estate tax attributable to the first 1.85 million dollars of taxable value (2025 indexed under § 6166(b)(7)) bears interest at a reduced 2% per annum. The mechanic warrants careful reading: the 1.85 million dollar figure is a ceiling on closely-held VALUE, not on the deferred-tax amount itself. The deferred tax attributable to that first 1.85 million dollars of value is determined by the estate tax rate at the decedent's marginal bracket — typically 40% under § 2001(c), the top rate. So the maximum tax that can qualify for the 2% rate is roughly 740 thousand dollars (1.85 million times 40%).

The portion of the deferred tax attributable to closely-held value ABOVE the 1.85 million dollar ceiling bears interest at 45% of the IRS § 6621 underpayment rate under § 6601(j)(2). At the current 3.6% underpayment rate, that works out to about 1.6% per annum on the regular-rate tranche. Both rates float annually with the § 6621 rate, which the IRS publishes quarterly.

The 1.85 million dollar figure is indexed annually under § 6166(b)(7). The IRS publishes the indexed amount each November for the following tax year in the annual inflation-adjustments revenue procedure. For 2025 the figure is set at 1.85 million dollars in Rev. Proc. 2024-40.

The 14-year payment schedule

Under IRC § 6166(a)(3) and § 6166(f), the schedule runs up to 14 years and divides into two phases.

Years 1 through 4 are interest-only. No principal is due. Interest accrues on the full deferred-tax balance, split between the 2% tranche and the regular tranche as described above. Interest payments are due annually on the anniversary of the Form 706 due date (nine months after death). The first interest payment is due 21 months after death — twelve months after the Form 706 was filed.

Years 5 through 14 are installment. Ten equal annual principal installments of one-tenth of the deferred tax, plus interest on the outstanding balance. The total payment in each installment year declines as the principal amortizes. The last installment is due 14 years and 9 months after the date of death.

The schedule is fixed. There is no provision for accelerating principal payments (although the executor may make them voluntarily, with no penalty), and there is no provision for stretching the schedule beyond 14 years (although a refinancing of the underlying tax via a private loan can effectively extend cash-flow obligations indefinitely at market rates).

Acceleration triggers under § 6166(g)

Three events accelerate the deferred balance to immediate payment. Each is a hard-stop: when the trigger fires, the full remaining balance becomes due immediately.

Aggregate disposition of 50% or more under § 6166(g)(1)(A). Sales, exchanges, withdrawals, or other dispositions totaling 50% or more of the decedent's interest in the closely-held business accelerate the balance. The percentage is computed cumulatively across all dispositions during the deferral — a 30% sale in year 3 and a 25% sale in year 6 cross the threshold and accelerate the balance. A transfer to a qualified heir under § 6166(i) is NOT a disposition (the heir takes over the schedule). A sale to a third party IS, even if the consideration received is reinvested in the family.

Late payment beyond the 6-month cure under § 6166(g)(3). Any installment or interest payment more than 6 months late accelerates the balance. Within the 6-month cure period, a late-payment surcharge under § 6651 applies but the deferral survives. Past 6 months, no cure is available — the executor must pay the full remaining balance immediately. This trigger is the most common acceleration in practice; estates with administrative-disorganization risk should put the payment dates on a calendar with multiple reminders.

Undistributed-income test under § 6166(g)(1)(B). If the closely-held business distributes 50% or more of its accumulated earnings and profits in a single year, the deferral accelerates. This trigger is rare in practice and is tested factually. It exists to prevent the family from extracting business value during deferral as a workaround for the disposition rules.

Multi-business aggregation under § 6166(c)

The executor may aggregate two or more closely-held businesses to meet the 35% qualifier, PROVIDED that the decedent's interest in EACH aggregated business is at least 20% of the value of that business. The 20% per-business floor is a critical and sometimes-overlooked rule: a 19.9% interest does NOT aggregate, and the executor must either accept exclusion of that business from the calculation or forgo § 6166 entirely.

Worked example: decedent owns 35% of Business A worth 3 million dollars, and 15% of Business B worth 4 million dollars. The AGE is 18 million dollars. Business A qualifies for aggregation (35% above the 20% floor). Business B does not (15% below). The aggregate counts only Business A's 3 million dollars, which is 16.7% of AGE — does not qualify. If Business B had been a 25% interest worth 4 million dollars, the aggregate would be 7 million dollars (38.9% of AGE) — qualifies.

The 20% floor is sometimes addressed pre-death by gifting shares to family members so the decedent's remaining interest stays at or above 20% in each holding. Sophisticated estate planning often runs the § 6166 qualifier test against multiple ownership-mix scenarios to identify the gifts that preserve eligibility while minimizing transfer-tax exposure.

