The Senior Deduction Can Raise the Real Tax Rate on a Roth Conversion

The 2025 tax law (the One Big Beautiful Bill Act) created an extra deduction of up to $6,000 for each eligible individual who is 65 or older by the end of the tax year. It applies to tax years 2025 through 2028 and then expires unless Congress extends it. It is claimed whether you itemize or take the standard deduction, it requires a valid Social Security number, and married couples must file jointly to claim it.

It is a third layer of deduction. For 2026 a single filer starts with a $16,100 standard deduction, adds $2,050 for being 65 or older, and can then add the senior deduction. That is up to $24,150 before any itemized expense. It is a deduction from income, not a credit, so its value is the deduction times your bracket rate.

It is claimed on Schedule 1-A of Form 1040 and it reduces taxable income, not adjusted gross income. That detail matters later: because adjusted gross income is unchanged, the deduction does not lower the income measure Medicare uses for IRMAA, and it does not reduce the amount used to decide how much of your Social Security is taxable.

The deduction is reduced by 6% of your modified adjusted gross income above $75,000 for a single filer or $150,000 for married filing jointly. For this purpose modified adjusted gross income is adjusted gross income with a few add-backs that rarely apply to retirees, such as certain foreign income exclusions. Tax-exempt interest is not added back for this test, unlike IRMAA. Confirm the definition on the Schedule 1-A instructions for your return.

For a single filer the math is simple. The deduction is $6,000 up to $75,000, falls by $60 for every $1,000 above that, and is gone at $175,000, because $100,000 of excess income times 6% equals $6,000. A Roth conversion counts as income for this test, so it is exactly the kind of income that pushes you into the range.

Married couples need more care. With one qualifying spouse on a joint return, the same 6% rule applies over a $100,000 range, from $150,000 to $250,000 of income. When both spouses qualify, the reduction applies to each spouse's $6,000 separately, so each dollar of income above $150,000 removes 12 cents of deduction and the full $12,000 is gone at $250,000, the same endpoint as with one qualifying spouse. Married filing separately cannot claim the deduction. The example below uses a single filer; couples should run their numbers on the Schedule 1-A worksheet.

Inside the phase-out range, each extra $1 of income does two things. It is taxed at your bracket rate, and it also removes 6 cents of deduction, which makes another 6 cents of income taxable. So $1 of conversion adds $1.06 of taxable income. At a 22% bracket that is 22% times 1.06, or 23.32%. At 24% it is 25.44%. For a married couple where both spouses qualify, each $1 removes 12 cents of deduction, so $1 of conversion adds $1.12 of taxable income: 22% acts like about 24.64% and 24% like about 26.88%. A filer who is still inside the phase-out range is already past the 12% bracket, so the 12% bracket rarely applies here.

This is not a new tax. It is the deduction you were already counting on being lost at a rate of 6 cents per dollar. The total extra cost is also capped, because there is only $6,000 of deduction to lose per person. At a 22% to 24% bracket that is roughly $1,300 to $1,440 of extra tax in total, and once the deduction is gone the multiplier disappears.

The multiplier only exists while some deduction is still left to lose. Below $75,000 of income (single), conversions are taxed at the plain bracket rate. Above $175,000 (single), the deduction is gone and the plain bracket rate applies again. The bump zone is the range in between.

• 22% bracket in the range: 23.32% effective (single, or a couple with one qualifying spouse).
• 24% bracket in the range: 25.44% effective (single, or a couple with one qualifying spouse).
• Married couple with both spouses qualifying: about 24.64% at 22% and 26.88% at 24%.
• Outside the range: the bracket rate, with no multiplier.

The phase-out is not the only line a conversion can cross. For a single filer in 2026, the first Medicare IRMAA tier starts above $109,000 of modified adjusted gross income, and the first married filing jointly tier starts above $218,000. Per the 2026 schedule, the first tier adds $81.20 a month to Part B and $14.50 a month to Part D, or $1,148.40 a year per person. It is a cliff: one dollar over costs the whole tier.

Medicare looks back two years. Income you create in 2026 is read when setting 2028 premiums, and the 2028 dollar thresholds are not published yet. Treat the 2026 figure as a stand-in and leave a cushion. Because the senior deduction does not lower adjusted gross income, it does not help you stay under an IRMAA line.

Social Security adds a third layer. If you are collecting benefits, a conversion raises the income used to decide how much of your benefit is taxable, up to 85%. The thresholds are not indexed ($25,000 and $34,000 single, $32,000 and $44,000 married filing jointly). That interaction can create its own bump and depends on your benefit amount, so the example below assumes benefits have not started. Arizona's 2026 treatment of this deduction should be confirmed against the 2026 Form 140 instructions before relying on it.

Start with your income before any conversion, then mark three lines on it: where the phase-out starts, where your current bracket ends, and where the first IRMAA tier begins. In the example below, income before conversion is $70,000, so the phase-out starts after $5,000 of conversion, the 24% bracket starts at about $126,700 of income, and the 2026 IRMAA line sits at $109,000. Those three numbers, not any single one, set the useful range.

If a conversion would land just over an IRMAA line, the honest comparison is between converting a little less this year and the extra tax plus surcharge of going over. In the example, the last $1,000 over the line costs about $1,382 once the first tier is counted. Trimming the conversion to the line and carrying the rest to next year is often cheaper, as long as next year is not worse for another reason.

The bump zone can also argue for converting more, not less. Once income is above $175,000 (single) the deduction is gone and the multiplier ends, so a very large conversion year pays the plain bracket rate on its top dollars. Whether that is better than spreading the conversion depends on the brackets you would hit and on how long the senior deduction is in force. The deduction ends after 2028, so the zone only matters for conversions made in 2026, 2027 and 2028.

Model the whole stack for each year you consider converting: bracket, deduction phase-out, IRMAA line, and Social Security taxation. A conversion sized to the top of a bracket can look fine and still land on a Medicare cliff or in the middle of the bump zone.

Do not build a multi-decade plan on this deduction. It is scheduled to end after 2028. The cost of the bump zone is small relative to a conversion that is otherwise worth doing, so it usually tweaks the amount or timing instead of cancelling the plan. A conversion may still make sense at 23.32% if your expected future rate is higher, and may not make sense at 12% if it is lower. That depends on your whole picture.

A practical habit is to ask for the marginal cost of the next $10,000 at several points, not just the average. The average rate on a conversion hides the bump. In the example, the average across a $105,000 conversion is 23.80%, but the stretch from $75,000 to about $126,700 of income runs at 23.32% and the stretch after it, up to $175,000, at 25.44%. Those are the numbers that tell you where to stop, and they change every year as brackets, income and the deduction itself change.

Other options include converting in a year before 65, spreading conversions over several years, or using charitable giving to lower income in a heavy year. Each has tradeoffs, and which one fits depends on the household. This page is educational and illustrative, not individualized tax advice.