Roth Conversion Tax Implications

How Federal Income Tax Works on Roth Conversions

A Roth conversion is treated as ordinary income by the IRS. The converted dollars are stacked on top of your existing income for the year, and you calculate tax as if you had earned that money. There's no special conversion tax rate—it's your marginal rate in whatever bracket the conversion pushes you into.

This is crucial because it means the tax cost of a conversion is not fixed. A $50,000 conversion might cost you $11,000 in federal tax (22% bracket) or $20,500 (41% bracket), depending on your total income. You need to know your exact income situation before converting.

Bracket filling example: You're single with $80,000 in wages. After the 2026 $16,100 standard deduction, your pre-conversion taxable income is $63,900 (already inside the 22% bracket). If you convert $30,000, your taxable income becomes $93,900 — still below the single 22% ceiling of $105,700 (per Rev. Proc. 2025-32). The entire $30,000 conversion sits in the 22% bracket, costing $6,600 in federal tax. Bracket planning matters.

State Income Tax on Roth Conversions

Nine states impose no state income tax: Alaska, Florida, Nevada, New Hampshire, South Dakota, Tennessee, Texas, Washington, and Wyoming. If you live in one of these states, you don't owe state tax on your conversion.

For everyone else, state taxes add significantly. California adds 9.3%, while other states offer special treatment. Illinois exempts qualified retirement plan distributions — including Roth conversions — from state income tax under 35 ILCS 5/203. Minnesota does not offer a general retirement-income deduction; IRA distributions and Roth conversions are fully MN-taxable (up to 9.85%). Narrow subtractions exist for Social Security benefits (AGI-phased) and for certain public pensions of retirees who didn't earn Social Security credits, but these do NOT cover IRA/Roth-conversion income. State location can make conversions 9% more expensive or cheaper depending on where you live.

Form 8606: How to Report Your Conversion

Roth conversions are reported on Form 8606, filed with your Form 1040. Part I handles conversions, walking through the pro-rata rule calculation. If you converted $50,000 and had $10,000 in non-deductible basis, the form calculates that $10,000 is non-taxable and $40,000 is taxable. Part IV creates a permanent record of your conversion basis for future reference when you withdraw from the Roth.

IRMAA Impact: The 2-Year Lookback

One of the most overlooked conversion impacts is Medicare's IRMAA (Income-Related Monthly Adjustment Amount). A 2026 conversion increases your 2026 income, which becomes the lookback for your 2028 Medicare premiums, triggering surcharges that can last 20+ years.

In 2026, IRMAA Tier 1 thresholds (using 2024 MAGI under the two-year lookback) are $109,001 (single) and $218,001 (married filing jointly). A $100,000 conversion could push a married couple past $218,000, triggering IRMAA Part B and Part D surcharges whose amount depends on which tier you cross (see CMS's 2026 fact sheet). This makes IRMAA avoidance a powerful planning tool for those age 55-65 (retired but not yet on Medicare).

ACA Marketplace Subsidy Impact (Early Retirees)

For early retirees purchasing ACA marketplace insurance before age 65, conversions directly reduce premium tax credits. The ACA subsidy is based on modified adjusted gross income (MAGI). A $40,000 conversion could reduce your subsidy by $5,000–$10,000 for that year alone. Early retirees should carefully time conversions during low-subsidy years.

The Inflation Reduction Act's "400% FPL cliff elimination" was extended through 2025 and remains in place for 2026 under current law, but if it lapses after 2026, a single dollar of conversion income that pushes you from 399% FPL to 401% FPL can cause you to lose all of your premium tax credit—a cliff that can cost $15,000+ for a family of four. Modeling conversions around FPL thresholds (rather than marginal brackets alone) is critical for anyone 50–64 who isn't yet Medicare-eligible.

The Social Security Tax Torpedo: Conversions and the Hidden 45.7% Bracket

Social Security's taxability is governed by "provisional income" (also called "combined income") under IRC §86: half of your SS benefit plus all other taxable income plus tax-exempt interest. If provisional income exceeds $25,000 (single) or $32,000 (married), up to 50% of benefits become taxable; above $34,000/$44,000, up to 85% become taxable. These thresholds have not been adjusted for inflation since 1983 and are not scheduled to be.

Because a Roth conversion adds dollar-for-dollar to provisional income, it can simultaneously (a) push you into a higher federal bracket, (b) pull an additional 50¢ or 85¢ of Social Security into taxation per dollar of conversion, and (c) trigger IRMAA. The result is a "tax torpedo" where a retiree in the nominal 22% bracket is actually paying 22% × 1.85 = 40.7% marginal on conversion dollars—and if state tax of 5% applies, the true marginal rate reaches 45.7% (40.7% federal marginal + 5% state; if the state also taxes the SS pickup, the true marginal reaches ~50%). This is why the gap between retirement and SS claiming (often ages 62–70) is the premier conversion window: SS hasn't started, so there's no torpedo.

