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Q: I've read a couple of your articles on converting a traditional individual retirement account to a Roth IRA. Would such a conversion add to the income included in calculating the tax I owe on my Social Security benefits?—Howard Groopman, Portland, Ore.

A: In most cases, money taken out of a traditional IRA is considered ordinary income. That means funds withdrawn as part of a Roth conversion will be included in your adjusted gross income and will be counted when calculating any tax owed on your Social Security benefits. The exception is if you made any nondeductible contributions, in which case a portion of your withdrawals wouldn’t be subject to tax.

As such, a large Roth conversion could result in a whopping tax bill that includes higher federal taxes on your Social Security benefits. If you live in one of the states that taxes Social Security income, you could also face higher state taxes on those benefits.

“I always tell clients to do a tax projection before they convert, so that they can see what the tax bill will be,” says Dana Levit, a financial planner with Paragon Financial Advisors in Newton, Mass. If that bill looks too high, Levit tells clients to consider converting gradually, moving only part of their traditional IRA assets to a Roth in any one year. (There is also an option to undo a Roth conversion, which could be particularly valuable this year.)

Depending on your income and filing status, up to 85% of your Social Security benefit can be taxable. However, figuring out exactly how much of your benefit will be taxed is complicated, since there are so many variables.

Start by considering the impact of your conversion on your tax bracket. Let’s assume, for example, you're a married couple filing jointly and have adjusted gross income of $50,000 and taxable income of $30,000. That would put you in the 15% marginal federal income tax bracket. If you were to convert a $50,000 traditional IRA to a Roth, your taxable income would more than double. It would also bump you into the 25% marginal bracket, which is the rate that will apply as you add income.

The portion of your Social Security income subject to federal tax is based on something called “combined income.” It’s calculated using your adjusted gross income plus nontaxable interest such as income from municipal bonds and half of your Social Security income, says Grafton “Cap” Willey, a managing director in the Providence, R.I., office of CBIZ Tofias, an accounting, tax and consulting firm. Your benefits can be taxable if your combined income is greater than $25,000 for individuals and $32,000 for those married and filing jointly.

IRS Publication 915 includes worksheets and examples of how the tax on Social Security benefits is calculated. There are also a variety of online calculators that will do the math for you, such as this one from CalcXML.

Of course, once you convert to a Roth—which is funded with after-tax dollars—qualified withdrawals are not subject to income tax and will not affect the tax owed on your Social Security benefits.