Dale retired from a federal agency in March at 58, after a buyout offer he couldn't turn down. His wife, Priya, was still working part-time. By December, their household income looked strange: a partial year of salary, a modest FERS annuity, and almost nothing else. The next year would be lower still. Dale's tax preparer looked at the return and said something that stuck: "You're in a tax bracket you may never see again."

That sentence is the whole idea behind strategic Roth conversions. Some years, your taxable income dips well below its long-term norm. Those years are cheap places to pay tax you would otherwise pay later at a higher rate.

What a conversion does

A Roth conversion moves money from a pre-tax account, like a traditional IRA or the traditional balance in your TSP, into a Roth account. You owe ordinary income tax on the amount converted in the year you do it. After that, qualified withdrawals are tax-free, and the money isn't subject to required minimum distributions in a Roth IRA or the Roth TSP.

The trade is simple. You pay tax now, at a rate you can see, to avoid paying tax later, at a rate you can only guess.

Why low-income years are the right moment

Tax brackets are stacked. The first slice of your taxable income is taxed at 10%, the next at 12%, and so on. In a low-income year, you may have unused room in the lower brackets. That room disappears when the year ends. You can't save it for later.

Think of it as filling a bucket. If Dale and Priya's taxable income after deductions is $40,000 and the top of the 12% bracket for joint filers sits in the mid-$90,000s (the exact figure changes every year, so check irs.gov), they have roughly $55,000 of room. Converting $55,000 from a traditional IRA would cost about $6,600 in federal tax at 12%.

Now picture Dale at 75. Social Security, his FERS annuity, Priya's pension, and required minimum distributions from a large traditional balance could push the couple into the 22% or 24% bracket. Converting at 12% today might save them ten or twelve cents on every dollar tomorrow. On $55,000, that's several thousand dollars. Repeat it for a few years and the savings compound.

Where the low-income years show up

They don't always announce themselves. Common ones include:

  • The gap between retirement and Social Security. Many federal retirees stop working in their late 50s or early 60s and delay benefits. Their income is just the annuity, if that.
  • The gap before RMDs begin. RMDs start at 73 for most people today and at 75 for those born in 1960 or later. Confirm the current rules at irs.gov.
  • Job loss, a sabbatical, or a career change.
  • Military separation. A service member leaving active duty for school or a slow civilian start may have a thin income year.
  • A year with large deductions or business losses.

Converting inside the TSP

The TSP now allows in-plan Roth conversions. You can move money from your traditional TSP balance to your Roth TSP balance, and the amount converted is taxable. There is a minimum conversion amount, and conversions can't be undone once processed. Read the current rules at tsp.gov before you act, because details and forms get updated.

Taxes matter here. If you're under 59½, don't have the TSP withhold the tax from the conversion itself. Money withheld is treated as a distribution and could trigger the 10% early withdrawal penalty. Paying the tax from savings outside the TSP keeps the full amount working in the Roth, which is the point.

The TSP doesn't let you convert in tiny, precise slices the way a large IRA custodian can, so some people prefer to convert from an IRA when they need to land on an exact figure. Others rolled their TSP into an IRA for that reason. That's a real tradeoff, because the TSP's low fees are hard to beat. Weigh them both.

The traps to check first

Medicare IRMAA

Medicare Part B and D premiums rise at certain income levels, and the government looks back two years. A large conversion at 63 can raise your premiums at 65. Check the current thresholds at medicare.gov or ssa.gov before converting near that age.

ACA subsidies

If you buy health insurance on the marketplace before Medicare, a conversion raises your modified adjusted gross income and can shrink or eliminate premium tax credits. For early retirees, this can cost more than the tax itself. Run the numbers before the end of the year, not after.

The pro-rata rule

If you hold both pre-tax and after-tax money in traditional IRAs, the IRS treats any conversion as a proportional mix of the two. You can't pick only the after-tax dollars. This catches people who made nondeductible contributions years ago.

Social Security taxation

Conversion income can push more of your Social Security benefit into the taxable column. If you're already collecting, include this in your estimate.

The five-year rule

Each conversion has its own five-year clock for penalty-free withdrawal of the converted amount if you're under 59½. Most people converting in their late 50s don't plan to touch the money soon, but know the rule exists.

State taxes

Some states tax conversions and some don't. If you may move in retirement, that matters.

A practical way to plan it

Start with a rough projection of your taxable income for the year. Your tax software or last year's return is a decent base. Subtract your deductions, then see how much room remains in the bracket you're willing to fill.

Don't aim to fill it exactly. Income estimates miss. A late dividend, a capital gain distribution, or a bonus can push you over. Many people convert in the fall, when the year is clearer, or do it in stages.

Then decide where the tax money will come from. Converting $55,000 and paying $6,600 out of a savings account is a very different move from converting $55,000 and using $6,600 of the converted funds to pay the bill.

Finally, think about the next ten years, not just this one. A string of moderate conversions often beats one big one. Spread across several low-income years, you stay in the lower brackets and avoid IRMAA cliffs.

When conversions don't make sense

If you expect your tax rate to be lower in retirement, conversion may lose money. If you'd have to pay the tax from the retirement account itself, the math weakens. If you plan to leave the account to a charity, the tax angle disappears, because charities don't pay income tax on inherited traditional accounts. And if you're near a cliff in health insurance or Medicare, the "cheap" conversion may not be cheap at all.

Dale and Priya ran the numbers, converted about $45,000 that year, and left a cushion in case Priya's hours changed. It wasn't dramatic. But every year, Dale looks at that Roth balance growing without a tax bill attached, and he's glad the lower bracket didn't go to waste.

Talk with a tax professional who understands federal retirement before you pull the trigger, and confirm the current figures at irs.gov and tsp.gov.