There is no dollar cap and no income ceiling on Roth IRA conversion limits: federal law lets you convert $5,000 or $5 million from a Traditional IRA or employer plan in a single year, regardless of what you earn. The real limit is the tax bill. Every pre-tax dollar you convert gets added to your ordinary income for the year, and the downstream effects on your bracket, Medicare premiums, and investment surtaxes can be much larger than the headline tax rate suggests.
No Dollar Cap and No Income Limit
Direct Roth contributions are income-restricted. For 2026, the ability to contribute phases out between $153,000 and $168,000 of modified adjusted gross income for single filers, and between $242,000 and $252,000 for married couples filing jointly.1Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500 Earn above those ranges and you cannot put a single dollar directly into a Roth.
Conversions play by different rules. The IRS applies no income limits and no dollar caps to amounts converted from a Traditional IRA, SEP IRA, SIMPLE IRA, or employer plan such as a 401(k) or 403(b) into a Roth IRA.2Internal Revenue Service. Retirement Topics – IRA Contribution Limits A taxpayer earning $50,000 and one earning $5 million have the same conversion access. There is also no limit on how many conversions you can do in a year, so a large balance can be split into several smaller pieces to manage the tax impact.
This absence of an income limit is what makes the Backdoor Roth possible: a high earner blocked from direct Roth contributions can make a non-deductible Traditional IRA contribution and immediately convert it, with the pro-rata rule described below being the main thing that can spoil the result.
The Tax Bill Is the Real Constraint
The converted amount is added to your ordinary income for the year. Convert $80,000 while already earning $120,000 and your taxable income jumps to at least $200,000 before deductions. That spike can push you into a higher federal bracket, and the conversion is taxed at whatever marginal rates apply to your total income for the year.
The portion that escapes tax is your basis, meaning the total of any non-deductible contributions you have made to your Traditional IRAs over the years. Because those contributions were made with money you already paid tax on, the IRS does not tax them again on the way out. You track basis by filing Form 8606 for every year you make a non-deductible contribution.3Internal Revenue Service. About Form 8606, Nondeductible IRAs If you never filed the form, the IRS assumes your basis is zero and every dollar you convert is taxable.
The taxable portion goes on line 4b of Form 1040, reported through Part II of Form 8606.4Internal Revenue Service. Form 8606 – Nondeductible IRAs One piece of good news: the conversion itself is not subject to the 10% early withdrawal penalty, even if you are under 59½, because it qualifies as a rollover.5Internal Revenue Service. Topic No. 557, Additional Tax on Early Distributions From Traditional and Roth IRAs
The Pro-Rata Rule
You cannot cherry-pick which dollars to convert. If your Traditional IRA holds a mix of deductible and non-deductible contributions, the IRS treats every distribution, including a conversion, as coming proportionally from both piles.
The math: divide your total non-deductible basis across all Traditional, SEP, and SIMPLE IRAs by the combined fair market value of all those accounts as of December 31 of the conversion year. The result is the tax-free percentage of anything you convert.6Internal Revenue Service. Publication 590-B, Distributions From Individual Retirement Arrangements (IRAs) Form 8606 walks through the calculation line by line.4Internal Revenue Service. Form 8606 – Nondeductible IRAs
Say you have $10,000 in non-deductible basis and $90,000 in pre-tax IRA money, for a total of $100,000 across all your non-Roth IRAs. Only 10% of any conversion is tax-free. Convert $20,000 and you owe tax on $18,000. The other $2,000 is a return of after-tax basis.
This is where the Backdoor Roth can quietly fail. Make a $7,500 non-deductible contribution and convert it, but keep a $200,000 rollover IRA at any custodian, and the IRS aggregates everything. Your tax-free slice becomes tiny. The common workaround is to roll existing pre-tax IRA balances into your employer’s 401(k) before doing the Backdoor conversion, since 401(k) balances are not counted in the pro-rata calculation.
Medicare Surcharges and the 3.8% Surtax
A large conversion doesn’t just raise your bracket. It can trigger two additional costs that catch people off guard.
