Traditional and designated Roth 401(k) contributions differ mainly in when federal income tax is generally paid. The better choice depends on current and future tax circumstances, plan rules and cash-flow needs—not a universal age rule.
Formula and decision rule
Simplified current federal tax reduction from a deductible traditional contribution = contribution × current marginal federal rate. Roth contributions generally do not create that current deduction; qualified Roth distributions follow different rules. This shortcut excludes state tax and deduction interactions.
Worked example
A worker making a $10,000 traditional contribution while in a 22% marginal federal bracket has a simplified current federal tax reduction of about $2,200. That does not make the contribution free, predict the retirement tax rate or include payroll tax, because traditional 401(k) contributions generally still face Social Security and Medicare tax.
Questions that change the choice
- Is the current marginal rate unusually high or low?
- Does the plan offer both contribution types and how are matches treated?
- Will retirement income, required distributions or state residency change?
- Can the worker afford the lower current take-home pay associated with Roth contributions?
Contribution limit discipline
The IRS employee deferral limit generally applies across traditional and designated Roth employee contributions to relevant plans, rather than giving a separate full limit to each type. Confirm current limits and plan administration.
Use the calculators
Project contributions and match with the 401(k) Calculator, then model current federal wages with the Income Tax Calculator.
Primary sources
Frequently asked questions
Do traditional 401(k) contributions reduce FICA?
Generally no. Social Security and Medicare wage treatment differs from federal income-tax deferral.
Can a person split contributions?
A plan may allow both types, subject to the combined applicable employee limit and plan terms.
Change log
Published August 18, 2026 with an explicit simplified tax example and no prediction of future tax rates.