This guide covers the 10 Roth conversion mistakes advisors make, plus the errors near-retirees and DIY converters make on their own, because the costliest ones rarely show up on the conversion form itself. They surface months later as a higher tax bill, a Medicare premium surcharge, a lost health-insurance credit, or a penalty sizing math would have caught. Each mistake below carries a 2026 number, the corrective rule, and a worked example of how one oversized conversion stacks costs at once.
The costliest of the 10 Roth conversion mistakes advisors make is converting more than a target bracket can absorb in one year, because that surplus income can also trip the IRMAA Medicare cliff, push up to 85% of Social Security into taxable income, and cost pre-65 filers their ACA premium credit. A conversion is taxed as ordinary income, has no dollar cap, and cannot be reversed after 2017 (Source: IRS, Retirement Plans FAQs).
What is the single biggest Roth conversion mistake?
The single biggest Roth conversion mistake is sizing the conversion without a multi-year plan, so the top dollars land in a higher bracket or cross a hidden income cliff. A Roth conversion is uncapped, taxable as ordinary income, and irreversible since 2017, which is why the amount and the year you convert carry more weight than the decision to convert at all (Source: IRS, Retirement Plans FAQs Regarding IRAs).
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A Roth conversion moves money from a traditional IRA, 401(k), or 403(b) into a Roth IRA. You pay ordinary income tax on the converted amount that year, and the balance then grows and comes out tax-free in qualified withdrawals, with no lifetime RMDs for the owner. There is no dollar or income limit, and the deadline is December 31, not the following April.
Mistakes fall into two groups: the errors savers make on their own, and the mistakes advisors make, usually single-year thinking instead of a multi-year plan. This page is the comprehensive 10-mistake pillar; for a shorter companion list, see our 5 costly Roth conversion mistakes.
What 2026 numbers should you check before you convert?
For 2026, a married couple filing jointly reaches the 22% bracket above $100,800 of taxable income and the first IRMAA Medicare surcharge above $218,000 of MAGI (Source: IRS Rev. Proc. 2025-32; CMS 2026 Medicare Parts A and B Fact Sheet). The standard deduction is $16,100 single and $32,200 joint. The IRA contribution limit is $7,500, but that cap does not apply to conversions, which have no dollar limit.
2026 federal income tax brackets (Source: IRS Rev. Proc. 2025-32):
| Rate | Single taxable income | Married filing jointly |
|---|---|---|
| 10% | $0 to $12,400 | $0 to $24,800 |
| 12% | $12,401 to $50,400 | $24,801 to $100,800 |
| 22% | $50,401 to $105,700 | $100,801 to $211,400 |
| 24% | $105,701 to $201,775 | $211,401 to $403,550 |
| 32% | $201,776 to $256,225 | $403,551 to $512,450 |
| 35% | $256,226 to $640,600 | $512,451 to $768,700 |
| 37% | $640,601+ | $768,701+ |
The One Big Beautiful Bill Act (P.L. 119-21, 2025) made this rate structure permanent, so the 37% top rate is not scheduled to sunset. Filers age 65 and older add roughly $2,050 (single) or $1,650 per spouse to the standard deduction, plus a temporary $6,000 senior deduction per person for 2025 through 2028, both of which widen the low-bracket room a conversion can fill (Source: OBBBA, 2025).
Mistake 1: Are you converting too much in a single year?
Converting a large lump sum can push the top slice into a higher bracket than you needed to pay. A common fix is bracket filling: convert only enough to reach the top of your current bracket. In 2026 a joint filer sits in the 12% bracket up to $100,800 of taxable income, jumps to 22% above it, then to 24% above $211,400 (Source: IRS Rev. Proc. 2025-32).
Convert too little and a large pre-tax balance keeps growing bigger RMDs later; convert too much and the last dollars can be taxed at 22% or 24% when a multi-year plan could have kept them at 12%. Our pages on how much to convert to a Roth and the Roth conversion break-even show how the future-rate assumption moves that target.
Mistake 2: Should you pay the conversion tax out of the IRA?
No, in most cases. Using the converted funds to pay the tax shrinks the amount that lands in the Roth, and if you are under 59 and a half, the withheld portion can count as an early distribution subject to a 10% penalty. Paying the tax from outside savings keeps the full converted balance growing tax-free (Source: IRS Publication 590-A).
