A Roth conversion can be one of the most effective tools in retirement tax planning. It can also be one of the most expensive mistakes you make if it isn't planned correctly.

At Summit Investment Advisors, our tax planning team runs multi-year tax projections for clients to model different Roth conversion strategies before a single dollar is converted. Over the years, we've seen the same handful of mistakes trip people up. These mistakes can mean paying tens of thousands of dollars more in taxes than necessary.

Here are seven of the most common ones, and what to do instead.

1. Converting Without a Multi-Year Tax Projection

People fixate on getting the conversion amount exactly right this year. That's not actually the question that matters. The real mistake is picking any number before you've looked at how it plays out over time.

A Roth conversion creates taxable income today in exchange for tax-advantaged growth potential later. Without a projection that maps out your income, tax brackets, and account balances for years into the future, you're just guessing. We've walked clients through side-by-side comparisons showing that paying more in taxes over the next few years can mean paying far less in taxes over the rest of retirement. It's not an easy number to look at upfront, but it's the number that matters.

What to do instead: Before converting anything, ask for a multi-year projection that shows your total lifetime tax liability under a few different conversion scenarios, not just next year's bill.

2. Ignoring How a Conversion Affects Your Withholding

Your retirement income doesn't show up the same way a paycheck does. It's coming from several places at once: Social Security, maybe a pension, required distributions, and if you're converting, a chunk of taxable income from your IRA. Nobody's coordinating the withholding across all of that for you. So it's easy to end up underpaying, especially that first year or two after you retire or do a conversion.

We see this all the time. People show up to a tax workshop because they got hit with a bill they didn't expect, and it's almost never because they did anything wrong on purpose. They just didn't realize withholding on one income source doesn't account for what's happening with the others.

What to do instead: Any year you do a conversion, review your withholding and estimated payments across every income source, not just the conversion itself.

3. Forgetting About the Social Security Tax Trap

Many retirees don't realize that converting can push more of their Social Security benefit into taxable territory. Under current IRS rules, once your combined income passes certain thresholds, up to 85% of your Social Security benefit can become taxable:

• Individual filers: 50% of benefits become taxable at $25,000 of combined income; 85% becomes taxable above $34,000
• Joint filers: 50% of benefits become taxable at $32,000 of combined income; 85% becomes taxable above $44,000

A large conversion in a single year can be the difference between a small portion of your Social Security being taxed and nearly all of it being taxed.

What to do instead: Model your Social Security taxation alongside any conversion amount, not as a separate calculation.

4. Overlooking Medicare (IRMAA) Surcharges

This one can catch you off guard. Medicare doesn't look at your income this year to set your premiums; it looks back two years. So if a conversion pushes you over an IRMAA threshold, you won't feel anything right away. The surcharge shows up two years later, and it sticks around for the whole year, gradually padding your Part B and Part D premiums.

For 2026, that first IRMAA tier kicks in at $109,000 for single filers and $218,000 for joint filers. And because IRMAA works on a cliff, not a gradual scale, going even a dollar over the line triggers the full surcharge for that tier. There's no partial penalty for barely crossing it.

What to do instead: If you're on Medicare or approaching it, check your projected income two years out before finalizing a conversion amount.

5. Doing One Large Conversion Instead of a Multi-Year Strategy

Doing the whole conversion in one year sounds simpler, but it can be more expensive. Convert everything at once and you're likely to shove yourself into a much higher tax bracket than if you'd spread it out over a few years. And it's not just the bracket. A big one-time conversion can trip the Social Security and IRMAA issues we just talked about, all in the same year, all stacking on top of each other.

That's why we don't usually recommend converting it all at once. We build it out year by year, filling up a specific bracket each year and stopping before it spills into the next one.

What to do instead: ask whether a multi-year strategy, one that fills your current bracket each year without pushing into the next, makes more sense than doing it all in one shot.

6. Waiting Too Long or Not Accounting for Required Minimum Distributions (RMDs)

Under current law, RMDs generally begin at age 73 (rising to 75 for those born in 1960 or later). Once RMDs start, they add to your taxable income automatically, which reduces your flexibility to control how much additional income you take on through a conversion in any given year.

Some of the best conversion opportunities happen in the years after retirement but before RMDs begin, when income (and tax brackets) may be temporarily lower.

What to do instead: If you're retired or newly retired and haven't started RMDs yet, this window is worth evaluating for conversions before it closes.

7. Going It Alone Instead of Coordinating Your Full Financial Picture

A Roth conversion doesn't happen in isolation. It touches your investment accounts, your Social Security timing, your Medicare costs, and your estate plan. When your financial advisor and tax preparer aren't working from the same information, decisions get made in a vacuum and that's when costly mistakes happen.

What to do instead: Make sure whoever is running your conversion numbers has visibility into your complete financial picture: investments, income sources, healthcare costs, and estate planning goals, not just your tax return. 

Frequently Asked Questions About Roth Conversions

Is a Roth conversion a good idea in retirement? It depends entirely on your individual tax situation, income sources, and timeline. A conversion that makes sense for one household can be the wrong move for another. A multi-year tax projection is the only way to know.

How much should I convert each year? There's no fixed number. Many conversion strategies are built around filling up your current tax bracket each year without spilling into a higher one, but this depends on your full income picture, including Social Security and Medicare considerations.

Do Roth conversions affect Medicare premiums? Yes. Because Medicare premiums are based on income from two years prior, a conversion this year could raise your Medicare premiums two years from now if it pushes you over an IRMAA threshold.

When is the best time to do a Roth conversion? Often, the years after retirement but before required minimum distributions begin (currently age 73) offer a window where income and tax brackets may be lower, making conversions more efficient.

Ready to See What a Roth Conversion Strategy Could Look Like for You?

Every household's tax situation is different, and the mistakes outlined above are the ones we see most often because they're easy to miss without a full picture of your finances. If you'd like to talk through your specific situation, contact us to start the conversation.

Disclosure: Traditional IRA account owners have considerations to make before performing a Roth IRA conversion. These primarily include income tax consequences on the converted amount in the year of conversion, withdrawal limitations from a Roth IRA, and income limitations for future contributions to a Roth IRA. In addition, if you are required to take a required minimum distribution (RMD) in the year you convert, you must do so before converting to a Roth IRA.