Past performance does not indicate future results. This is not investment advice.
TL;DR — Quick Verdict
- A Roth conversion is taxed as ordinary income in the year it executes — at your marginal federal rate, which ranges from 10% to 37% in 2026 under IRS Revenue Procedure 2025-32.
- A married couple converting $100,000 in the 22% bracket pays roughly $22,000 in federal tax now; the same conversion in the 12% bracket costs about $12,000 — a $10,000 difference that compounds tax-free for decades.
- The One Big Beautiful Bill Act (OBBBA), signed July 4, 2025, permanently extended TCJA tax rates, eliminating the “rate sunset” urgency — but today’s historically low brackets still make conversions attractive for most pre-retirees.
- Medicare’s IRMAA surcharge is a cliff system: $1 over the $218,000 MFJ threshold in the lookback year adds $2,297 annually per couple in 2026 Part B costs — a hidden conversion penalty most people miss.
- Paying conversion tax from outside IRA funds is the highest-leverage decision in the entire strategy; using IRA dollars to cover the tax erodes the compounding benefit by up to 30% over 25 years.
- Conversions make the most financial sense during low-income gap years — early retirement, sabbatical, before Social Security or RMDs begin — when your marginal rate is temporarily lower than your projected retirement rate.
Roughly $13 trillion sits in traditional IRAs and 401(k)s across the United States, according to the Investment Company Institute — and every dollar of it is pre-tax, meaning the federal government is a silent partner in every withdrawal. A Roth conversion severs that partnership early, on your schedule, at a known rate. The question is whether the tax you pay today is cheaper than the tax you would owe later. In 2026, that math has shifted in meaningful ways. The OBBBA permanently locked in historically low TCJA brackets, removing the sunset deadline that drove conversion urgency through 2025. But Medicare IRMAA cliffs, Social Security benefit taxation, state income taxes, and the new senior deduction phaseout have made the optimal conversion amount dramatically more complex than simply “fill the 22% bracket.” This article models four realistic household scenarios using confirmed 2026 IRS brackets from Revenue Procedure 2025-32, quantifies the real dollar cost at each rate, identifies the five most expensive planning mistakes, and provides a conditional decision framework for Fidelity and Vanguard account holders deciding whether to convert this year.
What a Roth Conversion Actually Costs: 2026 Federal Tax by Bracket
A Roth conversion carries no separate conversion tax rate. Under Internal Revenue Code §408A, the taxable portion of the conversion is added to your ordinary income for the calendar year and taxed at your marginal federal rate. The 2026 brackets, per IRS Revenue Procedure 2025-32 (released October 9, 2025), are the same seven-tier structure as 2025, adjusted upward for inflation by approximately 2–3%.
For married filing jointly filers, the 22% bracket spans $96,950 to $211,400 of taxable income in 2026. The 24% bracket runs from $211,401 to $383,900. These thresholds — not the conversion amount itself — determine what you pay. A couple with $150,000 in combined pension and Social Security income who converts $60,000 will have $210,000 in taxable income, bumping only the last slice into the 24% zone if it crosses the $211,400 ceiling. The math is marginal, not flat.
The table below shows the federal tax cost of a $50,000 and $100,000 conversion at each bracket rate, assuming all converted dollars fall within a single bracket. Real-world conversions often span two brackets, but single-bracket modeling illustrates the per-dollar cost clearly.
Source: IRS Revenue Procedure 2025-32 (irs.gov, verify at irs.gov/pub/irs-drop/rp-25-32.pdf). Tax figures represent federal tax only at a flat bracket rate for illustration; actual tax will differ based on full income stack.
State taxes layer on top. A $100,000 conversion in California — which taxes ordinary income at up to 13.3% — costs $35,300 or more in combined federal-plus-state tax at the 22% bracket. The same conversion in Florida or Texas costs $22,000. That $13,300 gap is the actual price of geography, and it should drive timing decisions for anyone planning to retire in a lower-tax state.
