The difference between Roth and traditional 401(k) on your paycheck is the income tax timing. With a traditional contribution, your income tax drops now — reducing the net paycheck cost. With a Roth contribution, you pay income tax on the amount now, getting no current break. At $70,000 salary and a $7,000 contribution, traditional costs $46 less per biweekly paycheck than Roth — $2,010 vs. $1,964 per period. The retirement accounts grow the same; the difference is when the IRS gets paid.
Use the 401(k) paycheck calculator to compare both options side by side with your salary and filing status.
Core Difference in One Table
| Feature | Traditional 401(k) | Roth 401(k) |
|---|---|---|
| Contributions | Pre-tax | After-tax |
| Current income tax | Reduced | No reduction |
| FICA impact | None | None |
| Withdrawals in retirement | Taxed as income | Tax-free |
| Income limits | None | None (unlike Roth IRA) |
| Annual limit (2026) | $24,500 combined | $24,500 combined |
Both types count toward the same $24,500 annual elective deferral limit. You can split contributions however you want — for example, $15,000 traditional and $9,500 Roth.
Worked Example: $7,000 Contribution at $70,000 Salary
Single filer, biweekly payroll, no state income tax. Numbers computed with the tax engine.
| No 401(k) | Traditional $7,000 | Roth $7,000 | |
|---|---|---|---|
| Annual gross | $70,000 | $70,000 | $70,000 |
| Pre-tax 401(k) | — | $7,000 | — |
| Federal taxable wages | $53,900 | $46,900 | $53,900 |
| Federal income tax | $6,570 | $5,380 | $6,570 |
| FICA (7.65% on $70,000) | $5,355 | $5,355 | $5,355 |
| Roth 401(k) (after tax) | — | — | $7,000 |
| Annual take-home | $58,075 | $52,265 | $51,075 |
| Annual take-home reduction | — | $5,810 | $7,000 |
| Per-biweekly-check reduction | — | $223 | $269 |
| Net/biweekly paycheck | — | $2,010 | $1,964 |
The Roth costs $46 more per biweekly paycheck for the same $7,000 contribution. The traditional saves $1,190 in federal income tax ($6,570 − $5,380); the Roth saves nothing now.
How Each Type Is Taxed at Retirement
This is where the long-term tradeoff lives:
Traditional: You pay income tax on withdrawals at your retirement tax rate. If you withdraw $50,000 in a year with no other income, the 2026 standard deduction covers $16,100, so only $33,900 is taxable — probably at 10% and 12% rates. Many retirees are in lower brackets than during their working years.
Roth: Qualified withdrawals (account at least 5 years old, age 59.5 or older) are tax-free. If your retirement income would push you into a high bracket — for example, Social Security, a pension, and traditional 401(k) withdrawals combined — Roth income does not add to that.
The break-even rule of thumb: If your tax bracket will be the same at retirement as now, the two options are mathematically equivalent. If higher later, Roth wins. If lower later, traditional wins.
The Marginal Rate on the Contribution Matters
The traditional 401(k) saves you at your marginal rate now. In the $70,000 example (single, standard deduction), taxable income without 401(k) is $53,900. The 22% bracket runs from $50,400 to $105,700. The $7,000 contribution removes $3,500 from the 22% bracket ($770 savings) and $3,500 from the 12% bracket ($420 savings), for a total of $1,190 in savings — not a flat 22%.
At lower incomes where the contribution stays entirely in the 12% bracket, the tax savings per dollar is lower, making Roth relatively more attractive (you pay 12% now to avoid 12% later — a wash — but Roth gives you certainty against future rate increases).
Who Should Lean Traditional
- Currently in the 24% or higher federal bracket
- Expecting a lower income in retirement (e.g., no pension, modest Social Security)
- Wanting to reduce MAGI now to preserve other deductions or credits that phase out with income
Who Should Lean Roth
- Currently in the 10% or 12% bracket — tax rates are low, locking in makes sense
- Young workers with many decades of tax-free growth ahead
- Expecting higher income in retirement
- Planning to leave the account to heirs (Roth has no required minimum distributions during the owner’s lifetime)
The Catch-Up Roth Requirement (SECURE 2.0)
Starting in 2026, SECURE 2.0 requires catch-up contributions to be made as Roth contributions if your FICA wages from the same employer were more than $150,000 in 2025 (IRS Notice 2025-67; IRS catch-up contributions page). This applies to the catch-up amount only, not the base $24,500. Check with your plan administrator whether this applies to your situation, as implementation details can vary by plan.
For how contributions affect your net pay alongside an HSA or FSA, see HSA and FSA paycheck savings. For how the base limits work, see 401(k) contributions and take-home pay.