Planning a Roth Conversion? Here's the One Thing You Need to Know.

A Roth conversion could be a savvy move in the context of your retirement. By moving funds from a traditional retirement account to a Roth IRA, you can set yourself up to benefit from tax-free gains and withdrawals.

Plus, Roth IRAs don't force savers to take required minimum distributions. That could spare you a world of headaches down the line, not to mention give you more options in the context of estate planning.

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But one thing you must understand about a Roth conversion is that it's a taxable event. And depending on your situation, that tax bill could have big consequences.

The problem with raising your taxable income

If you're not working or are working part-time and are doing a modest Roth conversion, the tax consequences may not be so extreme. But if you're doing a Roth conversion at the same time you're earning a decent salary, and you're moving hundreds of thousands of dollars into a Roth IRA, you could end up with a host of repercussions.

First, a large conversion could push you into a higher tax bracket, thereby costing you more. Let's say you're 65 and are still earning $90,000 a year. If you're single, that would place you in the 22% tax bracket.

Now, let's say you add a $200,000 Roth conversion to your income. Suddenly, you're in the 35% tax bracket, which means that conversion may be costing you more than expected.

Also, if you're receiving Social Security benefits, a larger Roth conversion could leave you on the hook for taxes on those monthly checks. That's because taxes on Social Security hinge on your income.

Finally, if you're still working, you may have health insurance through your employer. But if you're planning to stop working within the next year or two and switch over to Medicare, your Roth conversion could cause a problem.

Though there's a standard monthly premium for Medicare Part B, higher earners are assessed surcharges on those premiums known as IRMAAs, or income-related monthly adjustment amounts. If your taxable income this year is $290,000 between your salary and Roth conversion, you could end up paying more for Medicare two years from now, since IRMAAs have a two-year lookback period.

Make sure to look at the big picture

A Roth conversion can be a powerful financial planning tool. But it's important to recognize that it's a taxable event, and one that could have consequences beyond a larger tax bill in a single year.

If you're moving a large sum of money into a Roth IRA, it could pay to spread your conversion over a few tax years. That could help keep your tax bills more manageable and potentially allow you to avoid consequences like IRMAAs later on.

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