Roth Conversion Calculator: How Much to Convert

The optimal Roth conversion amount is the largest sum you can convert while staying below the binding constraint — whether that's a tax bracket boundary, an ACA subsidy threshold, or an IRMAA surcharge trigger. For a married couple filing jointly in 2025 with no other income, that ceiling is $126,950 to fill the 12% bracket, $236,700 to fill the 22% bracket, or as low as $84,600 if ACA subsidies are in play. The conversion tax is simple arithmetic. The constraints are where the real optimization happens.

Most people searching for a Roth conversion calculator want one number: how much should I convert this year? Fidelity, Schwab, and Vanguard each offer calculators that estimate the tax on a given conversion amount. But tax is only one variable. The optimal conversion also depends on ACA premium credits, Medicare IRMAA surcharges, and the interaction between conversion income and Social Security taxation — none of which those brokerage tools model.

The Bracket-Filling Math

The core Roth conversion calculation takes three inputs: your conversion amount, your other ordinary income, and the standard deduction. Add the conversion to your other income, subtract the standard deduction ($30,000 for married filing jointly in 2025, per Rev. Proc. 2024-40), and apply the marginal tax brackets. The goal is to fill your lowest available brackets — converting enough to use cheap tax space without pushing income into an expensive bracket.

Worked Example: $100,000 Conversion (MFJ, No Other Income)

A married couple, both 58, retired last year with no pensions and no Social Security yet. They convert $100,000 from a Traditional IRA to Roth.

• Gross income: $100,000
• Standard deduction: −$30,000
• Taxable income: $70,000
• 10% on first $23,850: $2,385
• 12% on remaining $46,150: $5,538
• Federal tax: $7,923 (7.9% effective rate)

They still have $26,950 of room in the 12% bracket. Converting that additional amount costs 12 cents per dollar. The next dollar after $126,950 jumps to 22 cents — nearly double the marginal rate.

Tax Cost at Different Conversion Levels

This table assumes married filing jointly in 2025 with zero other income (all brackets from Rev. Proc. 2024-40):

Conversion	Taxable Income	Federal Tax	Effective Rate	Marginal Rate
$30,000	$0	$0	0.0%	0% (std. deduction)
$50,000	$20,000	$2,000	4.0%	10%
$75,000	$45,000	$4,923	6.6%	12%
$100,000	$70,000	$7,923	7.9%	12%
$126,950	$96,950	$11,157	8.8%	12%
$150,000	$120,000	$16,228	10.8%	22%
$200,000	$170,000	$27,228	13.6%	22%
$236,700	$206,700	$35,302	14.9%	22%

The jump from 12% to 22% at $126,950 is the most consequential threshold for most retirees. Converting $126,950 costs $11,157 in tax. Converting $150,000 costs $16,228 — the extra $23,050 generates $5,071 in additional tax at the 22% marginal rate. Whether paying 22% now is worth it depends on whether your future RMDs will push you into the 22% bracket or higher anyway.

Three Constraints That Override Tax Brackets

Tax brackets set the floor for your conversion cost, but three income-sensitive programs set the ceiling: ACA premium subsidies, Medicare IRMAA surcharges, and Social Security benefit taxation. A conversion that looks cheap in pure bracket math can become expensive — or catastrophic — once these constraints bite.

The ACA Subsidy Cliff (Pre-Medicare)

If you buy health insurance through the ACA marketplace before Medicare eligibility at 65, your Roth conversion income counts toward Modified Adjusted Gross Income for premium tax credit calculations.

The Inflation Reduction Act eliminated this cliff for 2021–2025, capping marketplace premiums at 8.5% of income regardless of FPL percentage. If this provision expires after 2025, the cliff returns — and it becomes the single tightest constraint for every early retiree on marketplace coverage. [VERIFY: whether IRA enhanced subsidies were extended beyond 2025]

Even without the cliff, higher MAGI still reduces your subsidy. A $100,000 conversion produces a smaller premium credit than a $75,000 conversion. The analysis shifts from "will I lose everything?" to "how much subsidy am I giving up per dollar of conversion?" — a marginal calculation that brokerage tools don't perform.