Form 706 election mechanics

The § 6166 election is made on a TIMELY-FILED Form 706 (federal estate tax return) by attaching a written notice of election. The notice must identify the closely-held business interest, state that the executor is electing § 6166 installment payment, supply the value of the interest, demonstrate that the 35% qualifier is satisfied, and supply the information required by Treas. Reg. § 20.6166-1.

The Form 706 is due nine months after the date of death. A six-month extension is available under § 6081 (Form 4768), and the § 6166 election may be made on the extended return. There is NO LATE ELECTION absent § 9100 regulatory relief, which the IRS grants parsimoniously — only when the executor can show reasonable cause and good-faith reliance on a professional advisor.

The election is irrevocable once made and binds the estate and any qualified heirs to whom the business is transferred during deferral under § 6166(i). The IRS may require security for the deferred tax under § 6166(k): typically the § 6324A special estate-tax lien on the closely-held business assets, or a surety bond. The IRS evaluates each estate individually under Notice 2007-90 to determine whether to require the lien, the bond, or neither — historically, the IRS has been more aggressive about requiring security since 2007.

Worked example 1: typical qualifying estate

An estate has 20 million dollars of Adjusted Gross Estate and 8 million dollars of closely-held business value. The gross federal estate tax (after the unified credit) is 4 million dollars. The executor elects § 6166 on a timely-filed Form 706.

• Qualifier ratio: 8 million / 20 million = 40% — qualifies (above 35%).
• Deferrable tax: 4 million × 40% = 1.6 million dollars.
• Cash due now (with Form 706): 4 million − 1.6 million = 2.4 million dollars.
• 2% tranche: (1.85 million / 8 million) × 1.6 million = 370 thousand dollars at 2% per annum.
• Regular tranche: 1.6 million − 370 thousand = 1.23 million dollars at 45% × 3.6% = 1.62% per annum.
• Year-1 interest: (370K × 2%) + (1.23M × 1.62%) ≈ 27 thousand 326 dollars.
• Total nominal payments over 14 years: approximately 1.85 million dollars (1.6 million principal + 250 thousand interest).
• Present value at a 6% discount rate: approximately 1.27 million dollars.
• NPV savings vs. paying 1.6 million up front: approximately 330 thousand dollars.

The economic value of the election is meaningful — about 20% of the deferrable tax in PV terms. The strategic value is larger: avoiding a fire-sale of the business is the bigger driver, but cannot be reduced to a dollar figure.

Worked example 2: borderline non-qualifying estate

Same estate facts but with 6.5 million dollars of closely-held value instead of 8 million.

• Qualifier ratio: 6.5 million / 20 million = 32.5% — does NOT qualify (below 35%).
• Deferrable tax: zero.
• Cash due now: full 4 million dollars with Form 706, nine months after death.

The executor must either find 4 million dollars of liquidity by month 9 or commission a private financing arrangement against the closely-held business. Pre-death planning to lift the closely-held percentage above 35% (typically by gifting non-business assets, or by restructuring the entity to concentrate value) could have preserved the election.

Worked example 3: large estate with full deferral

A 50 million dollar AGE estate with 45 million dollars of closely-held value and a 16 million dollar gross estate tax.

• Qualifier ratio: 45 million / 50 million = 90% — qualifies.
• Deferrable tax: 16 million × 90% = 14.4 million dollars.
• Cash due now: 1.6 million dollars.
• 2% tranche: (1.85 million / 45 million) × 14.4 million = 592 thousand dollars at 2%.
• Regular tranche: 14.4 million − 592 thousand = 13.8 million dollars at 1.62%.
• NPV savings at 6% discount rate: approximately 3 million dollars.

For large closely-held business estates, the 14-year stretch generates seven-figure NPV savings even with modest discount-rate assumptions.

Acceleration in practice: the 50% disposition trap

The most common unintended acceleration in § 6166 elections comes from routine business activity that the executor does not realize counts as a disposition.

Common scenarios that trigger or contribute to acceleration:

• A sale of business divisions to a strategic acquirer.
• A redemption of the decedent's stock by the corporation (counted as a disposition under § 6166(g)(1)(A)).
• A like-kind exchange of business real estate (the IRS treats this as a disposition).
• A series of small sales over the deferral period that cumulatively cross 50%.
• A merger or reorganization that converts the closely-held interest into stock of a publicly-traded acquirer.

Common scenarios that do NOT trigger acceleration:

• A transfer to a qualified heir under § 6166(i) (spouse, lineal descendant, etc.).
• Sales of assets BY the business in the ordinary course (only sales of the decedent's interest count).
• Gifts of the decedent's interest to qualified heirs.