The inverse planning insight: If you delay Social Security to age 70 and convert aggressively between ages 62 and 70, you compress the torpedo. Once SS begins at 70, every post-70 conversion dollar hits the torpedo, so large late-career conversions are punishingly expensive. Modeling the tax torpedo is where the real multi-decade alpha of Roth conversions is captured—and is absent from virtually all generic conversion calculators.

Net Investment Income Tax (NIIT) 3.8%: Indirect Exposure

Under IRC §1411, investment income (dividends, interest, capital gains, rental income, annuity income) is hit with an additional 3.8% tax when modified adjusted gross income exceeds $200,000 (single) or $250,000 (married filing jointly). Roth conversions themselves are not subject to NIIT—retirement distributions are expressly excluded under §1411(c)(1)(A). But conversions raise MAGI, which can pull other investment income above the threshold.

Worked example: A couple earns $220,000 in wages and has $50,000 in dividends. Their MAGI is $270,000 before conversion—$20,000 over the NIIT threshold, so $20,000 of investment income is hit with NIIT ($760). They convert $100,000. MAGI becomes $370,000. Now the full $50,000 of investment income is subject to NIIT, raising the bill to $1,900. The conversion itself wasn't taxed the 3.8%, but it triggered an additional $1,140 in NIIT on existing investments—a hidden 1.14% "leak" on the conversion. NIIT thresholds, like provisional income, are not indexed to inflation, so their reach expands every year.

State Tax Mismatch: Where You Convert Matters as Much as When

Federal treats all conversions as ordinary income, but states treat retirement income wildly differently. A quick anatomy:

States That Don't Tax Roth Conversions

Beyond the nine states with no income tax at all, several states provide meaningful (but not uniform) relief for retirement-plan conversions. Illinois exempts qualified retirement plan distributions including Roth conversions under 35 ILCS 5/203. Mississippi exempts qualified retirement income once the recipient has separated from service and reached qualifying age. Pennsylvania does NOT tax traditional-to-Roth conversions at any age — the PA Department of Revenue (REV-636) treats a fully-rolled conversion as non-taxable. Only federal withholding not redeposited in the Roth becomes PA-taxable. Pre-59½ earnings distributions follow PA's cost-recovery method separately. Iowa (as of 2023) eliminated state tax on retirement income for taxpayers 55+. Confirm with a state-licensed CPA before relying on this for your fact pattern.

High-Tax States and the Move-Before-Converting Strategy

California (up to 13.3%), New York (up to 10.9%), New Jersey (up to 10.75%), Oregon (up to 9.9%), and Minnesota (up to 9.85%) levy full state income tax on conversions. On a $500,000 multi-year conversion, a move from California to Nevada or Florida before converting can save $50,000–$66,500 in state tax. The catch: you must establish genuine domicile (driver's license, voter registration, physical presence >183 days, no California home) or California will audit you aggressively under the "temporary absence" doctrine.

The Trailing-State-Tax Trap

Some states (notably California) have historically asserted "source income" claims on retirement distributions earned while the taxpayer was a resident—but this was largely preempted by federal law (4 U.S.C. §114, the "Source Tax" statute of 1996), which prohibits states from taxing retirement income of former residents. However, pre-retirement conversions don't qualify for 4 U.S.C. §114 protection in all fact patterns; deferred-comp and 457 plan conversions have faced state challenges. If your domicile change is close in time to the conversion, a tax attorney review is worth the cost.

New Jersey's "Already Taxed" Phenomenon

New Jersey did not allow deductions for traditional IRA contributions for many years, so NJ residents often have substantial state-level basis in their traditional IRAs that federal pro-rata doesn't acknowledge. When converting, NJ treats a portion of the conversion as "already taxed at the state level" and exempts it. The "IRA Worksheet" in the NJ-1040 instructions tracks basis (Part I for first-year withdrawals; Part II ["Unrecovered Contributions"] for later years), per NJ Division of Taxation GIT-2. Residents who've always filed in NJ often discover they owe substantially less NJ tax on conversions than they expected—but the tracking requires careful record-keeping going back decades.

Paying the Conversion Tax: From Conversion or Outside Funds?

Always pay the conversion tax from outside funds, never from the conversion itself. Here's the math: A $50,000 conversion at 22% tax = $11,000 owed. If you pay from the conversion, only $39,000 grows tax-free in your Roth (at 7% for 30 years = $297,000). If you pay from outside funds, the full $50,000 grows tax-free ($381,000). The difference: $84,000 in lost wealth. For someone converting $200,000 over years, this difference approaches $400,000+.

Estimated Tax Payments

If your conversion creates a large tax liability without withholding, you may owe quarterly estimated tax payments (April 15, June 15, September 15, January 15) to avoid an underpayment penalty. Most people use the safe harbor of paying 100% of their prior year's tax quarterly. Work with a tax professional to calculate the requirement.

