The optimum marital deduction is the smallest amount of marital deduction that, combined with the unified credit, reduces the estate tax at the first spouse’s death to zero. It is not the maximum deduction (which would be the entire estate), but the minimum amount needed to eliminate tax. The difference between these two concepts drives the structure of most estate plans for married couples.
Claiming the maximum marital deduction wastes the first spouse’s applicable exclusion amount. Claiming the optimum deduction preserves it. The goal is to shelter as much as possible with the first spouse’s exemption while deferring tax on the rest through the marital deduction.
How the Optimum Marital Deduction Works
The optimum marital deduction divides the estate into two shares at the first death. The first share, equal to the decedent’s remaining applicable exclusion amount ($15 million in 2026), passes to a credit shelter trust where it is sheltered from estate tax by the unified credit of $5,945,800. The second share, equal to the balance, passes to the surviving spouse (or a marital trust) and qualifies for the marital deduction.
The result is zero estate tax at the first death. The credit shelter trust assets are not taxed because the unified credit absorbs the entire tax liability on that share. The marital trust assets are not taxed because the marital deduction offsets the tax on that share. At the surviving spouse’s subsequent death, the marital trust property is taxed in the surviving spouse’s estate, but the credit shelter trust property passes to the next generation without further estate tax.
Why the Formula Approach Is Essential
Estate planning documents cannot specify a fixed dollar amount for the marital deduction. The exact amount that produces zero tax depends on variables that cannot be known when the documents are drafted: the size of the estate at death, the applicable exclusion amount in effect at death, allowable deductions under IRC §§ 2053 and 2054, any prior taxable gifts, and the interaction between these components and the rate structure.
Instead, estate planners use a formula bequest. The standard formula instructs the personal representative to give the surviving spouse “the smallest amount that, if allowed as a federal estate tax marital deduction, would result in the least possible federal estate tax being payable by reason of my death.” The personal representative then computes the amount after death, when all the variables are known.
The computation requires the personal representative to determine the gross estate, subtract all non-marital deductions (debts, administration expenses, charitable gifts), subtract the applicable exclusion amount, and calculate the marital deduction needed to reduce the remaining tax to zero. The formula must also account for state death taxes, prior adjusted taxable gifts, and the interaction between these components.
For details on how these formulas are expressed in estate planning documents, see our explanation of how funding formula clauses divide estates between trusts.
The Circular Computation Problem
Computing the optimum marital deduction involves a circular calculation: the deduction amount depends on the tax amount, and the tax amount depends on the deduction. Estate planners resolve this through what practitioners call the “fudge formula,” an iterative approach that accounts for the interaction between deduction, tax, and available credits.
The formula must also address the IRC § 642(g) “swing item” election. Administration expenses can be deducted on the estate tax return (Form 706) or the estate’s income tax return (Form 1041), but not both. The personal representative’s choice affects the taxable estate size, which in turn affects the optimum marital deduction. Some formulas explicitly reference this election; others leave it to the personal representative’s discretion.
Two Directions for the Formula Bequest
The formula bequest can be expressed in two directions, and the choice has significant consequences for funding:
- Marital formula bequest (traditional). The marital trust receives the formula-determined amount (the optimum marital deduction). The credit shelter trust receives the residue. This is the traditional approach, but it means the larger bequest (the marital share) is the pecuniary amount, which can create administrative complexity during funding.
- Nonmarital formula bequest (reverse). The credit shelter trust receives a formula-determined amount equal to the applicable exclusion. The marital trust receives the residue. This “reverse” or “credit-consuming” approach applies the formula to the smaller bequest, reducing funding complexity. It has become increasingly popular among practitioners.
Both approaches produce the same optimum marital deduction. The difference lies in which trust receives the fixed amount and which receives the fluctuating residue, with downstream implications for gain or loss recognition, income tax, and administrative burden.
What the Optimum Marital Deduction Does Not Account For
The optimum marital deduction calculation focuses exclusively on federal estate tax at the first death. It does not address several related factors that may matter to the family:
- State estate taxes. Many states impose their own estate tax with lower exemption thresholds than the federal exclusion. The formula may require adjustment to account for state tax, particularly where the state exemption is significantly lower than the federal exclusion.
- Income tax basis. Property in the marital trust receives a stepped-up basis at both the first death (under IRC § 1014) and the second death (because it is included in the surviving spouse’s estate). Property in the credit shelter trust receives a stepped-up basis only at the first death. For highly appreciated assets, this difference can make the marital trust a better home for assets with significant unrealized gains.
- GST exemption allocation. The generation-skipping transfer tax exemption ($15 million in 2026) must be coordinated with the estate plan structure. Currently, the GST exemption equals the applicable exclusion, so the credit shelter trust can be fully sheltered from both estate tax and GST tax. If Congress decouples these amounts, the allocation strategy becomes more complex.
Estate planners who focus only on the federal estate tax computation at the first death may produce a suboptimal result when these additional factors are considered.
When the Optimum Marital Deduction Is Not the Best Strategy
For very large estates, the optimum marital deduction (which produces zero tax at the first death) may not minimize the combined taxes across both deaths. An “equalizer” approach, which deliberately pays some tax at the first death to reduce the total tax across both estates, can produce better results. The equalizer is most effective when both spouses die within a relatively short period.
For an analysis of when this alternative strategy applies, see our discussion of equalizer planning for larger estates.
Portability: An Alternative That Has Limits
Congress created portability of the deceased spouse’s unused exclusion (DSUE) in 2010, made permanent in 2013, under IRC § 2010(c)(4). Portability allows the surviving spouse to use the first spouse’s unused exclusion amount without creating a credit shelter trust. For smaller estates, portability can simplify planning by eliminating the need for a two-trust structure.
Portability has four significant limitations. First, it does not apply to the GST exemption, so families with multi-generational planning goals still need a credit shelter trust. Second, it requires a timely filed estate tax return for the first spouse’s estate, even if no tax is owed. Third, the DSUE amount is not indexed for inflation, so it loses purchasing power over time. Fourth, the DSUE is lost if the surviving spouse remarries; the surviving spouse retains only the DSUE of the most recent deceased spouse.
For these reasons, the credit shelter trust and the optimum marital deduction remain the preferred approach for most estate planning practitioners, particularly for larger estates where the GST exemption, inflation protection, and asset protection benefits justify the additional complexity.