Phase-Outs in a Big Sale Year: The Hidden Taxes on a Capital Gain

Why the rate table understates the sale-year bill

The headline federal rates on long-term gain are 0%, 15% and 20% (Rev. Proc. 2025-32, 2026; see the capital gains guide). Those rates are honest about the gain itself. What they leave out is that the gain also lands in adjusted gross income (AGI), and dozens of provisions use AGI, or a modified version of it (MAGI), as an on and off switch. A sale year flips many switches at once.

Our first two examples isolate the effect. A 60-year-old couple with $90,000 of other income owes $62,955 on a $400,000 gain. A couple over 65 with the same income plus $48,000 of Social Security owes $71,570 on the same gain, $8,615 more, because the gain also makes more of their benefits taxable and erases their senior deduction. None of that shows up in the 15% rate, and the Medicare surcharge two years later comes on top.

Medicare IRMAA: the bill that arrives two years later

Medicare Part B and Part D premiums include an income-related monthly adjustment amount (IRMAA) based on MAGI, which is AGI plus tax-exempt interest, from the tax return two years earlier (20 CFR 418.1135). That means a 2026 sale sets 2028 premiums, and 2024 income set 2026 premiums.

On the 2026 table from CMS, joint filers above $218,000 of MAGI start paying surcharges, and the top tier, at $750,000 or more, raises Part B to $689.90 a month instead of the standard $202.90, plus a Part D surcharge of $91.00 a month, per person (CMS 2026 fact sheet). For a married couple both on Medicare, the top tier costs about $13,900 more for that year than the standard premiums. The tiers are cliffs: one dollar over a threshold triggers the full step.

A sale is not one of the life-changing events in 20 CFR 418.1205 that let you ask the Social Security Administration to use a more recent year. Retirement (work stoppage) is, but the gain stays in the income figure. Spreading the gain with an installment sale can keep each year below a tier; the year-end timing page covers moving the closing date.

Social Security taxation and the disappearing 0% bracket

Up to 85% of Social Security benefits become taxable once provisional income (AGI, tax-exempt interest and half of benefits) passes $44,000 on a joint return or $34,000 for single filers (IRC 86; IRS Publication 915, 2025). Those thresholds were set in 1983 and 1993 and are not indexed.

For a modest seller this matters more than the gain rate. Our retired couple with $40,000 of IRA income and $50,000 of benefits owes $8,537 on a $100,000 gain, an effective 8.5%. The same $100,000 gain costs a couple without benefits only $1,335, or 1.3%. Much of the gain would sit in the 0% band, which ends at $98,900 of taxable income for joint filers (Rev. Proc. 2025-32, 2026), but the gain pulls more benefits into taxable income, and that income fills the 0% band and pushes gain into the 15% band. If you plan to harvest gains at 0%, check the 0% capital gains harvesting analysis with your benefits included.

The $6,000 senior deduction phase-out

The 2025 budget law, P.L. 119-21, added a deduction of $6,000 for each taxpayer aged 65 or older for tax years 2025 through 2028 (IRC 151(d)(5)(C)). It shrinks by 6% of MAGI above $75,000, or $150,000 on a joint return, so each spouse's $6,000 is gone at $250,000 of joint MAGI and at $175,000 for a single filer. Married couples must file jointly to claim it.

In dollars the loss is modest, up to $12,000 of deduction for a couple, worth $2,640 to $4,440 at the 22% to 37% brackets (2026). It still belongs in the sale-year math because it stacks on the Social Security effect at the same income levels. A sale in 2029 or later avoids it only because the deduction expires after 2028.

SALT cap phase-down and itemized deductions

The state and local tax deduction cap rose to $40,000 for 2025 and $40,400 for 2026, but it is reduced by 30% of MAGI above $500,000 for 2025 and $505,000 for 2026, never below $10,000 (IRC 164(b)(7)). The thresholds grow 1% a year through 2029, and the cap reverts to $10,000 for 2030. A $1 million gain in a high-tax state therefore costs most of the higher cap in exactly the year your state income tax bill is largest.

Two other 2026 changes reduce itemized deductions in a big year: charitable gifts count only above 0.5% of AGI (IRS Publication 505, 2026), and for taxpayers in the 37% bracket itemized deductions are valued at a maximum 35% rate (IRC 68 as amended by P.L. 119-21). In our New Jersey example, the itemizing couple owes $512,630 on the sale, $6,383 more than the same sale computed on the standard deduction, because the gain strips away deductions they used in a normal year. A large gift of appreciated property, covered on the donor-advised fund page, interacts with these limits too.

AMT returns for large sellers

Long-term gain keeps its 15% and 20% rates inside the alternative minimum tax, so the gain itself is not taxed more. The problem is the exemption. For 2026 the AMT exemption is $140,200 on a joint return, and P.L. 119-21 reset the phase-out to start at $1,000,000 of alternative minimum taxable income ($500,000 for single filers) and doubled its speed to 50 cents per dollar, so it is gone at $1,280,400 joint (IRC 55(d)(4); Rev. Proc. 2025-32, 2026). The gain counts toward that income, so it removes the exemption and exposes your salary to the 26% and 28% AMT rates.

In our California example with $600,000 of salary, the $2,000,000 gain causes $26,842 of AMT on top of regular tax, part of a total sale-year bill of $748,886. The New Jersey couple shows $27,642 of AMT caused by the sale. Owners with large incentive stock options or private activity bond interest are most exposed.

ACA premium credits and other cliffs

Early retirees buying Marketplace coverage feel the sharpest cliff. For 2026 the premium tax credit applies only up to 400% of the federal poverty line, with a top required contribution of 9.96% of income (IRC 36B; Rev. Proc. 2025-25). Using the 2025 HHS guideline of $21,150 for a household of two, that line is $84,600, and one dollar over it can end the whole credit for the year. A sale year for a couple under 65 can cost thousands in lost premium help before any income tax.

Other switches keyed to income include the net investment income tax at $250,000 joint MAGI (IRC 1411, not indexed; see the net investment income tax page), the $25,000 rental loss allowance that phases out between $100,000 and $150,000 of AGI (IRC 469(i)), and college aid formulas that look back at prior-year income.

Managing the switches

Three levers move MAGI between years. Spreading the gain over several years with seller financing keeps each year's income lower. Moving the closing date shifts which premium year IRMAA hits. Roth conversions are best moved out of the sale year, since a conversion stacks on the same switches; the Roth conversion in a sale year page covers that trade-off. The right mix depends on which cliffs you sit near, which is a modeling question, not a rule of thumb. Get the Big Sale Tax Analysis to see each cliff mapped against each exit path.

Run your own numbers

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