Roth Conversion Tax Calculator

Roth Conversions as Deliberate Tax Arbitrage

The bracket-fill discipline

A conversion stacks on top of your other income, filling brackets upward — the calculator shows where it tops out and how much room remains before the next rate jump. The strategy practitioners run: convert annually to the top of the 12% or 22% or 24% bracket (whichever fits the long-term plan), never spilling into the next. Over a decade of retirement gap years, this systematically moves six figures from future-taxed to never-taxed at rates chosen in advance rather than imposed by RMDs later.

The ripple effects beyond the bracket

Conversion income counts toward more than income tax: it can trigger Medicare IRMAA surcharges two years later (cliffs, not phase-ins — one dollar over costs the full surcharge), tax up to 85% of Social Security benefits, and reduce ACA premium credits for early retirees. For anyone 63+, the IRMAA lookback makes conversion sizing a genuinely multi-variable problem worth modeling carefully or professionally.

Rules that shape execution

Conversions are irreversible since 2018 — recharacterization is gone, so size them late in the year when income is knowable. Each conversion starts its own 5-year clock for penalty-free access to that principal before 59½ (the basis of 'Roth conversion ladder' early-retirement plans). And pay the tax from taxable savings: paying from the IRA itself converts less, and under 59½, the withheld portion is a taxed and penalized distribution.

Conversion cost by bracket: the fill-up map

The cost of converting depends entirely on which brackets the conversion fills. For a single filer, here's what each bracket's full width costs to convert at 2025 thresholds:

The map's planning insight is the 24% bracket's unusual width: nearly $94,000 of conversion room at a rate only 2 points above the 22% floor, which is why "fill through 24%" became the default aggressive-but-sane strategy for large-IRA retirees in the gap years. The jump to 32% is where most planners stop — an 8-point step that demands strong conviction about future rates to justify.

The gap years: retirement's tax-planning window

The structural opportunity this calculator prices lives between two dates: retirement (wages stop) and the arrival of RMDs at 73 plus, often, Social Security at 70. A retiree in that window can have taxable income near zero — meaning conversions fill brackets from the very bottom, harvesting the 10% and 12% rates that working years never see. Worked example: a 62-year-old couple with $1.5M in traditional IRAs, living on cash and taxable accounts, converts $96,950 annually (topping the 12% bracket, married) for eight years — roughly $775K moved at an 11% blended cost, versus the 22–24%+ those dollars would face as stacked RMDs and Social Security later. The sequencing subtleties: delay Social Security while converting (benefits would fill the cheap brackets otherwise, and delaying grows them 8%/year anyway); watch the ACA premium-credit cliff before Medicare at 65 if buying marketplace insurance (conversion income counts, and credits phase with it); and after 63, size conversions against IRMAA thresholds, since Medicare looks back two years. The window closes at 73 — which is the whole argument for planning it rather than discovering it.

What people get wrong

• Paying conversion tax from the IRA. It shrinks the converted amount and, under 59½, the withheld tax is itself a penalized distribution. Outside cash is the only clean fuel.
• Converting in January. Income for the year is unknowable in January and conversions are irreversible. November–December, with the year's income visible, is when sizing to a bracket edge is possible.
• Ignoring state taxes — twice. The conversion pays your current state's rate; a planned retirement move to a no-tax state flips the math toward waiting, while the reverse move argues for converting before leaving.
• All-or-nothing thinking. The strategy is a dial, not a switch: annual partial conversions sized to bracket room beat both zero conversion and any single giant one in nearly every modeled scenario.

Market timing that actually works: converting into drawdowns

Roth conversion is the rare context where reacting to market drops is strategy rather than panic. Converting shares while depressed moves more shares per bracket dollar — a $50,000 conversion during a 25% drawdown transfers what was recently $66,667 of position, and the entire recovery then compounds tax-free on the Roth side. The 2020 March window and the 2022 bear were, in hindsight, the decade's best conversion moments, and the retirees who executed had a standing plan rather than fresh nerve: a written trigger ("convert $X additional if the portfolio drops 20%+ in a year I have bracket room") decided in calm months. The same logic applies at security level — converting the beaten-down position specifically, rather than pro-rata across the account, concentrates the recovery where it's untaxed. The guardrails: the conversion still stacks on income (a drawdown year with a layoff may be a low-income year, doubling the opportunity), irreversibility means sizing conservatively, and nobody knows the bottom — tranching a planned drawdown conversion into two or three pieces buys average-down pricing on the tax itself.