How the Tax Is Calculated: Pro-Rata Rule, the 5-Year Clock, and the OBBBA Senior Deduction
Three mechanics determine your actual taxable amount — and getting any one wrong produces a costly surprise.
The pro-rata rule. Under IRC §408(d)(2), the IRS aggregates all your traditional, SEP, and SIMPLE IRA balances into a single pool when calculating the taxable portion of a conversion. You cannot designate which dollars — pre-tax contributions, after-tax contributions, or earnings — are being converted. If your combined IRA balance is $200,000 and $20,000 of it is after-tax basis (from nondeductible contributions reported on Form 8606), then 10% of any conversion is tax-free and 90% is taxable. Converting $50,000 in this scenario generates $45,000 in ordinary income, not $50,000. This rule does not apply to workplace 401(k) plans, which is why the “reverse rollover” — moving IRA assets into a current employer 401(k) before executing a backdoor Roth — can clear the pro-rata deck entirely.
The 5-year clock. Each Roth conversion carries its own 5-year holding requirement for penalty-free withdrawal of the converted principal, under IRC §408A(d)(2). Roth account owners under age 59½ who withdraw converted funds before the 5-year window closes pay a 10% early withdrawal penalty on those dollars (the tax was already paid at conversion). Owners over 59½ face only the earnings 5-year rule — and only if their Roth account itself is less than five years old. Opening a token Roth contribution account early in life starts the clock permanently.
The OBBBA senior deduction. For tax years 2025–2028, taxpayers aged 65 or older may claim an additional $6,000 deduction per qualifying individual ($12,000 per couple), per the Tax Foundation’s analysis of IRS Rev. Proc. 2025-32. This deduction phases out at a 6% rate beginning at $150,000 MAGI for joint filers. The phaseout itself creates a shadow marginal rate: for every $1,000 in additional Roth conversion income above $150,000, a joint filer over 65 loses $60 of this deduction — effectively adding 1.3–1.4 percentage points to their true marginal rate. A couple converting at what appears to be the 22% rate in this income range is actually paying closer to 23.4% on each marginal dollar.
Four Real-Dollar Conversion Scenarios for 2026
Generic advice says “convert to the top of your bracket.” Real planning requires projecting the full income stack. The four scenarios below use confirmed 2026 parameters. All scenarios assume federal tax paid from outside IRA funds — the single highest-leverage assumption in conversion math.
Sources: IRS Revenue Procedure 2025-32 (irs.gov); CMS 2026 Medicare Parts A & B Premiums (cms.gov, verify at cms.gov); Kiplinger 2026 IRMAA analysis. Tax cost figures are federal only; state taxes not reflected.
Scenario 3 is the one that generates the most expensive surprises in practice. The IRMAA cliff operates like a tax bracket, but it is not a bracket — it is a cliff. A couple whose 2026 MAGI lands at $218,001 — one dollar above the joint threshold — pays the full Tier 1 surcharge of $81.20 per person per month in Part B costs, plus $14.50 per person per month in Part D. Per year, per couple, that is $2,297 in additional Medicare costs for two years, since IRMAA uses a two-year lookback on MAGI. A $25,000 conversion that crosses this cliff costs not just the $5,750 in income tax, but potentially an additional $4,594 in IRMAA penalties across 2028 and 2029 — making the true all-in cost nearly $10,344, not $5,750.
Roth Conversion vs. Leaving Money in a Traditional IRA: Which Wins?
The core question is whether paying tax now — at a known rate — beats paying tax later at an unknown rate. This comparison depends on three variables: your current marginal rate, your projected retirement marginal rate, and how long the Roth assets compound before withdrawal.
The math is symmetric when current rate equals future rate: converting $100,000 at 22% today and paying $22,000 produces exactly the same after-tax outcome as leaving $100,000 to grow and distributing it at 22% in retirement — assuming identical growth rates and a zero-tax payment source. The conversion wins only when the future rate is higher than today’s rate, or when non-tax benefits (RMD reduction, estate planning, tax-free inheritance) add independent value. Below are the three realistic outcome scenarios.