The 50% test is cumulative across the deferral. The executor should track every disposition and the qualified-heir status of every transferee. A surprise acceleration can require liquidating the very business the deferral was protecting.

Security under § 6324A: the special estate-tax lien

Under IRC § 6166(k), the IRS may require security for the deferred tax. The primary mechanism is the § 6324A special estate-tax lien — a statutory lien on the closely-held business property that secures the deferred balance for the duration of the deferral. The lien is recorded in the local land or business records and runs ahead of competing claims by the lien-recording date.

If the IRS requests the § 6324A lien, the executor must consent — refusal terminates the election. The lien can impair business operations: lender covenants that require the business to be free of liens, vendor relationships that require lien-free status, and any sale or refinancing of the underlying assets must work around the lien. A surety bond is an alternative under § 6166(k), but bond premiums typically run 0.5% to 2% of the bonded amount per annum, which can erode the NPV savings of the election.

The IRS's lien-vs-bond decision under Notice 2007-90 considers the executor's compliance history, the value of the underlying business, the alternative security available, and the estate's projected ability to make the installment payments. Engage estate counsel to negotiate the form of security before the election is finalized — the IRS is open to negotiation on the choice, but the timing window is short.

Common errors

The most consequential errors in § 6166 elections:

Missing the 35% qualifier. Failing to include all qualifying closely-held value, or aggregating businesses that fail the 20% per-business floor under § 6166(c). A pre-death valuation review can identify aggregation opportunities and gifting strategies that preserve the qualifier.

Missing the timely-filed requirement. The election can only be made on a Form 706 filed within nine months of death (or fifteen with the § 6081 extension). Late returns lose the election absent § 9100 relief, which is granted parsimoniously.

Unintended acceleration. Routine business activity that the executor did not realize would trigger § 6166(g)(1)(A). The 50% disposition test runs cumulatively, and small sales can add up over a 14-year deferral.

Underestimating the security cost. The § 6324A lien or surety bond can impair operations or cost meaningfully more than executors anticipate.

Confusing AGE with the taxable estate. The AGE under § 6166(b)(6) does NOT subtract the marital or charitable deductions. A large marital bequest can make the qualifier easier to satisfy than a naive look at the taxable estate would suggest.

What this calculator does not do

This is a planning and screening tool. It does NOT substitute for engagement of estate counsel and a CPA in the actual Form 706 preparation. It does NOT compute the gross federal estate tax itself (the unified-credit and rate-schedule calculation is a separate prior step). It does NOT model the § 6166(b)(8) and § 6166(b)(9) holding-company lookthrough rules, which require careful entity-by-entity analysis. It does NOT advise on the choice between the § 6324A lien and a surety bond under § 6166(k); that decision turns on business-specific operational considerations. It applies only to federal estate tax; state-level estate-tax conformity (a handful of states have § 6166 lookthrough) is out of scope.

How this page is maintained

The § 6166 framework has been stable since the 2010 Tax Relief Act. The 35% qualifier, the 14-year schedule, the 2% special interest rate, and the acceleration triggers have not changed. The indexed ceiling under § 6166(b)(7) is updated annually each November in the IRS inflation-adjustments revenue procedure; this page is refreshed when a new ceiling is published. The IRS § 6621 underpayment rate is updated quarterly; the calculator accepts the current rate as an input so practitioners can run scenarios against the published quarterly figure.

Last reviewed: 2026-05-16 against IRC § 6166 (full section), IRC § 6166(a) (election + schedule), IRC § 6166(a)(1) (35% qualifier), IRC § 6166(a)(3) (10-year installment count), IRC § 6166(b)(6) (AGE definition), IRC § 6166(b)(7) (annual indexing), IRC § 6166(c) (multi-business aggregation), IRC § 6166(f) (4-year interest-only), IRC § 6166(g) (acceleration triggers), IRC § 6166(i) (qualified-heir transfer), IRC § 6166(k) (security), IRC § 6601(j) (2% special rate + 45% formula), IRC § 6621 (underpayment rate), IRC § 6324A (special estate-tax lien), IRC § 2001(c) (estate tax rate schedule), IRC § 2053 and § 2054 (AGE deductions), Rev. Proc. 2024-40 (2025 indexed ceiling), Form 706 (election attached), IRS Notice 89-32 (election mechanics), IRS Notice 2007-90 (lien-vs-bond decision), IRS Publication 559 (Survivors, Executors, and Administrators), the Tax Reform Act of 1976 (original § 6166), the Economic Growth and Tax Relief Reconciliation Act of 2001 (ceiling raise), and the Tax Relief Act of 2010.