If a 58-year-old converts $50,000 and has $12,000 withheld for taxes, only $38,000 reaches the Roth and the $12,000 withheld can be a penalized early withdrawal. Conversions generally make the most sense when the tax is paid from a taxable brokerage or bank account, keeping every converted dollar inside the Roth.
Mistake 3: Do you have to take your RMD before converting?
Yes, if you are 73 or older. Your required minimum distribution for the year must come out first, because an RMD cannot be converted (Source: IRS, Retirement Topics: Required Minimum Distributions). Converting before taking the RMD can create an excess Roth contribution that carries a 6% annual excise tax until corrected.
RMDs begin at age 73 under current law, rising to 75 for those born in 1960 or later, whose earliest age-75 RMD year is 2035 (Source: SECURE 2.0 Act, 2022). The first dollars leaving a traditional IRA in an RMD year satisfy the RMD and cannot be converted, one reason many people concentrate conversions in the gap years before RMDs start. See our overview of required minimum distributions for 2026.
Mistake 4: Will the conversion trigger the IRMAA Medicare surcharge?
It can. IRMAA is a Medicare Part B and D premium surcharge that starts above $109,000 of MAGI for single filers and $218,000 for joint filers in 2026 (Source: CMS 2026 Medicare Parts A and B Fact Sheet). It is a cliff: one dollar over a threshold triggers the full tier. It also uses a two-year lookback, so 2026 income sets 2028 premiums.
The standard 2026 Part B premium is $202.90 per month and climbs from there. The added AGI is not itself net investment income, but it can push other investment income over the 3.8% net investment income tax threshold of $200,000 single or $250,000 joint.
| Tier | Single MAGI (2024) | Joint MAGI (2024) | 2026 Part B/mo | Part D surcharge/mo |
|---|---|---|---|---|
| Base | ≤ $109,000 | ≤ $218,000 | $202.90 | $0 |
| 1 | $109,001 to $137,000 | $218,001 to $274,000 | $284.10 | $14.50 |
| 2 | $137,001 to $171,000 | $274,001 to $342,000 | $405.80 | $37.50 |
| 3 | $171,001 to $205,000 | $342,001 to $410,000 | $527.50 | $60.40 |
| 4 | $205,001 to $499,999 | $410,001 to $749,999 | $649.20 | $83.30 |
| 5 | ≥ $500,000 | ≥ $750,000 | $689.80 | $91.00 |
Mistake 5: Could it make more of your Social Security taxable?
Yes. Roth conversion income raises your provisional income, which can push the taxable share of Social Security benefits up toward the 85% maximum (Source: IRS Publication 915). Once you cross the upper threshold, each added dollar can be taxed while also making an extra portion of your benefits taxable, raising the effective rate on those conversion dollars.
Provisional income is roughly your adjusted gross income plus tax-exempt interest plus half of your benefits, and once a conversion pushes you past the upper threshold the effective rate can run well above the headline bracket, a major reason many plans front-load conversions into the years before Social Security starts.
Mistake 6: Will you lose your ACA premium tax credit before 65?
You can. If you buy Affordable Care Act marketplace coverage before Medicare starts, conversion income raises the MAGI that determines your premium tax credit, and a large conversion can reduce or eliminate the subsidy. The exact clawback depends on then-current ACA rules, so pre-65 converters should model the subsidy alongside the tax before converting.
For early retirees between 60 and 65 on a marketplace plan, any advance credit received during the year may have to be repaid at filing if a conversion lifts MAGI, so the subsidy belongs in every projection.
Mistake 7: Do you understand the two 5-year rules?
There are two separate 5-year clocks. One governs tax-free earnings on your first Roth IRA. The other applies to each conversion: withdraw converted principal within five years and before age 59 and a half, and a 10% penalty can apply. Each conversion starts its own clock (Source: IRS Publication 590-B).
A saver who converts at 57 and withdraws that principal at 59 could owe the 10% penalty, while the same saver who waits past 59 and a half would not. Keeping a record of each conversion date and amount is the practical defense.
Mistake 8: Are you ignoring the pro-rata rule and Form 8606?
