The standard advice — “young people should use a Roth, older people should use a Traditional” — gets the surface right and the mechanism wrong. Once you understand the mechanism, you can make the call for your own situation in about 60 seconds.

What each one actually does

Traditional IRA:

  • You contribute pre-tax money (deducted from your taxable income this year)
  • Money grows tax-deferred
  • You pay ordinary income tax on every dollar you withdraw in retirement

Roth IRA:

  • You contribute after-tax money (no deduction this year)
  • Money grows tax-free
  • You pay zero tax on withdrawals in retirement

These two are mathematically identical if your tax rate is the same when you contribute as when you withdraw. They’re only different when those rates differ.

The one question that decides it

Will my marginal tax rate in retirement be higher or lower than it is today?

  • Higher in retirement → Roth wins. You’d rather pay tax at today’s lower rate.
  • Lower in retirement → Traditional wins. You’d rather pay tax at the future lower rate.
  • About the same → It doesn’t matter mathematically. Pick based on other factors (Roth’s flexibility, RMD rules, etc.).

The whole game is estimating “marginal tax rate in retirement.”

How to estimate that

Your retirement-year marginal rate depends on three things:

  1. How much taxable income you’ll have in retirement. Mostly: Social Security + traditional 401k/IRA withdrawals + any pension + taxable brokerage dividends + part-time work. Roth withdrawals don’t count.
  2. What tax brackets look like then. This is the unknowable part. Default assumption: similar to today. If you think rates will go up structurally, Roth gets more attractive.
  3. Your filing status. Married couples have wider brackets at the same dollar amount.

A rough way to estimate: pick a retirement spending target (say $80k/year for a single person, $120k for a couple). Subtract Social Security (estimate from ssa.gov, usually $25-35k for a single late-career earner). The rest is what you need from retirement accounts. That’s your taxable income in retirement. Look up the marginal rate today on that amount.

Real example

Say you’re 32, married, both spouses working, household income $180k. Marginal rate today: 22%.

You expect to retire at 65, spending $120k/year. Social Security at 67 might give you and your spouse $50k/year combined. So you’d need about $70k/year from accounts. With the standard deduction (~$30k for married filing jointly in current dollars), taxable income is about $40k. That puts you in the 12% bracket.

Today: 22%. Retirement: 12%. Difference: 10 percentage points in favor of Traditional.

For this couple, Traditional wins by a noticeable margin: contribute pre-tax now, pay 12% in retirement, save 10% of every dollar contributed.

When Roth wins anyway

Even when the math says Traditional, Roth has structural advantages worth considering:

  • No required minimum distributions (RMDs). Traditional forces you to start withdrawing at 73; Roth doesn’t. If you expect to leave money to heirs, Roth lets it grow longer.
  • Tax-rate insurance. If you’re wrong about future tax rates being lower, Roth caps your downside.
  • Penalty-free contribution withdrawal. You can pull out Roth contributions (not earnings) at any age without penalty. This makes Roth a more flexible vehicle if you’re not 100% sure you’ll keep the money locked up until 59½.
  • Income-limit-free via the backdoor. High earners can’t contribute to Roth directly but can use the “backdoor Roth” conversion. Traditional has no such workaround beyond the deductibility phase-out.

The shortcut

If you’re in the 22% bracket or higher and married, Traditional is usually right.

If you’re in the 12% bracket or lower, Roth is almost always right (you’re paying tax at near-bottom rates now; retirement won’t be lower).

If you’re in the 22-24% bracket and single, it’s genuinely close. Default to Roth for the flexibility and the tax-rate insurance unless you’re confident your retirement spending will be modest.

The lazy correct answer

Split. Half of your IRA contribution to each. Your future self will be glad to have both buckets — Traditional for the year you want to keep taxable income low (large medical expense, deciding when to start Social Security), Roth for the year you want a big tax-free chunk (down payment for a second home, paying off a mortgage early). Diversification of tax treatment matters at retirement as much as diversification of assets.

What this does not cover

  • Backdoor Roth mechanics (separate article)
  • 401k vs IRA priority (different question, covered separately)
  • Roth conversions in lower-income years (the powerful intermediate move)
  • State income tax (varies; can shift the math by a few percentage points)

If any of those affect your situation materially, the simple decision rule above is a starting point, not a stopping point. But for the basic IRA question, it’s enough.