Conversion wins when: Your retirement marginal rate (including RMDs, Social Security, pension, and investment income) exceeds your current conversion rate by at least 3–5 percentage points, you pay the tax from outside IRA funds, and you have 10 or more years for Roth assets to compound before withdrawal.
It’s a wash when: Your effective rate is roughly equal in both periods, you are forced to withhold tax from the converted IRA funds, and your time horizon is under 8 years.
Conversion hurts when: You convert at 32% or 35% in peak earning years and retire into a 22% bracket. It also hurts when IRMAA surcharges, ACA premium clawbacks, or increased Social Security taxation from the conversion add hidden costs that conventional “fill the bracket” calculators don’t model.
Verdict
For pre-retirees in a temporary low-income gap year — sabbatical, early retirement before Social Security, or between business ventures — the Roth conversion wins almost every time the conversion rate is 22% or below. For active high earners in the 32%+ bracket with no near-term income drop, leaving the IRA intact and planning a multi-year conversion ladder starting at retirement is typically superior. For Medicare-eligible retirees, the IRMAA cliff is the decisive variable: model the two-year lookback before converting a single dollar over $109,000 (single) or $218,000 (joint).
What Most People Get Wrong About Roth Conversions
Five mistakes account for the majority of conversion regrets — and three of them involve costs that standard online calculators never surface.
Mistake 1: Paying conversion tax from IRA funds. When you convert $100,000 and instruct your custodian — Fidelity, Vanguard, Schwab — to withhold $22,000 for federal taxes, only $78,000 reaches the Roth. The $22,000 withheld is treated as a distribution, not a conversion, and that money never compounds tax-free. At 7% annual growth over 25 years, $22,000 grows to approximately $119,258. Paying that $22,000 from a taxable brokerage account instead preserves the full $100,000 in Roth — a difference that compounds to $41,258 in lost tax-free growth on that one decision alone.
Mistake 2: Ignoring the IRMAA two-year lookback. The Social Security Administration determines your Medicare IRMAA surcharge using MAGI from two years prior. A large conversion executed in 2026 does not affect 2026 Medicare costs — it affects 2028 costs. Many retirees convert aggressively in the years they enroll in Medicare, unaware that a 2024 or 2025 conversion is what drove their 2026 premiums. The correct behavior is to model IRMAA exposure for two years forward, every single year.
Mistake 3: Assuming the pro-rata rule doesn’t apply to them. Taxpayers who made nondeductible IRA contributions and track basis on Form 8606 often believe they can simply convert the nondeductible portion tax-free. They can — but only if their total traditional/SEP/SIMPLE IRA balance is zero after the conversion, or if the pre-tax dollars are rolled into an employer plan first. Executing a partial nondeductible conversion without clearing pre-tax IRA balances generates a taxable event on the pro-rata portion, often catching holders by surprise at filing.
Mistake 4: Converting in the wrong state, in the wrong year. California taxes Roth conversions as ordinary income at rates up to 13.3%. A resident planning to move to Nevada or Florida in two years who converts $200,000 at the 22% federal rate pays roughly $26,600 in California state tax — money that would have been saved entirely by waiting. The breakeven calculation must include state tax in the numerator.
Mistake 5: Forgetting Social Security benefit taxation. For households receiving Social Security, up to 85% of benefits become taxable once MAGI crosses $44,000 (joint filers). A Roth conversion that pushes MAGI from $42,000 to $56,000 — a $14,000 increment — can also trigger taxation on thousands of dollars of Social Security income that was previously excluded. The effective marginal rate on conversion income in this range can reach 40.7% for a couple in the 22% bracket, because each converted dollar effectively taxes an additional $0.85 of Social Security at the same 22% rate.
