7 Roth Conversion Mistakes That Cost Pre-Retirees Thousands

Roth conversions are among the most powerful tax moves available to pre-retirees — and among the easiest to execute badly. The mistakes below aren't hypothetical: they're the patterns fee-only advisors encounter repeatedly when clients arrive after converting on their own. Each one is quantifiable, and each one is avoidable.

Mistake 1: Converting Too Much in a Single Year

The most common error. A large one-time conversion feels efficient — get it done, move on. The problem is that conversion income stacks on top of all other ordinary income, and the federal bracket boundaries are exact cliffs, not ramps.

The math: Take a couple with $60,000 of other income (interest, dividends, pension) in 2026. After the $32,200 standard deduction,¹ their taxable income before any conversion is $27,800. The 22% bracket ceiling is $201,050 of taxable income,² so their optimal conversion stays at or below $173,250 this year.

If they convert $500,000 instead:

• $173,250 is taxed at 22% — the rate they planned on.
• $182,850 of the excess falls in the 24% bracket (ceiling $383,900).
• $143,900 of the excess falls in the 32% bracket — paying 10 percentage points more than necessary.

Cost of the bracket spillover: roughly $18,000 of federal tax that a multi-year spread would have avoided entirely. For a $2M traditional IRA, a single-year "get it over with" conversion can waste $40–60K in unnecessary tax vs. a calibrated annual plan.

The right approach: calculate bracket headroom each year, convert to that limit, and repeat. The bracket-filling calculator does this arithmetic in real time.

Mistake 2: Paying the Conversion Tax from the IRA Itself

Custodians let you withhold taxes directly from the IRA during conversion. It feels convenient. It's usually a mistake.

When you withhold 22% from a $200,000 conversion, only $156,000 reaches the Roth. The $44,000 withheld goes directly to the IRS — and it's also treated as a taxable distribution, so you effectively converted just $156,000 while paying taxes on $200,000 of income. You get less Roth growth and the same tax bill.

If you pay the $44,000 from outside savings instead, the full $200,000 earns tax-free inside the Roth. At 7% annual return over 20 years, that $44,000 compounds to roughly $170,000 inside the Roth — versus approximately $135,000 net if the same $44,000 had been left in a taxable account and grown subject to capital gains tax.

Mistake 3: Forgetting IRMAA's 2-Year Lookback

IRMAA (Income-Related Monthly Adjustment Amount) surcharges Medicare Part B and Part D premiums based on your MAGI from two years prior. A 2026 Roth conversion affects your 2028 Medicare premiums. Most people don't track this connection.

The 2026 IRMAA Tier 1 threshold is $218,000 MAGI for MFJ / $109,000 for single filers.³ Crossing it adds $81.20/month/person in Part B surcharges alone — $1,948.80/year for a couple, before Part D surcharges.

The trap in numbers: A couple with $80,000 of other income has $138,000 of IRMAA headroom ($218K − $80K). Converting $155,000 crosses Tier 1 by $17,000. The tax "savings" on that last $17,000 of conversion (say, avoiding 32% RMDs later) might be worth $5,440 over time. The extra IRMAA cost: ~$2,000/year for 2028 and potentially 2029 — roughly equal or worse. The $17,000 conversion that tipped the tier was a net loser.

The correct calculation compares the lifetime NPV of the additional conversion after subtracting the IRMAA surcharge cost — not just the tax rate differential. Use the IRMAA-aware calculator to find your precise headroom.

Mistake 4: Converting During the Social Security Phase-In Zone

Social Security benefits aren't fully taxable — but they're also not fully exempt. Up to 85% of benefits become ordinary income once your "combined income" (AGI + nontaxable interest + ½ of SS benefits) exceeds $44,000 for MFJ.⁴

The dangerous zone is the phase-in range between $32,000 and $44,000 of combined income. Inside this band, each additional dollar of Roth conversion income does two things at once: it adds $1 of taxable income directly, and it makes an additional $0.85 of SS benefits taxable. At a 22% marginal rate, the effective cost per dollar of conversion in this zone is 22% × 1.85 = 40.7% — nearly double the headline rate.

This "SS tax torpedo" is especially common for couples who have modest baseline income but start the year just below the $44K combined income threshold. A $30,000 conversion that looks cheap can trigger $25,500 of additional SS income in the calculation, with a combined tax cost well above the stated bracket rate.

See the full mechanics in the SS and Roth conversion guide. The fix: model the SS taxable income formula before sizing the annual conversion, and target conversions that land below — or substantially above — the phase-in cliff rather than landing in the middle of it.

Mistake 5: Converting Before Moving to a No-Income-Tax State

Nine states impose no income tax on ordinary income.⁵ Many pre-retirees plan to move from California, New York, or New Jersey to Florida, Texas, or Nevada. If that move is coming within a year or two, converting before the move means paying state income tax on every dollar converted — unnecessarily.

The cost: California's top rate runs to 13.3%; New York to 10.9%. On a $200,000 Roth conversion in California vs. one year later in Florida: $200,000 × ~10.3% (CA rate at this income) = $20,600 of avoidable state tax. Waiting 12 months for the move saves that entire amount.

The calculus reverses if the move is far away and the conversion window is short — deferring 5 years to save state tax might cost more in federal bracket opportunity than it saves. But for a confirmed move within 12–24 months, delaying conversions is almost always the right call. Be precise about when residency changes, since states have specific rules about domicile — a partial year of California residency can still generate California tax on that year's conversions.

Mistake 6: Skipping the Pro-Rata Check

Many pre-retirees have made nondeductible (after-tax) contributions to traditional IRAs over the years. They assume they can convert those after-tax dollars tax-free. The IRS doesn't see it that way.

Under IRC § 408(d)(2), all traditional IRAs you own are treated as a single pool for conversion tax purposes. The ratio of after-tax basis to total IRA value determines what fraction of each conversion is tax-free — regardless of which specific IRA you draw from.⁶

Example: You have a $950,000 rollover IRA (entirely pre-tax) and a $50,000 IRA with $50,000 of nondeductible basis. Total pool: $1,000,000. After-tax ratio: 5%. Converting $100,000: only $5,000 is tax-free — not $50,000. The remaining $45,000 of your basis is still trapped, spread across the whole pool.

The solution, if you want to convert the after-tax portion cleanly: roll your pre-tax IRA into a 401(k) that accepts incoming rollovers (most employer 401(k)s and many solo 401(k)s do). That leaves the IRA holding only after-tax basis, which you can then convert tax-free. This must be done in the same tax year the conversion is planned. See the full explanation in the pro-rata rule guide.

Mistake 7: The OBBBA Senior Deduction Phase-Out Trap (2026–2028)

This is a new wrinkle most advisors haven't fully priced in yet. The One Big Beautiful Bill Act (OBBBA, July 2025) created a $6,000 per-person deduction for taxpayers age 65 and older, available for tax years 2025–2028. For a married couple both 65+, that's $12,000 of additional deduction space — reducing taxable income and potentially increasing conversion headroom.

The catch: the deduction phases out between $150,000 and $250,000 of MAGI for joint filers. Every dollar of income in that range reduces the deduction by $0.12. At a 22% marginal rate, the effective marginal tax on income in the $150K–$250K MAGI band is 22% × 1.12 = 24.6% — not 22%.

The correct approach: identify whether you're eligible for the OBBBA senior deduction, calculate whether your planned conversion MAGI lands in the $150K–$250K phase-out zone, and adjust the conversion amount to either stay below $150K or push above $250K where the phase-out no longer applies.