Marcus spent 31 years as a civilian engineer at an Army installation and put every TSP dollar into the traditional balance. The tax deduction felt good every payday. At 62, with a FERS pension, a FERS supplement, and a $1.1 million TSP balance, he sat down with a tax preparer and learned something that stung. His first-year retirement income was close to what he had earned while working. Worse, his TSP withdrawals pulled more of his Social Security into taxable income.

Nobody did anything wrong. Marcus just never asked a question that matters more than most investment choices: Is a tax break worth more to me now, or later?

The basic trade

With traditional TSP, you skip taxes today and pay them on every dollar you withdraw. With Roth TSP, you pay taxes today and, if you meet the rules, withdraw everything tax-free, including decades of growth.

Same account, same funds, same investment options. The only thing that changes is when the IRS takes its share.

If your tax rate were identical now and in retirement, the two would land in roughly the same place. That is the part people miss. The choice is not about which is "better." It is a bet on your future tax rate compared with your current one.

A quick example with real numbers

Say you are married, filing jointly, and your marginal bracket is 22%. You want to put $10,000 toward the TSP this year.

  • Traditional: Your take-home pay drops by about $7,800 (the $10,000 less the roughly $2,200 in tax you avoid).
  • Roth: Your take-home drops by the full $10,000.

Assume the money triples over the years. The traditional balance becomes $30,000, and if you withdraw it at 12%, you keep $26,400. If you withdraw it at 22%, you keep $23,400. The Roth balance of $30,000 is yours entirely, but you paid $2,200 more up front.

So traditional wins when your retirement bracket is lower than your working bracket. Roth wins when it is higher. When they match, it is a wash.

Bracket thresholds and standard deductions change every year, so check irs.gov for the current tables before you run your own numbers.

When Roth tends to make more sense

You are early in your career. A GS-7 in the 12% bracket has a good chance of earning more later. Paying 12% now to avoid 22% or 24% down the road is a fair trade.

You expect a big retirement income. A FERS annuity, a Social Security benefit, and a large TSP can stack up fast. Many federal retirees are surprised to find they land in the same bracket they had while working.

You want to keep taxable income low in retirement. Traditional withdrawals count as income. That can make more of your Social Security taxable and can push you into higher Medicare Part B and D premiums, known as IRMAA. Qualified Roth withdrawals do not count toward either calculation. For a retiree trying to stay under certain thresholds, that flexibility has real value.

You serve in a combat zone. Service members who contribute from tax-exempt combat zone pay already skip tax on the way in. Directing that money to Roth means the earnings can come out tax-free too, assuming the withdrawal is qualified. That is hard to beat. Check tsp.gov for the exact treatment of tax-exempt contributions.

You plan to leave money behind. Heirs generally inherit Roth balances free of income tax. Traditional balances come with a tax bill for them, and most non-spouse beneficiaries have to empty inherited accounts within ten years.

When traditional tends to make more sense

You are at peak earning power. A GS-15 or a senior officer in a 32% or 35% bracket who expects a lower bracket in retirement gets a bigger deduction now than they are likely to pay later.

You live in a high-tax state and plan to retire to a low-tax one. The deduction saves you state tax today, and you might owe little or none when you withdraw.

You need the tax cut right now. If paying down debt or covering a mortgage depends on that bigger paycheck, the traditional deduction is not a small thing. A plan you can sustain beats a perfect plan you abandon.

You will have modest retirement income. If your pension is small and your TSP is your main income, you may fill the lower brackets for years. In that case, the deduction at a higher rate now is a bargain.

Rules specific to the TSP

A few details trip people up.

  • Agency matching always goes to traditional. Even if all your own contributions are Roth, the 1% automatic and up to 4% matching contributions land in your traditional balance. So you will end up with some of both no matter what.
  • No income limit. Unlike a Roth IRA, a Roth TSP is open to high earners.
  • The five-year rule. To withdraw earnings tax-free, you must be at least 59½ and at least five years must have passed since January 1 of the year you made your first Roth TSP contribution. Disability and death also qualify.
  • No required minimum distributions on Roth TSP balances. Starting in 2024, Roth balances in employer plans no longer face RMDs during your lifetime. Traditional balances still do, at an age set by your birth year, so confirm the current rule at irs.gov.
  • Conversions are possible. The TSP now allows in-plan Roth conversions, and you can also move money to a Roth IRA after separation. Either way, you owe income tax on the converted amount in the year you do it.
  • Contribution limits change yearly. The elective deferral limit and the age-50 catch-up amount are on tsp.gov.

A sensible way to decide

If you feel stuck, stop trying to predict the future. Nobody knows what Congress will do with tax rates, and your own circumstances will shift too.

Start by estimating your likely retirement income: pension, Social Security, any rental or part-time income, and a realistic TSP withdrawal. Then compare that bracket with your current one. If the gap is large in either direction, lean that way. If it is close, split.

A split is not a cop-out. Holding both Roth and traditional balances gives you control later. In a year when you have extra deductions, you can draw more from traditional. In a year when income spikes, you can pull from Roth and avoid tipping into a higher bracket. Accountants call this tax diversification, and it is worth more than it sounds.

Then revisit your election every few years, or after a promotion, marriage, divorce, or move. You can change your contribution mix at any time, so none of this is permanent.

Marcus did not make a mistake. He made a one-sided bet. The fix for most people is not to guess better, but to spread the bet while the decision is still easy to change.