7 Roth Conversion Mistakes That Cost Retirees the Most

A Roth conversion is one of the few tax moves a retiree can control from year to year. It is also one of the easiest to get wrong. Most of the costly errors come from rules that sit outside the conversion itself: how IRA balances are added together, Medicare premium brackets, how Social Security benefits are taxed, and timing rules that leave no room to fix a mistake. Below are seven of the most expensive ones, each with a example household and a way to avoid it.

1. Ignoring the Pro-Rata Rule Taxes Money You Thought Was Tax-Free

When the IRS figures the tax on an IRA distribution, it handles all individual retirement plans as one contract and values them as of the close of the calendar year (IRC Section 408(d)(2)). You can’t convert only your after-tax dollars.

Hypothetical: Pat has $100,000 across traditional IRAs, including $20,000 of nondeductible basis, and converts $20,000. Step 1: $20,000 ÷ $100,000 = 20% of the conversion is tax-free. Step 2: $20,000 − ($20,000 × 20%) = $16,000 is taxable. The fix: Before converting, add up every traditional, SEP and SIMPLE IRA balance you hold, and track your basis on Form 8606.

2. Withholding the Tax From the Conversion Costs Younger Retirees Extra

Any amount the custodian withholds never reaches the Roth. For someone under age 59½, IRS guidance warns that a 10% additional tax may apply to the taxable part (including an amount equal to the tax withheld) that isn’t rolled over. Hypothetical: A 57-year-old converts $50,000 and has $10,000 withheld. That $10,000 is taxed as income and can also face the penalty. The fix: Pay the tax from cash held outside the IRA, and make estimated payments if needed so you don’t owe an underpayment penalty.

3. Converting Before Taking the RMD Breaks the Rules

Treasury Regulation 1.408A-4, Q&A-6, says that the first dollars distributed during that year are treated as consisting of the required minimum distribution, and that an RMD can’t be rolled over. If you put an RMD into a Roth, it is handles as a regular contribution. Hypothetical: A retiree with a $15,000 RMD converts $40,000 in January. The first $15,000 counts as the RMD and is not a valid conversion. The fix: Take the full RMD first, then convert.

4. Crossing an IRMAA Line Raises Medicare Premiums

The standard Part B premium is $202.90 a month in 2026. Joint filers with modified AGI above $218,000 and up to $274,000 pay $284.10, plus a Part D surcharge of $14.50. Hypothetical: A couple with $200,000 of MAGI converts $50,000. Step 1: $200,000 + $50,000 = $250,000, which falls in that level. Step 2: ($284.10 − $202.90) × 12 months × 2 spouses = $1,948.80 in extra Part B premiums at 2026 rates. The fix: Size each conversion so it stops just below a bracket limit.

5. Extra Income Makes More Social Security Taxable

A converted amount counts as income for all purposes, which includes the formula for taxing Social Security benefits. For 2025 returns, the joint-filer base amount is $32,000. Up to 85% of benefits can be taxed once half of benefits plus other income tops $44,000. Hypothetical: A couple receives $30,000 in benefits and has $10,000 of other income.

Step 1: $15,000 + $10,000 = $25,000, which is below the base amount, so none of the benefits are taxed. Step 2: Adding a $20,000 conversion takes the total to $45,000, past the 85% limit. The fix: Run the Social Security worksheet with and without the conversion before you decide on an amount.

6. Assuming a Conversion Can Be Undone Leaves No Exit

IRS Publication 590-A says conversions made in tax years beginning after December 31, 2017, cannot be recharacterized, and IRC Section 408A(d)(6)(B)(iii) excludes conversions from the recharacterization rule. Hypothetical: A retiree converts $80,000 and then sees the market fall 20%. The tax is still owed on the full $80,000. The fix: Convert in stages over several years instead of all at once.

7. Converting Too Much in One Year Pushes Income Into Higher Brackets

In tax year 2026, joint filers pay 22% on taxable income over $100,800 and 24% over $211,400. Hypothetical: A couple with $80,000 of taxable income converts $100,000. Step 1: $100,800 − $80,000 = $20,800 is taxed at 12%. Step 2: $180,000 − $100,800 = $79,200 is taxed at 22%. The fix: Fill a target bracket each year. Remember that converted amounts count as income for the taxable year in which the amount is distributed or transferred, so the transfer has to be complete by December 31. State income tax, where it applies, comes on top of the federal bill.

Decide Whether a Conversion Fits Before the Year Closes

Conversions usually to make the most sense for people whose current bracket is lower than the one they expect later, who can pay the tax from cash outside the IRA, and who have years left for the Roth to grow. Those quiet years between a last paycheck and the first required withdrawal are often the cheapest window to convert, which is the whole subject of our free Roth guide. They often don’t suit:

• People who would have to use IRA money to pay the tax
• Retirees sitting just below an IRMAA or Social Security limit
• Anyone who expects to be in a lower bracket later or plans to leave the IRA to charity
• People who will need the money within a few years

Roth IRAs have no required distributions while the owner is alive, which helps with estate planning. That benefit only holds up if the tax cost was planned before the account owner signed the conversion paperwork.

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