Direct Answer

Compare the marginal tax rate on the dollars changed by the IRA decision today with a range of marginal rates that could apply to Traditional IRA withdrawals later. Do not compare average tax rates, and do not assume today's statutory bracket is your retirement bracket. Build the future rate from expected income sources, filing status, state tax, and required distributions, then test whether the Roth-versus-Traditional result changes across plausible scenarios.

The Eight Steps

Step 1: Identify the current marginal rate

For 2026, federal individual marginal tax rates remain 10%, 12%, 22%, 24%, 32%, 35%, and 37%. See IRS: 2026 Tax Inflation Adjustments.

Your marginal rate is the rate that applies to the next affected dollar of taxable income. It is different from the average or effective rate calculated across all income.

A deductible Traditional IRA contribution changes taxable income at the margin. If the deduction crosses a bracket boundary, different portions can save tax at different rates.

Step 2: Verify that a Traditional deduction actually exists

Do not assign a current tax benefit until Traditional IRA deductibility is known.

For 2026, workplace-plan coverage plus modified adjusted gross income (MAGI) can reduce or eliminate the Traditional deduction. See IRS Publication 590-A.

If the contribution is nondeductible, the current marginal tax rate does not produce the same upfront Traditional benefit. A nondeductible Traditional contribution requires tracking basis on Form 8606.

Step 3: Build the retirement income stack

Estimate future taxable income sources that may arrive before the IRA withdrawal you are modeling:

• pension income;
• wages or consulting;
• taxable portions of Social Security;
• interest and other ordinary income;
• Traditional 401(k) withdrawals;
• Traditional IRA withdrawals;
• required minimum distributions;
• spouse income.

Then ask: which marginal bracket could the next Traditional IRA withdrawal occupy? That is more useful than comparing current salary with retirement spending.

Step 4: Use at least three future scenarios

A practical model uses:

1. lower-rate retirement;
2. similar-rate retirement;
3. higher-rate retirement.

Example:

• current combined marginal rate: 24%;
• future low case: 15%;
• future middle case: 22%;
• future high case: 29%.

The exact numbers are assumptions. The purpose is to see whether the account choice is robust or fragile across those scenarios.

Step 5: Add state tax

If current and retirement states differ, state tax can materially change the rate spread.

A 24% federal rate plus a 5% state marginal rate today is a different tax price from 24% federal in a state with no individual income tax later.

Use state-specific rules rather than assuming every state taxes IRA deductions and withdrawals identically. Some states exempt pension or retirement income entirely; others tax it in full.

Step 6: Include a survivor scenario when relevant

Married couples can eventually face single-filer tax brackets after one spouse dies.

If much of the household's income continues but filing status changes, the survivor can face higher marginal rates on the same dollar amount of income, because joint brackets are wider than single brackets.

Large retirement balances should therefore be tested under both joint and survivor scenarios.

Step 7: Treat tax law as uncertain, not unknowable

Current law is the appropriate base case.

Older retirement articles often assumed individual federal rates would automatically revert after 2025. IRS 2026 guidance reflects current legislation keeping the seven-rate structure in place. Future Congresses can still change rates.

The useful response is not "nobody knows, so ignore tax rates." It is: use current law as the base case and stress-test a reasonable range of possible future outcomes.

Step 8: Compare equal economic cost

Run the Swoopr Roth vs Traditional Calculator twice:

• equal account contribution;
• equal pre-tax household cost.

If Traditional is deductible, decide what happens to the current tax savings. If the tax savings are invested alongside the Traditional contribution, they belong in the Traditional strategy's future-value calculation. If they are spent, the household has effectively saved less for retirement.

Worked Example

Assume:

• current federal marginal rate: 24%;
• current state marginal rate: 5%;
• current combined approximation: 29%;
• full Traditional deduction available.

Retirement scenarios:

• Low: 12% federal + 0% state;
• Middle: 22% federal + 0% state;
• High: 24% federal + 5% state.

Traditional has a strong rate-timing advantage in the low scenario, a smaller advantage in the middle scenario, and little pure rate advantage in the high scenario.

If Roth structural factors are important to the household (no required minimum distributions, estate flexibility, income in conversion windows), they may outweigh a small Traditional advantage in the middle case. If the low case is highly credible, the current deduction becomes more compelling.

Questions That Improve the Forecast

• Will a pension fill lower tax brackets before IRA withdrawals begin?
• How large could pre-tax balances become, and what will required minimum distributions look like at 73+?
• When will Social Security begin, and how much will be taxable?
• Could retirement include paid consulting or part-time work?
• Which state is likely to be home in retirement?
• Could a surviving spouse eventually file single with similar assets and income?
• Is there a planned low-income period before Social Security begins that could allow Roth conversions?
• Will the household need to withdraw more than any required minimum distribution anyway?

Common Errors

• Using effective tax rate instead of marginal rate. The deduction or contribution affects dollars at the margin, not all income.
• Comparing current salary with future spending. The relevant comparison is current taxable income vs. future taxable income, not gross salary vs. spending level.
• Ignoring Traditional deduction eligibility. A nondeductible contribution does not carry the same upfront benefit as a deductible one.
• Ignoring pensions and required minimum distributions. These fill lower brackets and push additional withdrawals into higher ones.
• Ignoring state tax. Moving to a no-income-tax state in retirement can shift the rate spread materially.
• Treating a single forecast as certain. Use a range and evaluate robustness.
• Using outdated tax-law assumptions. Base the forecast on current law, not expired sunsets or anticipated future legislation.
• Forgetting survivor filing status. Single-filer brackets are narrower than joint brackets on the same income.
• Assuming Traditional tax savings are invested without modeling them. If savings are spent rather than invested, the Traditional strategy loses part of its compounding advantage.

Decision Rule

In the clean tax-timing model:

• Current marginal rate lower than future withdrawal rate: Roth factors strengthen. Paying tax now at a lower rate and growing tax-free is more favorable.
• Current marginal rate higher than future withdrawal rate: Deductible Traditional factors strengthen. Deferring tax to a lower rate is more favorable.
• Rates similar or uncertain: Contribution-cap, flexibility, and tax diversification factors deserve more weight than rate timing alone.

This rule applies to the tax-rate dimension only. Other factors, including access rules, required minimum distributions, estate treatment, and the ability to invest Traditional tax savings, can shift the result independent of the rate comparison.

Educational information only. A tax-rate forecast is a scenario, not a promise. Verify current rules with a qualified tax professional.

References

1. IRS: 2026 Tax Inflation Adjustments (including One Big Beautiful Bill amendments). Accessed 2026-09-05.
2. IRS: Publication 590-A, Contributions to Individual Retirement Arrangements. Accessed 2026-09-05.
3. IRS: Publication 590-B, Distributions from Individual Retirement Arrangements. Accessed 2026-09-05.
4. IRS: Retirement Plan and IRA Required Minimum Distributions FAQs. Accessed 2026-09-05.
5. Swoopr: Roth vs Traditional Calculator.