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How to Calculate GRAT vs. IDGT Savings on a $17 Million Estate in September 2026: The Step-Up Basis Trade-Off Formula

The $95 fee people optimize vs. the $3 million question they don't

This week Navy Federal rolled out its new Flagship Premier Visa — a $95-annual-fee metal card with travel perks that, according to NerdWallet, can offset the fee if you use them right. If you're the kind of person eligible for that card, you probably spent a genuine hour comparing lounge access against the annual fee before deciding.

Now here's the uncomfortable part: a lot of people who run that math on a $95 card have never run the equivalent math on the far bigger number sitting in their estate. If you're holding a concentrated, low-basis position — company stock, an early equity grant, real estate bought decades ago — the difference between doing nothing and structuring a transfer correctly can be worth more than the entire cost of the credit card fee, repeated 30,000 times over.

This post walks through that math on a specific, worked example: a $17 million estate with a $5 million concentrated stock position. Your numbers will be different — different basis, different growth assumption, different state, different family structure — but the formula is the same, and you can run your own version at Voritanel.

Why September 2026's rate environment matters right now

The IRS 7520 rate — the "hurdle rate" that governs GRAT and CRT actuarial math — is derived from mid-term Treasury yields, and those yields have been moving. On September 9, 2026, NerdWallet reported mortgage rates ticking higher as markets reacted to escalating conflict in the Middle East. That's the same rate pressure that feeds into the 7520 rate calculation.

Meanwhile, the underlying economy is still running hot enough to matter: the Bureau of Labor Statistics reported August 2026 unemployment at 4.1%, payroll growth of +162,000, and July CPI up just 0.1% month-over-month. Translation: inflation is cooling, but the labor market hasn't cracked, which is exactly the kind of backdrop that keeps mid-term rates elevated rather than falling fast. If you've read how falling rates shifted GRAT vs. IDGT math by $85K+ on a $10M estate earlier this year, this is the mirror image — rates drifting up, not down.

For the walkthrough below, I'm using a September 2026 IRS 7520 rate of 4.6% as a labeled example assumption, consistent with the 4.5–4.8% range these posts have tracked through 2026, and a mid-term AFR of 4.4% for IDGT note pricing (the AFR curve typically sits slightly below the 7520 rate). Both change monthly — always confirm the actual published rate before finalizing anything.

The scenario

An individual with a $17 million estate holds $5 million of it in a single concentrated stock position with a $500,000 cost basis — a classic low-basis, high-growth asset. Under 2026 law, the federal estate tax exemption sits at roughly $15 million per person. That means $2 million of this estate is already exposed to the 40% federal rate today — an $800,000 tax bill if nothing changes.

But the real risk isn't today's $2 million excess. It's what happens if the $5 million position keeps compounding at, say, 9% annually (a labeled growth assumption, not a guarantee) while sitting inside a taxable estate. Every dollar of future growth on that position gets taxed at 40% at death unless it's moved out first.

Option 1: Do nothing (keep it in the estate, take the step-up)

The upside of doing nothing is real: assets held until death get a step-up in basis, wiping out capital gains tax entirely. If the $5 million position grows to $5M × 1.09⁵ = $7,693,120 over five years and the owner dies holding it, heirs inherit at that value with a fresh basis — $0 capital gains tax if they sell immediately.

But that entire $7,693,120 sits inside the taxable estate. If it's pushing the estate further past the $15 million exemption, the marginal dollar is taxed at 40% — up to $3,077,248 in federal estate tax exposure on this position alone, before even accounting for state estate tax in a non-exempt jurisdiction.

Option 2: A two-year zeroed-out GRAT

A GRAT lets the grantor contribute the $5 million asset, retain annuity payments structured to return the full value plus the 4.6% hurdle rate, and pass anything above that hurdle to heirs gift-tax-free. The approximate wealth transferred over the term:

(1.09)² = 1.1881 (1.046)² = 1.094116 Difference = 0.093984 × $5,000,000 = $469,920 transferred tax-free over two years

That's real money moved out of the estate with essentially no gift tax exposure — the annuity is sized so the taxable gift at funding is close to zero. The trade-off: a GRAT is a short bet. If the grantor dies during the term, most or all of the benefit is clawed back into the estate. It also doesn't shield future generations from generation-skipping transfer (GST) tax, since GST exemption generally can't be reliably allocated to a GRAT until the term ends.

