How to Calculate Estate Tax Portability Between Spouses (With Real Numbers)

What Portability Actually Means

Portability allows the surviving spouse to absorb the deceased spouse's unused federal estate and gift tax exemption. Without portability, that unused exemption evaporates at death. With it, the surviving spouse can shelter up to two full exemption amounts from federal estate tax.

Congress made portability permanent in the American Taxpayer Relief Act of 2012. It applies only to married couples. It does not extend to state estate taxes in most jurisdictions. And it is not automatic. The executor of the deceased spouse's estate must elect portability by filing IRS Form 706 even if no estate tax is owed.

The DSUE Formula

The Deceased Spousal Unused Exclusion (DSUE) amount equals the deceased spouse's applicable exclusion amount minus the sum of the taxable gifts made during the deceased spouse's lifetime and the taxable estate reported at death.

Written as plain text:

DSUE = Applicable Exclusion Amount - (Taxable Lifetime Gifts + Taxable Estate at Death)

The surviving spouse then adds the DSUE to their own applicable exclusion amount. The combined figure is the surviving spouse's total exemption ceiling for future transfers.

Surviving Spouse Total Exemption = Own Applicable Exclusion + DSUE

Worked Example 1: Clean Estate, Large DSUE

Margaret dies in 2026. Her gross estate is $4,200,000. She made no taxable lifetime gifts. Her executor files IRS Form 706 and reports a taxable estate of $4,200,000 after applicable deductions.

The 2026 applicable exclusion amount is $13,990,000.

DSUE = $13,990,000 - ($0 + $4,200,000) = $9,790,000

Her surviving spouse, David, has his own $13,990,000 exemption. He adds Margaret's DSUE.

David's Total Exemption = $13,990,000 + $9,790,000 = $23,780,000

David now controls $23,780,000 in federal estate tax exemption. If David's estate at his death is worth $21,000,000, his estate owes zero federal estate tax. Without the DSUE election, his taxable estate would have exceeded his $13,990,000 individual exemption by $7,010,000, generating a federal estate tax bill of $2,804,000 at the 40% rate.

Worked Example 2: Deceased Spouse Had Prior Taxable Gifts

Robert dies in 2026 with a gross estate of $2,800,000. During his lifetime, he made taxable gifts totaling $3,500,000 that were reported on IRS Form 709 and applied against his lifetime exemption.

DSUE = $13,990,000 - ($3,500,000 + $2,800,000) = $7,690,000

His surviving spouse, Catherine, adds this amount to her own exemption.

Catherine's Total Exemption = $13,990,000 + $7,690,000 = $21,680,000

The calculation is straightforward, but the prior gift history matters. The IRS requires the executor to account for all prior taxable gifts made by the deceased spouse when computing the DSUE. Omitting a $3,500,000 gift history from Form 706 overstates the DSUE and creates exposure to IRS audit and penalties.

The 9-Month Filing Deadline and the Late Election Option

The executor must file Form 706 within 9 months of the date of death to elect portability. A 6-month automatic extension is available via IRS Form 4768, extending the deadline to 15 months.

If the deadline passes, the surviving spouse is not always permanently locked out. IRS Revenue Procedure 2022-32 allows estates not otherwise required to file Form 706 to make a late portability election up to 5 years after the date of death. The estate must qualify: the decedent must be a U.S. citizen or resident, and the estate must not have been required to file a federal estate tax return based on its value.

Beyond 5 years, the DSUE is gone with no administrative remedy available. The surviving spouse would need to pursue a private letter ruling at significant legal cost, with no guarantee of success.

How the DSUE Interacts With Future Gifts

The surviving spouse can use the DSUE for both gift tax and estate tax purposes. If Catherine from Worked Example 2 decides to make a $5,000,000 gift to her children in 2027, her combined exemption of $21,680,000 absorbs it entirely.

One important restriction: the DSUE is fixed at the dollar amount determined on the deceased spouse's Form 706. If Congress increases the exemption after the first spouse's death, the surviving spouse's own exemption may rise, but the DSUE does not adjust upward. Conversely, if the exemption decreases, the DSUE locks in at its original computed amount.

If the surviving spouse remarries and that second spouse also predeceases them, only the most recently deceased spouse's DSUE applies. A surviving spouse cannot stack DSUE amounts from multiple deceased spouses.

State Estate Tax Does Not Follow Federal Portability Rules

Twelve states and the District of Columbia impose their own estate taxes, and most do not recognize federal portability. Massachusetts, for example, imposes estate tax on estates above $2,000,000 with no portability provision. A surviving spouse in Massachusetts with a combined estate of $10,000,000 may owe zero federal estate tax after capturing the DSUE, while still facing a six-figure state estate tax bill.

Estates with significant assets in states with separate estate taxes require separate planning, often involving a credit shelter trust (also called a bypass trust) to shelter assets from state tax at the first spouse's death.

What to Do With This Calculation

Run the DSUE formula on the deceased spouse's actual numbers before making any assumption about the surviving spouse's exemption. Pull the gross estate, deduct allowable deductions under IRC Section 2053 and Section 2054, factor in prior taxable gifts from Form 709 filings, and compute the residual DSUE.

Then add that figure to the surviving spouse's applicable exclusion amount and compare the total against the projected size of the surviving spouse's estate. If that projected estate exceeds the combined exemption, additional planning tools, including irrevocable trusts, annual gift exclusions ($18,000 per recipient in 2026), and qualified opportunity zone investments, come into consideration.

The CalcMoney estate tax calculator lets you input the deceased spouse's taxable estate, prior gift history, and surviving spouse's projected estate value to produce an estimated federal estate tax liability under both the with-portability and without-portability scenarios. Use the calculator to quantify the dollar value of the DSUE before your estate attorney bills another hour.

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Results are estimates for informational purposes only. Consult a licensed financial professional before making financial decisions.