IRMAA on Part B and Part D Premiums
Medicare Part B and Part D premiums are income-adjusted. If your modified AGI crosses certain thresholds, you pay an Income-Related Monthly Adjustment Amount. For 2026, the standard Part B premium is $202.90 per month. Push a married couple’s joint income above $218,000 and the premium jumps to $284.10, with surcharges escalating in tiers up to $689.90 per month at income of $750,000 or more.7Centers for Medicare & Medicaid Services. 2026 Medicare Parts A & B Premiums and Deductibles For single filers, the first surcharge kicks in above $109,000.
The timing is the trap: Medicare uses your tax return from two years prior. A conversion in 2026 could raise your 2028 premiums. If you are already on Medicare or approaching 65, that two-year lag is worth modeling before converting.
Net Investment Income Tax
The Net Investment Income Tax adds 3.8% on top of regular tax for taxpayers whose modified AGI exceeds $200,000 (single) or $250,000 (married filing jointly).8Office of the Law Revision Counsel. 26 U.S. Code 1411 – Imposition of Tax It applies to the lesser of your net investment income or the amount by which your AGI exceeds the threshold. The conversion itself is not investment income for this purpose, but the AGI boost it creates can expose capital gains, dividends, and interest that would otherwise have stayed below the threshold. Someone with $150,000 in wages and $60,000 in investment income normally owes no NIIT. Add a $100,000 conversion, AGI hits $310,000, and the investment income is suddenly subject to the surtax.
Take Your RMD Before Converting
If you have reached the age where required minimum distributions apply, you must take the full RMD for the year before converting any additional balance to a Roth. RMD amounts cannot be rolled over into any tax-deferred or Roth account.9Internal Revenue Service. Retirement Plan and IRA Required Minimum Distributions FAQs Try to convert before satisfying the RMD and the IRS treats the RMD portion as an ineligible rollover, creating penalties and a mess to unwind.
In practice, your conversion year starts with a forced taxable distribution, and only the balance above the RMD amount is eligible for conversion. The combined income from the RMD plus the conversion can be substantial, which is why the years between retirement and the start of RMDs at age 73 are often the best window for conversions.
The SIMPLE IRA Two-Year Rule
SIMPLE IRAs carry a unique timing restriction. During the first two years after you begin participating in your employer’s SIMPLE IRA plan, you cannot move those funds into anything other than another SIMPLE IRA. Convert or roll over to a Roth IRA during that window and the IRS treats the entire amount as a taxable distribution and applies a 25% early withdrawal penalty instead of the usual 10%.10Internal Revenue Service. Retirement Plans FAQs Regarding SIMPLE IRA Plans Once the two years end, SIMPLE IRA funds convert under the same rules as any Traditional IRA.
The December 31 Deadline and No Undo
Conversions must be completed by December 31 of the tax year you want them to count for. Unlike contributions, which can be made up to the April filing deadline for the prior year, conversions get no extension. If you are planning a year-end conversion, confirm your custodian’s cutoff, since some require submission by mid-afternoon on the last business day of the year.
Before 2018, taxpayers could undo a Roth conversion through recharacterization. The Tax Cuts and Jobs Act eliminated that option, making every Roth conversion irrevocable.11Office of the Law Revision Counsel. 26 U.S. Code 408A – Roth IRAs If the market drops 30% the month after you convert, you still owe tax on the full value at the time of conversion. That permanence is a strong argument for running the numbers carefully before pulling the trigger.
Why Partial Conversions Usually Beat a Single Large One
Nothing forces you to convert everything at once, and for most people a series of smaller conversions over several years produces a better after-tax result than one large event. The goal is to fill up your current tax bracket each year without spilling into the next, without tripping an IRMAA tier, and without dragging investment income into the 3.8% surtax.
The years between retirement and age 73 are often the sweet spot. Income tends to be lower, so meaningful amounts can be converted at relatively low rates. Once RMDs begin, they consume part of your bracket space and leave less room for conversions. Taxpayers living in a state with no personal income tax get an additional edge, since they avoid state-level tax on the converted amount entirely.