Option 3: A five-year installment sale to an IDGT

An intentionally defective grantor trust (IDGT) works differently: the grantor sells the $5 million position to the trust in exchange for a note priced at the 4.4% AFR. Any growth above that rate accrues entirely to the trust, outside the estate, for the full sale term — not just a two-year window.

(1.09)⁵ = 1.538624 (1.044)⁵ = 1.240215 Difference = 0.298409 × $5,000,000 = $1,492,045 transferred tax-free over five years

That's more than triple the GRAT's five-year-equivalent transfer, mainly because the sale isn't capped by an annuity structure and there's no mortality clawback risk. The costs: a seed gift (typically ~10% of the asset value, or $500,000, using part of the lifetime exemption) to validate the trust as a real economic entity, ongoing income tax paid by the grantor on trust earnings (a feature, not a bug — it's an additional tax-free gift to heirs), and less liquidity flexibility than a GRAT.

This is the kind of comparison Voritanel runs automatically against your actual basis, growth assumption, and published monthly rate — instead of you rebuilding this spreadsheet by hand every time the 7520 rate moves.

The step-up basis trade-off nobody puts in the same table

Here's the number that changes the decision. The IDGT trust holds the full $7,693,120 in appreciated stock at the end of five years, but its basis is the carryover basis of $500,000 — no step-up, because it was sold out of the estate, not inherited.

If the trust (or its beneficiaries) eventually sells:

Capital gain = $7,693,120 − $500,000 = $7,193,120 Tax at 23.8% (20% long-term capital gains + 3.8% NIIT) = $1,711,963

Compare the two end states on this single asset:

PathTax exposureWhat triggers it
Kept in estate, stepped-up basis$3,077,248 (40% federal estate tax)Death, if pushing past the exemption
Sold to IDGT, carryover basis$1,711,963 (23.8% capital gains, only if/when sold)A future sale, deferrable indefinitely

Even after losing the step-up, the IDGT path saves roughly $1.37 million on this position alone — because 40% applied to the full value beats 23.8% applied only to the gain, especially on a low-basis asset. This is precisely the calculation covered in more depth in what a $12M estate really costs in federal tax, state tax, and hidden wealth transfer traps — the step-up is a real benefit, but it's not automatically the bigger number.

Where GST tax and charitable intent change the answer

If the ultimate beneficiaries are grandchildren, the IDGT sale can be paired with GST exemption allocation (also roughly $15 million per person in 2026, since the exemptions are unified), shielding the trust's growth from an additional 40% GST tax layer across generations — a benefit that compounds well past the five-year window modeled here.

If there's charitable intent instead, a charitable remainder trust is worth modeling as a third path: it lets the grantor sell the appreciated stock inside a tax-exempt vehicle with no immediate capital gains hit, take an income tax deduction now, and receive an income stream — at the cost of the remainder ultimately going to charity rather than heirs. GRAT vs. charitable remainder trust math on a Massachusetts estate walks through that comparison in detail, including how state-level estate tax exemptions (often far below the federal $15 million) change the calculus.

What if the cash just sits in a savings account instead?

One more comparison worth running: what if the $5 million simply sits in cash earning a competitive but not table-topping yield — the kind NerdWallet describes for American Express's savings account — rather than being deployed into a GRAT or IDGT at all? Using an illustrative 4.00% APY, taxed annually at a roughly 41% marginal rate, the after-tax growth rate falls to about 2.37%. Over five years that's $5M × 1.02368⁵ ≈ $5.62 million — still fully inside the taxable estate, still exposed to the full 40% marginal rate, with none of the appreciation ever leaving. It's the lowest-effort option and, on this math, the most expensive one.

Run your own numbers

Every input here — the 7520 rate, the growth assumption, the cost basis, the state of residence, whether GST or charitable goals apply — is a variable, not a constant. Change the growth rate from 9% to 6% and the GRAT/IDGT advantage shrinks fast. Change the state to one with its own estate tax and the "do nothing" column gets worse. You can model this for your specific situation at Voritanel rather than approximating it with a generic rule of thumb — the same way you'd actually compare a card's perks against its $95 fee instead of guessing.

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