IRMAA Medicare Surcharges (Age 65+)

Once you're on Medicare, IRMAA replaces ACA as the income-sensitive constraint. IRMAA adds surcharges to Part B and Part D premiums based on your MAGI from two years prior.

For married filing jointly in 2025, the first IRMAA tier triggers at $206,000 in MAGI. Exceeding it adds $74.00 per month per person to Part B premiums — $1,776 per year for a couple. The second tier at $258,000 adds $185.00 per month per person ($4,440 per year for a couple). [VERIFY: exact 2025 IRMAA surcharge amounts per tier]

The two-year lookback creates a planning trap. A large conversion at age 63 hits your Medicare premiums at 65. Conversions at 64 affect premiums at 66. If you're planning aggressive conversions in your early 60s, map the IRMAA impact two years forward.

The Social Security Tax Torpedo

Social Security benefits are taxed based on "provisional income" — your AGI (excluding Social Security) plus tax-exempt interest plus half your Social Security benefit. For married couples filing jointly, the thresholds are $32,000 and $44,000 (IRC Section 86). These thresholds have not been adjusted for inflation since 1993.

Between $32,000 and $44,000 in provisional income, up to 50% of Social Security benefits become taxable. Above $44,000, up to 85% become taxable. This creates a hidden tax multiplier on Roth conversion income.

This interaction is exactly why most retirement calculators get taxes wrong. They model Social Security as either "taxable" or "not taxable," missing the 40.7% marginal zone entirely.

What Brokerage Calculators Miss

Fidelity, Schwab, and Vanguard each offer Roth conversion calculators, and all three model the same thing: the tax on the conversion amount and a comparison of future growth in Traditional versus Roth. None models the constraints that actually determine the optimal conversion amount for most retirees.

Feature	Fidelity	Schwab	Vanguard	Full Optimization
Conversion tax estimate	✓	✓	✓	✓
Future growth projection	✓	✓	✓	✓
Marginal bracket calculation	Partial	Partial	✗	✓
ACA subsidy impact	✗	✗	✗	✓
IRMAA threshold modeling	✗	✗	✗	✓
SS taxation interaction	✗	✗	✗	✓
Multi-year optimization	✗	✗	✗	✓
State tax integration	✗	✗	✗	Varies

Fidelity's calculator, available in their Planning & Guidance Center, handles basic bracket math and projects after-tax growth over a user-specified time horizon. Schwab's tool compares Traditional-vs-Roth outcomes given your tax rate assumptions. Vanguard provides general retirement planning tools but lacks a dedicated conversion calculator with bracket-level modeling.

The gap is the same across all three: they answer "what will I pay in tax on this conversion?" but not "what is the most I should convert given everything else in my financial picture?" These are different questions. The first is arithmetic. The second requires modeling constraints that interact with each other across multiple years.

CoastIQ's Roth Conversion Optimizer models the full constraint set — brackets, ACA, IRMAA, and Social Security taxation — year-by-year from your current age through the end of the projection.

Conversion Windows by Age

The optimal conversion amount changes every year because the binding constraint changes. Before 65, ACA subsidies cap your conversion. After 65, IRMAA thresholds take over. After 73, Required Minimum Distributions consume your lower brackets and shrink the room available for conversions.

Pre-62: The ACA-Constrained Window

Early retirees buying marketplace insurance face the tightest conversion window. If the ACA subsidy cliff applies, total MAGI must stay below 400% FPL for your household size. After subtracting any other income — part-time earnings, rental income, taxable brokerage dividends — the remaining space is what's available for conversions. For a couple with $20,000 in other income, that leaves $50,000–$70,000 per year for conversions.

