Guide
What are the most common Roth conversion mistakes?
By Secure Retirement PlannerUpdated
If you convert to a Roth in 2026, your most costly mistakes are jumping into a higher tax bracket or Medicare surcharge tier. Paying the tax out of the converted money is the other big one.
Since 2018 you can't undo a conversion. So a mistake you make in 2026 stays on your 2026 tax return.
Most of these mistakes come from not seeing your whole picture before December 31. The table below lists the common ones, what they can cost you, and how to avoid them.
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The short version
- Size the conversion to a bracket or Medicare line. Don't guess.
- Pay the tax from savings if you can. Withholding from the conversion shrinks the Roth, and under 59½ it can add a 10% penalty.
- Take your RMD first. An RMD can't be converted.
- If you have after-tax money in any IRA, the pro-rata rule decides what's taxable.
- You can't undo a conversion, so check IRMAA, Social Security taxes and the senior deduction before you convert.
What are the biggest Roth conversion mistakes?
The biggest one is converting without knowing where your income will land. Every dollar you convert is added to your other income for the year and taxed at your top rate.
Say you and your spouse have $90,000 of 2026 taxable income and convert $50,000. You pay 12% on the first $10,800 and 22% on the other $39,200. That's about $9,920 in federal tax. If you had stopped at $10,800, your tax would have been about $1,296.
| Mistake | What it can cost | How to avoid it |
|---|---|---|
| Converting past a bracket line | The top dollars taxed at 22%, 24% or more | Find your room in the bracket before converting |
| Going $1 over a Medicare IRMAA tier (age 63+) | About $1,148 per person for one year at the first 2026 tier | Leave a cushion below the tier line |
| Paying the tax from the conversion | Less money in the Roth, and a 10% penalty under 59½ | Pay from savings or other withholding |
| Trying to convert your RMD | The RMD can't be converted; excess goes in as a 6% excess contribution | Take the RMD first, then convert |
| Ignoring the pro-rata rule | Tax on money you thought was tax-free | Add up all traditional, SEP and SIMPLE IRAs first |
| Forgetting estimated tax | IRS underpayment penalty | Make a payment or raise withholding |
| Waiting until late December | Missing your custodian's cutoff | Start by early December |
Should I pay the conversion tax from the IRA?
If you can, pay it from savings or a brokerage account. Then the full amount goes into the Roth and grows tax-free.
If you have tax withheld from the conversion, the withheld part never reaches your Roth. It counts as a regular IRA withdrawal. If you're under 59½, that part can also owe the 10% early withdrawal tax. Your IRA custodian often withholds 10% by default unless you choose otherwise, so check your form.
Can I convert my required minimum distribution?
No. Your RMD age is 73, or 75 if you were born in 1960 or later. Once you reach it, the first dollars out of your IRA each year count toward your RMD. RMDs aren't eligible for rollover, so they can't go into a Roth.
If you convert in January before taking your RMD, part of the conversion is treated as the RMD. That part lands in your Roth as an excess contribution. It owes a 6% tax each year until you remove it. Take your full RMD first, then convert.
What is the pro-rata rule and why does it trip people up?
If you ever made nondeductible (after-tax) contributions to a traditional IRA, you can't convert just that money tax-free. The IRS treats all your traditional, SEP and SIMPLE IRAs as one pot, valued on December 31 of the conversion year. Each of your conversions is part tax-free and part taxable in the same ratio.
Example: you have $90,000 of pre-tax money and $10,000 of after-tax basis across your IRAs. You convert $10,000. Only 10% of it ($1,000) is tax-free. The other $9,000 is taxable.
Watch for two related traps. If you roll an old 401(k) into an IRA before December 31, your pot gets bigger and your tax-free share gets smaller. And you must file Form 8606 for each year you convert or have basis. If you don't, the IRS may not know about your after-tax money.
What hidden costs of a Roth conversion do people forget?
Your federal bracket is only part of the cost. A conversion raises your adjusted gross income, and several other things key off that number.
- Your Medicare premiums two years later, through IRMAA. See Roth conversions and IRMAA.
- Up to 85% of your Social Security becoming taxable. See Conversions and Social Security.
- The senior deduction for 2025 to 2028: $6,000 per person 65+, cut by 6% of income above $75,000 (single) or $150,000 (joint), per person.
- ACA premium tax credits if you're under 65 and buy marketplace coverage.