The Roth Conversion Sweet Spot

The years between retirement and RMD age (73 for those born 1951–1959, rising to 75 for those born 1960+ per SECURE 2.0 §107) are the cheapest time to convert. Your income is artificially low because you're not yet taking Social Security, you don't have RMDs, and other assets support your lifestyle. A $50,000 conversion at age 58 might cost 22% tax. At age 73+ with RMDs, that same conversion costs 32% + IRMAA surcharges. The gap years are where you capture the tax arbitrage.

Bracket-Filling Strategy

"Bracket-filling" means converting just enough each year to fill your current bracket without moving to the next. Example: Single filer with $30,000 income and the 2026 standard deduction of $16,100 → taxable income $13,900. Room in the 12% bracket: $50,400 − $13,900 = $36,500. Converting $36,500 at 12% = $4,380 federal tax. This lets the taxpayer top off the entire 12% bracket without slipping into 22% territory. This requires knowing your exact income and bracket limits but is a tactical approach that works well for retirees with predictable income.

The Widow's Penalty: Why Converting While Both Spouses Are Alive Matters

Married-filing-jointly brackets are roughly twice as wide as single-filer brackets up through 32%, but the surviving spouse files as single the year after the first spouse's death (with a narrow two-year qualifying-surviving-spouse filing status if there's a dependent child, under IRC §2(a)). Same RMDs. Same pensions. Same Social Security (now reduced to the higher of the two benefits). But the brackets have been cut in half, and the standard deduction has been cut roughly in half. This is the "widow's penalty."

Numerical illustration: A couple with $150,000 of retirement income pays 22% on the top dollars (MFJ 22% bracket runs to $211,400 in 2026). After one spouse dies, the survivor with $110,000 of remaining income (reduced SS and single pension) is now in the 24% bracket (single 22% bracket ends at $105,700). The survivor's IRMAA Tier 1 threshold drops from $218,001 (MFJ) to $109,001 (single) for 2026 (based on 2024 MAGI; per CMS 2025 annual notice), automatically adding ~$2,500/year in Medicare surcharges. Same assets, same basic lifestyle—thousands more in tax every year for the rest of the survivor's life.

Roth conversions executed while both spouses are alive (and in the wider MFJ brackets) pre-pay tax at lower rates than the surviving spouse would pay on the same distributions. This is one of the strongest arguments for aggressive conversions in late 60s and early 70s for couples with disparate ages or health profiles—and why the conversion analysis should always account for expected survivorship years, not just combined longevity.

QBI Deduction Phase-Out: Conversions Can Cost Business Owners Twice

Under IRC §199A, owners of pass-through businesses (S-corps, partnerships, sole proprietorships) can deduct up to 20% of qualified business income. But for specified service trades or businesses (SSTBs—medicine, law, accounting, consulting, financial services), the deduction phases out between taxable income of $241,950–$291,950 (single) and $483,900–$583,900 (MFJ) for 2026. Every dollar of Roth conversion that pushes you into the phase-out range reduces your QBI deduction, which has a multiplier effect: you're not just paying tax on the conversion dollar, you're also losing a 20% deduction on business income.

The doctor's conversion math: An orthopedic surgeon earns $450,000 as an S-corp and has $50,000 of QBI. Before conversion, she's below the phase-out and gets a $10,000 QBI deduction (saving $3,500 in tax). She converts $50,000 of a legacy IRA, pushing her taxable income into the middle of the phase-out range. Her QBI deduction shrinks by $5,000, raising her federal tax by $1,750 on top of the conversion tax. Effective cost of the conversion: 35% federal bracket + 3.5% QBI loss = 38.5% effective. SSTB owners near the phase-out threshold should time conversions for low-income years or spread them across years that keep them below the phase-out entirely.

Step-Up in Basis: Why Traditional IRAs Don't Get It (and Why That Matters for Conversion Planning)

Under IRC §1014, inherited capital assets receive a step-up in basis at the decedent's death—their cost basis resets to fair market value, eliminating all unrealized capital gains tax. But IRC §1014(c) specifically excludes "income in respect of a decedent" (IRD), which includes traditional IRAs, 401(k)s, and deferred compensation. Your children inherit a traditional IRA at your cost basis (usually zero), and they owe ordinary income tax on every dollar they withdraw.

This creates a planning asymmetry that conversion analyses often miss: a $1M taxable brokerage account left to heirs costs them zero in tax (step-up wipes out all gains); a $1M traditional IRA left to heirs costs them ~$300K+ in federal tax under the 10-year rule. Converting while you're alive shifts the asset from IRD (no step-up, taxed as ordinary income) to Roth (no step-up needed because distributions are tax-free). When comparing "convert now at my rate" vs. "let heirs inherit and withdraw at their rate," the step-up unavailability on traditional IRAs tilts the math toward conversion for anyone with substantial non-retirement wealth to leave. The heir tax is real and typically higher than the post-2017 estate tax for the middle-class.