A Roth conversion ladder is the standard FIRE strategy here: convert a fixed amount each year during the five-year seasoning period, funding living expenses from taxable brokerage accounts while the converted dollars become penalty-free.

Ages 62–64: Social Security Timing Intersects

If you claim Social Security at 62, the torpedo zone activates alongside your conversions. Each dollar of conversion income can face an effective rate of 22.2% (in the 12% bracket with the 50% torpedo) to 40.7% (in the 22% bracket with the 85% torpedo). Delaying Social Security to 70 eliminates the torpedo during your prime conversion years and increases your benefit by 77% compared to claiming at 62 (for those with a Full Retirement Age of 67).

Ages 65–72: The Widest Window

Medicare replaces ACA at 65, removing the subsidy cliff entirely. IRMAA thresholds ($206,000 MFJ) are high enough that aggressive conversions through the full 22% bracket — up to $236,700 with no other income — stay below the first surcharge tier. RMDs haven't started. This seven-year window is where the largest tax-efficient conversions happen.

For a couple with $1.5M in Traditional IRAs at 65, converting $200,000 per year for seven years moves $1.4M to Roth at a 13.6% average effective rate — well below the 22–24% marginal rates that RMDs would otherwise trigger at 73.

Age 73+: RMDs Compete for Bracket Space

Required Minimum Distributions begin at 73 under SECURE 2.0. A $2 million Traditional IRA at 73 requires roughly $75,500 in the first year (Uniform Lifetime Table divisor of 26.5). That RMD fills the standard deduction and most of the 10% and 12% brackets before you convert a single additional dollar. Your available conversion space drops to whatever room remains between the RMD and the top of your target bracket. See the RMD guide for the year-by-year distribution schedule.

How to Calculate Your Optimal Conversion Amount

Start with your target: the top of the tax bracket you want to fill. Apply each constraint as a cap. The lowest number wins.

Step 1 — Set the bracket target. Decide the highest marginal rate you're willing to pay. For most early retirees, the 12% bracket is the target. For those with large Traditional balances and looming RMDs, filling the 22% bracket produces better lifetime outcomes.

Step 2 — Calculate the bracket-fill amount. (Top of target bracket) + (standard deduction) − (other ordinary income). Targeting the 12% bracket with no other income: $96,950 + $30,000 = $126,950.

Step 3 — Apply the ACA cap. If you're on marketplace insurance and the subsidy cliff is in effect, cap total MAGI at 400% FPL for your household size. Subtract other income to find available conversion room.

Step 4 — Apply the IRMAA cap. If you're on Medicare, cap MAGI at $206,000 (MFJ) to avoid the first surcharge tier. Or accept the surcharge if the conversion savings over your remaining lifetime exceed $1,776 per year.

Step 5 — Check the Social Security torpedo. If you're receiving benefits, calculate provisional income (AGI + tax-exempt interest + 50% of SS benefit). If it falls in the $32,000–$44,000 range (MFJ), each conversion dollar costs 1.5× your marginal rate. Above $44,000, each dollar costs 1.85× your marginal rate, until 85% of benefits are taxable.

Step 6 — Take the minimum. Your optimal conversion is the smallest of: the bracket-fill amount, the ACA cap, and the IRMAA cap — adjusted for the torpedo effect on your true effective rate.

This is a tax-efficient withdrawal problem at its core. The conversion decision shares bracket space with your withdrawal sequence — you can't optimize one without accounting for the other.

For spreadsheet users, the core calculation is five formulas: taxable income, bracket tax, ACA MAGI check, IRMAA MAGI check, and provisional income. Most "Roth conversion calculator Excel" templates available online handle the first two and skip the last three. A complete spreadsheet also requires projecting the Traditional IRA balance year by year (factoring in returns, conversions, and eventual RMDs), which is where dedicated tools like CoastIQ's Roth Conversion Optimizer or a purpose-built Excel model save significant effort.