Kiplinger has reported that the Treasury Department is studying whether it can index the cost basis of capital gains to inflation through regulation, without an act of Congress. For a retiree liquidating a long-held taxable account, the dollar impact is large enough to change retirement planning.
Model the case the Kiplinger piece centers on: a $500,000 portfolio with an average cost basis established around 2015, average annual CPI of 3%, and a full liquidation. Under current law, the entire nominal gain is taxed at long-term capital gains rates: 15% for most retirees, 20% plus the 3.8% net investment income tax for higher-income sellers. Indexing replaces the original purchase price with a basis grown by cumulative inflation, shrinking the taxable gain.
What the math looks like
A position bought in 2015 has roughly an 11-year inflation tail by 2026. At 3% compounded, a sizeable share of the nominal gain is purchasing power that never compounded in real terms. On the $500,000 liquidation, the difference between the unindexed and indexed tax bill runs into the tens of thousands of dollars, depending on the original basis and the seller’s bracket. The savings scale directly with holding period and gain size.
Inflation is still doing the damage in real time. CPI registered 333.020 in April 2026, up from 325.252 in January 2026 and 308.417 in January 2024. Headline PCE ran at 3.5% year-over-year in March 2026, with core PCE at 3.2%. Every month a long-held position sits taxable, more of its “gain” is phantom.
Context Kiplinger does not provide
Indexing benefits one group disproportionately: investors with large unrealized gains and long holding periods. Retirees holding appreciated index funds, founders selling a 25-year-old business, landlords exiting rentals bought in the early 2000s. Younger investors with shorter horizons see modest relief.
Durability matters too. A Treasury regulation can be reversed by a future Treasury. Basis adjustments already claimed are unlikely to be clawed back, but the mechanism could disappear before a new long-term position matures.
How to act on it
Deferring realizations carries real option value where the rule is in play. Selling a long-held position in 2026, before any regulation is finalized, locks in the unindexed tax bill and forfeits the optionality.
For inherited assets, indexing changes little. The step-up at death already zeros out embedded gains, so heirs gain nothing extra. Owners who can hold to death still win on that score.
For new contributions, indexing narrows the after-tax penalty on taxable brokerage accounts versus Roth. Roth still wins on tax-free growth and no required distributions, but the gap closes once inflation no longer counts as taxable income.
The takeaway
If Treasury moves, a $500,000 liquidation becomes a tens-of-thousands-of-dollars decision based on timing alone. For investors with long-held positions, the indexed basis is worth modeling before any 2026 sale, and the rulemaking docket is worth tracking.
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