- State income tax, which depends on your state.
What timing mistakes should I avoid?
Your 2026 conversion must be finished by December 31, 2026, and your custodian may want paperwork earlier. There's no April extension.
If you are under 59½ and need the money within five years, watch the clock. Each conversion has its own five-year clock for the 10% penalty. See The 5-year rules.
Plan your tax payment too. If your withholding and estimated payments fall short, the IRS can charge you an underpayment penalty. Your fourth-quarter estimated payment for 2026 is due January 15, 2027.
When is doing a Roth conversion the mistake?
Sometimes the conversion itself is the wrong move for you. Converting makes sense when your tax rate now is lower than the rate the money would face later. It may not pay off if:
- Your income in retirement will be low enough that RMDs stay in the 10% or 12% bracket.
- You plan to leave the IRA to charity, which pays no income tax on it.
- Your heirs will likely be in a lower bracket than you.
- You'd have to use IRA money to pay the tax.
- You need the money soon, or you're moving to a state with lower or no income tax.
Example: a 58-year-old pays the tax from the conversion
Bob is 58 and converts $50,000 from his traditional IRA. He asks the custodian to withhold 20%, or $10,000, for taxes.
Only $40,000 lands in the Roth. The $10,000 withheld is a regular withdrawal. Because he's under 59½, it can owe a $1,000 early withdrawal tax. He owes income tax on all $50,000 either way.
If Bob had paid the tax from savings, the full $50,000 would have gone into the Roth.
If you have savings outside your IRA, using them to pay the tax keeps more of your money growing tax-free in the Roth.
| Tax withheld from conversion | Tax paid from savings | |
|---|---|---|
| Amount in the Roth | $40,000 | $50,000 |
| 10% early withdrawal tax | $1,000 | $0 |
| Roth after 15 years at 6% | about $95,862 | about $119,828 |
Hypothetical example. Assumes 6% yearly growth with no withdrawals and no exception to the 10% tax. It doesn't count what the $10,000 of savings would have earned if Bob had kept it. Your results will differ.
Common questions
Can I undo a Roth conversion if I made a mistake?
No. Recharacterizing (undoing) a conversion has not been allowed since 2018. Once the conversion is done, the tax for that year is owed.
Is it a mistake to convert my whole IRA at once?
Often, for larger balances. One big conversion can push the top dollars into the 24% bracket or higher. Spreading it over several years can keep more of it in lower brackets.
Do I need to file Form 8606 for a Roth conversion?
Yes, generally. Form 8606 reports conversions from traditional, SEP and SIMPLE IRAs, and tracks any after-tax basis.
Is it a mistake to convert when the market is down?
Not by itself. Converting when values are lower means less tax for the same number of shares. The risk is that the market can keep falling after you convert, and you can't undo it.
Can I convert before taking my RMD if I take the RMD later in the year?
No. The first dollars out of the IRA count toward the RMD. Take the full RMD first, then convert.
Does a Roth conversion trigger the 10% penalty if I'm under 59½?
Not on the amount converted. The penalty can apply to any amount withheld for taxes, and to converted money you take out within five years.
Sources
Facts on this page were last checked on November 9, 2026.
- IRS Instructions for Form 8606 · Who must file, December 31 value of all traditional, SEP and SIMPLE IRAs for the pro-rata rule. Read Nov 9, 2026.
- IRS Publication 590-A, Contributions to IRAs · Conversion rules, excess contributions and the 6% tax. Read Nov 9, 2026.
- IRS Publication 590-B, Distributions from IRAs · RMDs not eligible for rollover; 10% additional tax on early distributions. Read Nov 9, 2026.
- IRS: Rollovers of retirement plan and IRA distributions · Default 10% IRA withholding, RMDs can't be rolled over. Read Nov 9, 2026.
- IRS: IRA FAQs, recharacterization of IRA contributions · Conversions can't be recharacterized after 2017. Read Nov 9, 2026.
- IRS Rev. Proc. 2025-32 (2026 inflation adjustments) · 2026 brackets used in the bracket example. Read Nov 9, 2026.
- CMS: 2026 Medicare Parts A & B premiums and deductibles · 2026 IRMAA tiers. Read Nov 9, 2026.
Who wrote this
This guide was written and fact-checked by the Secure Retirement Planner editorial team using IRS, CMS and Social Security sources listed above. We update guides when the law or yearly limits change. How we research.