The one thing that actually decides it
Everything you read about Roth vs Traditional boils down to a single comparison: your marginal tax rate today vs. your marginal tax rate when you withdraw. If your rate today is higher than your rate in retirement, Traditional wins. If your rate today is lower, Roth wins. If they are identical, they are mathematically identical — the "tax-free growth" of a Roth and the "tax-deferred growth" of a Traditional produce the same after-tax dollars when rates are equal.
Most households underestimate future taxes. Our last 20 years have seen the lowest federal brackets in US peacetime history. The 22% bracket today may be 28% in 15 years. That is why Roth often wins for people in the 22% and 24% brackets even when the math "on paper" favors Traditional — the scenario insurance against higher future rates is valuable.
Worked example: 35-year-old, $7,000/year, 25 years
At 24% today and 22% in retirement, $7,000/year for 25 years at 7% grows to $442,726 in either account. The Roth delivers the full $442,726 tax-free. The Traditional is worth $345,326 after 22% tax on withdrawal — about $97,000 less. But the Traditional also got a $1,680/year tax deduction ($42,000 total over 25 years). If that $1,680 was invested in a taxable account at 7% (with a 15% drag on returns), it would grow to roughly $97,000 — almost exactly the tax difference.
Flip the tax rates (15% now, 24% later, typical for someone in their 20s): Traditional is now worth $336,472 after tax, Roth still $442,726. The Traditional "tax savings" from the deduction only total $26,250 ($1,050/year) — far short of the $106,000 tax gap. Roth wins by $80,000+.
When Traditional still makes sense
Three situations where Traditional genuinely beats Roth:
- Peak-earning years, 32% bracket or higher. You fully expect a lower retirement bracket and want the deduction now.
- You need the deduction to afford the contribution. If Roth means contributing $4,000 and Traditional means contributing $7,000 (because the deduction makes it painless), Traditional wins on raw dollars saved.
- You plan Roth conversions in retirement. Park pre-tax money now, convert in low-income years between retirement and RMDs (age 73) at 12% or 22% rates.
For most working-age Americans below the 32% bracket, the rational move is split the difference: Roth the IRA for tax flexibility and scenario insurance, Traditional the 401(k) for the deduction (since 401(k) limits are higher and Roth 401(k) adoption is spotty). That gives you three tax pools in retirement — taxable, tax-deferred, and tax-free — and the ability to blend withdrawals for the lowest bracket each year.
What the comparison tool does not capture
Four real-world effects that tilt further toward Roth beyond the pure math:
- No RMDs on Roth. Traditional forces withdrawals at 73; Roth never does. If you have other income, Roth lets you skip a year and let it grow.
- Roth is better for heirs. Inherited Roth IRAs are tax-free to the beneficiary; inherited Traditional is fully taxable and must be drained in 10 years (SECURE Act) at the heir's top rate.
- Roth protects against legislative risk. Congress can raise rates or change rules on pre-tax accounts; a Roth is pre-paid and much harder to un-tax.
- Medicare premiums (IRMAA) key off MAGI. Traditional withdrawals raise MAGI and can cost $1,000–$6,000/year in Medicare surcharges. Roth withdrawals don't.
Those four are each worth 1–2% of expected annual return in retirement for a typical retiree — not captured in the spreadsheet, but real money.