Is a Roth Conversion Worth It in 2026? Decision Framework by Household Type
The OBBBA’s permanent extension of TCJA rates eliminated the “convert now before rates jump” urgency that defined planning through 2024. Rates will not automatically sunset. That changes the calculus in one important way: patience is no longer punished. You can now model multi-year ladders without racing a legislative clock.
Convert now if: You are between ages 59 and 72, in a gap year with meaningful reduction in W-2 or business income, your 2026 marginal rate is 12% or 22%, you have taxable funds outside the IRA sufficient to pay the tax bill, and your projected retirement income from RMDs, Social Security, and pensions will push you into a higher bracket. This profile describes the classic Roth conversion sweet spot, and the math is unambiguous in its favor.
Convert partially and ladder if: You are within 2 years of Medicare enrollment, your total IRA balance exceeds $500,000, and your income is near — but not yet above — an IRMAA threshold. A series of $20,000–$40,000 annual conversions calibrated to stay below the first IRMAA cliff ($218,000 MFJ, $109,000 single in 2026) delivers meaningful tax-free conversion volume without triggering cliff surcharges. Northern Trust’s analysis of the OBBBA estimates that a couple converting a $500,000 IRA in 10 annual installments of $50,000 each pays substantially less cumulative tax than converting the full amount in a single year, even when accounting for growth.
Do not convert if: Your current marginal rate equals or exceeds your expected retirement rate, your IRA is your primary or only liquidity source (meaning you would have to pay the tax from IRA funds), you plan to donate the IRA to charity via qualified charitable distributions (QCDs), which are untaxed distributions to charity for owners over 70½, or you are in a high-tax state and moving to a no-income-tax state within three years. Each of these conditions materially changes the net present value of the conversion.
One underused option: the conversion of a Self-Directed IRA holding alternative assets (real estate LLCs, private equity) at a valuation discount. IRS-supported qualified appraisal can establish a fair market value 15%–35% below the underlying asset value at the time of conversion, reducing the taxable base. This strategy requires a qualified appraiser and experienced tax counsel, but it is grounded in established case law and represents one of the few opportunities to reduce the per-dollar conversion cost below the stated bracket rate.
How We Researched This Article
This article was researched and modeled in May 2026. All federal tax bracket figures, standard deduction amounts, and IRMAA thresholds were drawn from primary government sources and cross-verified against independent tax publishers before publication.
The 2026 federal income tax brackets and standard deduction amounts ($16,100 single, $32,200 MFJ) were sourced directly from the IRS announcement of Revenue Procedure 2025-32 (IR-2025-103, released October 9, 2025). The 2026 IRMAA thresholds and Part B premium amounts were sourced from the Kiplinger 2026 IRMAA analysis and verified against CMS Medicare Parts A & B premium release data (verify at cms.gov). The Tax Foundation’s 2026 tax bracket analysis was used to confirm bracket ceiling values and the senior deduction phaseout mechanics. The OBBBA’s impact on Roth conversion strategy was analyzed using Northern Trust Institute’s post-OBBBA article (verify at northerntrust.com). The pro-rata rule citations reference IRC §408(d)(2) and §408A(d)(2) as published by the IRS in Revenue Procedure 2025-32.
Tax cost scenarios were modeled using marginal rates applied to hypothetical conversion amounts, assuming all converted dollars fall within the stated bracket. Real household outcomes will differ based on total income composition, itemized deductions, state taxes, and the pro-rata ratio of pre-tax to after-tax IRA basis. IRMAA impact calculations used 2026 Tier 1 surcharge values confirmed by CMS ($81.20/person/month Part B + $14.50/person/month Part D for joint filers). The long-term compounding example ($22,000 growing to $119,258 over 25 years at 7%) was calculated independently using compound interest formula A = P(1+r)^t; no real portfolio performance was implied. Limitations: state income tax rates were not modeled for each scenario. Social Security benefit taxation phase-in was modeled conceptually rather than with specific household income values. All figures were verified against named primary sources before publication.