Conversions as an estate instrument

For balances likely to outlive their owners, the conversion question changes shape: the comparison is your bracket now versus your heirs' brackets during their 10-year emptying window — which typically lands in their peak earning years. A parent converting at 22–24% spares children distributions taxed at 32–37% stacked on their salaries; the conversion is, functionally, a tax-free gift of the tax payment, made without touching gift-tax limits. The widow's-bracket problem argues the same direction earlier: after a spouse's death, the survivor files single with compressed brackets on largely unchanged income, so conversions during joint-filing years pre-pay tax at rates that will never be available again. Against these, the one estate argument for not converting: traditional IRA balances destined for charity should stay traditional, since charities pay no income tax and QCDs or estate charitable bequests extract the money untaxed — converting charity-bound dollars pays tax nobody needed to pay. Mapping which dollars are for spending, heirs, and charity, then converting only the middle tranche, is the version of this strategy estate attorneys actually draw up.

Withholding and estimated payments on conversions

The conversion's tax is due in the year it happens, and the payment mechanics have a trap and a trick. The trap: electing withholding on the conversion itself shrinks the amount reaching the Roth and, under 59½, the withheld slice is a taxed-and-penalized distribution — so decline conversion withholding and pay from outside funds. The trick: IRA and 401(k) withholding from other distributions is deemed paid evenly across the year, so a December RMD or small IRA distribution with heavy withholding retroactively covers a conversion made in July, curing estimated-payment underpayment without penalty. Converters without that lever use quarterly estimates under the safe harbors (100/110% of last year's tax, or 90% of this year's), with the conversion-quarter payment sized to the marginal cost this calculator shows. Either way, the tax on a November conversion planned in October is a scheduled bill, not an April ambush — which is the administrative half of why late-year conversion is the professional default.

Where the rules come from

Conversion taxation is IRC §408A(d)(3) — converted amounts are gross income in the conversion year, with no income limits since TIPRA took effect in 2010. Irreversibility dates to TCJA's elimination of recharacterization for conversions (2018), the change that moved best practice to late-year sizing. The RMD-first ordering rule in RMD years is §401(a)(9) regulations; each conversion's 5-year penalty clock is §72(t) as applied through §408A(d)(3)(F) — the machinery behind Roth conversion ladders in early-retirement planning. IRMAA's two-year lookback is 42 CFR §418; the ACA premium-credit interaction runs through §36B's MAGI definition, which includes conversion income. Brackets are the 2025 figures from Rev. Proc. 2024-40, current-law permanent after the 2025 tax act.

Frequently asked questions

Does a conversion make sense if tax rates might drop?

Rate uncertainty cuts both ways — 2025's law made current brackets 'permanent,' which in tax law means 'until changed.' The safer framing is personal: convert when YOUR rate is unusually low relative to your own future, which is knowable, rather than betting on Congress, which isn't.

Is there a limit on how much I can convert?

No dollar limit and no income limit — you could convert a $2M IRA tomorrow. The bracket math is the practical limiter: giant conversions pay top rates on the upper slices, which usually argues for spreading across years.

Do conversions count toward RMDs?

No — in RMD years the RMD must come out first, and RMD dollars can't be converted. This is precisely why the prime conversion window is the years after retirement but before RMDs begin at 73.

Do conversions make sense after RMDs have already started?

Sometimes, but the economics narrow: the RMD must come out first (unconvertible), and conversions stack on top of it, usually at higher brackets. The remaining cases are down-market years (converting depressed shares), legacy planning for heirs in high brackets, and widowhood-ahead planning — the survivor files single, compressing brackets, which conversions during joint-filing years pre-empt.

How do conversions interact with the standard deduction in low-income years?

Beautifully — the deduction shelters the first $15,000/$30,000 of income entirely, so a retiree with no other income can convert that much at a true 0% federal rate before the 10% bracket even begins. Gap-year converters who skip this are leaving the code's only free conversions unclaimed.