The pro-rata rule treats all your traditional, SEP, and SIMPLE IRAs as one pool, so if you hold pre-tax IRA money, a backdoor Roth conversion is mostly taxable rather than tax-free (Source: IRS Form 8606 Instructions). Reporting nondeductible basis correctly on Form 8606 each year is what keeps already-taxed dollars from being taxed twice.
The taxable share is the ratio of pre-tax dollars to total non-Roth IRA dollars on December 31, so a large existing rollover IRA can make almost the entire backdoor conversion taxable. Clearing pre-tax IRA balances into a workplace plan first is one way many savers sidestep it.
Mistake 9: Did you under-withhold or skip estimated taxes?
A conversion is not automatically covered by paycheck withholding, so failing to send estimated or withheld taxes can trigger an underpayment penalty even though you will owe the tax at filing (Source: IRS, Estimated Taxes). Meeting a safe-harbor amount through timely payments generally avoids the penalty.
The safe harbor generally means paying at least the prior year’s tax liability, or more for higher earners, through withholding or quarterly estimates. As noted in Mistake 2, for those under 59 and a half, a portion withheld from the conversion itself can be a penalized early distribution.
Mistake 10: Are you overlooking state tax, heirs, and the no-undo rule?
Most states tax a conversion as ordinary income, so a move to a lower-tax state before converting can change the cost. Conversions are also irreversible since 2017, and the inherited-IRA 10-year rule means the heir’s bracket matters (Source: IRS, Retirement Plans FAQs; SECURE Act, 2019).
Most non-spouse heirs must empty an inherited IRA within 10 years, so a traditional IRA can land large distributions on an heir in their peak earning years, while a Roth generally comes out tax-free. For charitably inclined savers past age 70 and a half, a qualified charitable distribution, available only from an IRA and capped at $111,000 in 2026, is an alternative (Source: IRS, 2026). A conversion also cannot be undone after 2017 and the window closes December 31, as our Roth conversion deadline page covers.
What Roth conversion mistakes do advisors themselves make?
Not every Roth conversion mistake belongs to the saver. Advisors make their own: waiting until required minimum distributions have already begun and the low-bracket window has closed, treating a conversion as a one-year decision rather than a multi-year bracket-filling plan, and converting without coordinating the move against RMD projections, charitable intent, and the estate plan.
Waiting too long is the most common: the widest low-tax window sits in the gap years between retirement and age 73, and once RMDs begin they add taxable income every year. Single-year thinking is next: spreading conversions across years can keep more of the balance in the 12% or 22% bracket than one large conversion that spills into 24%, which is why a multi-year Roth conversion analysis often beats a single decision. Failing to coordinate is the third: a conversion sized in isolation can undercut a qualified charitable distribution or ignore a future survivor year, when the widow or widower files single.
Worked example: how a $100,000 conversion stacks costs at once
In this hypothetical, a married couple, both 63 and retired, convert $100,000 in 2026. Sized to fill the 12% bracket with a small 22% spill, the conversion adds roughly $13,560 of federal tax and stays under the $218,000 IRMAA and the Social Security thresholds. A conversion large enough to push MAGI just over $218,000 would cross the IRMAA cliff for 2028 premiums. This is hypothetical, not a projection or a real client.
Consider a hypothetical joint-filing couple, both age 63, fully retired, deferring Social Security to 70, with $50,000 of other ordinary income in 2026.
| Item | Before conversion | After $100,000 conversion |
|---|---|---|
| Ordinary income | $50,000 | $150,000 |
| Less 2026 standard deduction | $32,200 | $32,200 |
| Taxable income | $17,800 | $117,800 |
| Federal income tax (approx.) | $1,780 | $15,340 |
| Bracket reached | 10% | 22% (small spill) |
The conversion adds about $13,560 of federal tax on $100,000, an average of roughly 13.6%, because most of it fills the 12% bracket and only the top slice reaches 22% (Source: IRS Rev. Proc. 2025-32). Post-conversion MAGI of about $150,000 stays below the $218,000 joint IRMAA threshold, and because they are not yet collecting Social Security, no benefits become taxable. A flat 5% state income tax would add about $5,000 more.
The cliff sits just out of frame. Converting enough to push MAGI just over $218,000 would move their Part B and D premiums two years later to IRMAA Tier 1. Had they been collecting Social Security, up to 85% of benefits could have become taxable; on ACA coverage, the higher MAGI could have clawed back their premium credit. Sizing the conversion to fill the bracket while staying under the cliffs is why a multi-year projection often beats one large conversion.
Roth conversion mistakes at a glance
The table below pairs each common Roth conversion mistake with who it tends to hit and the corrective rule. It summarizes the sections above and is not a substitute for modeling your own situation, which depends on your current bracket, your RMD trajectory, and your income relative to the IRMAA, Social Security, and ACA thresholds.
| Roth conversion mistake | Who it tends to hit | The corrective rule |
|---|---|---|
| Converting too much in one year | Anyone with a large pre-tax balance | Fill a target bracket; spread across years |
| Paying the tax from the IRA | Converters under 59 and a half | Pay the tax from outside savings |
| Not taking the RMD first | Owners age 73 or older | Take the full RMD before converting |
| Triggering the IRMAA surcharge | Near-Medicare converters (2-year lookback) | Keep MAGI below the next tier edge |
| Making Social Security taxable | Benefit recipients | Front-load conversions before benefits begin |
| Losing the ACA premium credit | Pre-65 marketplace enrollees | Model the subsidy alongside the tax |
| Misreading the two 5-year rules | Under-59.5 savers who may need the money | Track each conversion’s date and clock |
| Ignoring the pro-rata rule | Backdoor-Roth users with pre-tax IRAs | File Form 8606; clear pre-tax IRAs first |
| Under-withholding on the conversion | Retirees without wage withholding | Pay estimated tax to meet a safe harbor |
| Overlooking state tax and heirs | Relocators and those leaving IRAs to heirs | Weigh residency timing and the heir’s bracket |
| Single-year thinking (advisor error) | Anyone converting without a multi-year plan | Model conversions across several years first |
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Q3 Advisors is a registered investment adviser focused on retirement tax planning. This page is educational and is not advice; consult a qualified professional.
Frequently asked questions
What is the biggest mistake to avoid with a Roth conversion?
The most costly Roth conversion mistake is converting more in one year than your target bracket can absorb, because the surplus income can also trigger the IRMAA Medicare surcharge, tax more of your Social Security, and reduce a pre-65 ACA credit at the same time. Sizing the conversion to fill a chosen bracket without crossing those thresholds addresses most of the risk at once.
What are the most common Roth IRA mistakes?
The most common Roth IRA mistakes are converting too much in a single year, paying the conversion tax out of the IRA rather than from outside savings, converting an amount that trips the IRMAA, Social Security, or ACA thresholds, mishandling the pro-rata rule and Form 8606 on a backdoor Roth, and withdrawing converted principal within five years while under 59 and a half.
How can I avoid paying too much tax on a Roth IRA conversion?
Estimate your taxable income before converting, then convert only enough to fill a chosen bracket. In 2026 a joint filer stays in the 12% bracket up to $100,800 of taxable income and a single filer up to $50,400 (Source: IRS Rev. Proc. 2025-32). Paying the tax from outside savings and spreading conversions across low-income years before RMDs and Social Security begin further lowers the total tax.
When is the best time to convert to a Roth IRA?
The lowest-tax window is usually a year when your other income is temporarily low, most often the gap years between retirement and age 73, before required minimum distributions and Social Security add taxable income. Many plans convert during those years to fill a lower bracket. A market decline can also let you convert the same shares at a smaller taxable value.
Can Roth IRA mistakes affect my Social Security benefits?
Yes. Roth conversion income raises your provisional income, and up to 85% of Social Security benefits can become taxable as that figure rises (Source: IRS Publication 915). A conversion timed while you are already collecting benefits can make an extra portion of those benefits taxable, which is why many plans front-load conversions into the years before benefits begin.
When should you not do a Roth conversion?
A conversion is often reconsidered when the tax can only be paid from the retirement account itself, when you expect a clearly lower tax bracket soon, when you are close to a Medicare, Social Security, or ACA income threshold you would cross, or when you may need the money within five years and are under 59 and a half. Each situation is fact-specific and worth modeling before deciding.
