Should you contribute pre-tax to a Traditional 401(k) or after-tax to a Roth 401(k)? Compare both options side by side based on your current tax rate, expected retirement tax rate, contribution amount, and time horizon to see which leaves you with more spendable income.
The Roth vs. Traditional 401(k) decision is the IRA decision at workplace-401(k) scale. Same mechanics: Traditional contributions reduce your current-year taxable income and grow tax-deferred (taxed on withdrawal in retirement); Roth contributions are made with after-tax dollars and grow tax-free (no tax on withdrawal). The right choice depends primarily on whether your marginal tax rate today is higher or lower than what you expect in retirement.
The 401(k) scale changes the stakes. The 2025 employee contribution limit is $23,500 ($31,000 if 50+, with super-catch-up for ages 60–63 raising it further). At those contribution levels, a 5–10 percentage point difference between current and retirement tax rates compounds into hundreds of thousands of dollars over a 25–30 year career. The decision matters more in a 401(k) than in an IRA simply because the dollars are larger.
This calculator handles the comparison carefully, including the often-overlooked "fair comparison" issue: if the Traditional path produces upfront tax savings (because $23,500 in Traditional costs less out of pocket than $23,500 in Roth), invest those savings in a taxable account at the same rate. Without that adjustment, the comparison is unfair — Traditional looks worse than it actually is. With it, the comparison shows the true mathematical equivalence (or lack thereof) between the two paths under your specific rate assumptions.
Age 30, $23,500/year for 35 years, 7% return. Current marginal rate: 32% (federal) + 5% (state) = 37% Expected retirement rate: 22% Roth projected after-tax: $3,475,000 Traditional projected after-tax (with tax-savings invested): $3,920,000 Traditional wins by ~$445,000. The 15-point rate gap (37% now → 22% later) makes Traditional dominate. This counterintuitive result (Roth often favored for young people) flips here because the current bracket is unusually high.
Age 40, $23,500/year for 25 years, 7% return. Current marginal rate: 22% (federal) + 5% (state) = 27% Expected retirement rate: 22% Roth projected after-tax: $1,485,500 Traditional projected after-tax (with tax-savings invested): $1,535,000 Essentially a tie ($50K difference on $1.5M). At similar rates, the mathematical advantage is negligible. The tiebreaker often comes down to non-mathematical factors: optionality of tax-free Roth in retirement, RMD avoidance, estate planning.
Age 25, $15,000/year for 35 years, 7% return. Current marginal rate: 12% federal + 4% state = 16% Expected retirement rate: 24% federal + 4% state = 28% Roth projected after-tax: $2,215,000 Traditional projected after-tax: $1,755,000 (even with tax-savings invested) Roth wins by ~$460,000. The low current rate (16%) gives up only a small tax break now, while the higher retirement rate (28%) makes the Traditional withdrawal much more expensive later. Classic young-low-bracket Roth case.
Use this calculator at every annual benefits enrollment to confirm your 401(k) contribution type still matches your tax situation. The right answer can shift over time: a Roth choice that made sense at age 25 (low bracket) may no longer be optimal at 40 (higher bracket). Reviewing annually catches drift.
Also use it when planning major income changes: promotions that push you into a higher bracket, sabbaticals or partial retirement that drop you into lower brackets, geographic moves that change state tax rates. Any meaningful change to current or future tax rate makes the calculation worth re-running.
Pair this with the 401(k) calculator (the broader retirement projection including employer match), the IRA calculator (which has its own Roth-vs-Traditional decision), the Roth-vs-Traditional calculator (the simpler comparison without specific 401(k) limits), and the income-tax-estimator (to confirm your current marginal rate).
A subtle but important point about the employer match: matching contributions are always made on a pre-tax basis, regardless of whether your contribution is Roth or Traditional. The match grows tax-deferred and is taxed on withdrawal. This means even pure-Roth contributors end up with some Traditional balance (the match portion). Total retirement balance is usually a hybrid by default.
Tax diversification is the often-overlooked best practice. Doing 50/50 (or some other split) between Roth and Traditional creates two buckets in retirement. In low-tax years (early retirement before Social Security, gap years), withdraw from Traditional to fill low brackets. In high-tax years (big RMDs, large purchases, expected rate increases), withdraw tax-free from Roth. The flexibility is worth more than perfectly optimizing one bucket given uncertain future rates.
Estimate your 401(k) balance at retirement with employer matching and investment growth.
Compare Roth and Traditional IRA/401(k) to see which is better for you.
Compare Traditional vs Roth IRA growth and tax implications over time.
Calculate when you can achieve Financial Independence and Retire Early (FIRE).
Estimate your pension benefit based on years of service, salary, and benefit formula.
Calculate substantially equal periodic payments for early retirement distributions without penalty.
Roth 401(k) After-Tax
$1,353,529
Traditional After-Tax
$1,404,014
Winner
Traditional
Difference
$50,485
| Year | Traditional Balance | Roth Balance | Taxable Savings |
|---|---|---|---|
| 1 | $21,400.00 | $21,400.00 | $5,136.00 |
| 2 | $44,298.00 | $44,298.00 | $10,631.52 |
| 3 | $68,798.86 | $68,798.86 | $16,511.73 |
| 4 | $95,014.78 | $95,014.78 | $22,803.55 |
| 5 | $123,065.81 | $123,065.81 | $29,535.80 |
| 6 | $153,080.42 | $153,080.42 | $36,739.30 |
| 7 | $185,196.05 | $185,196.05 | $44,447.05 |
| 8 | $219,559.77 | $219,559.77 | $52,694.35 |
| 9 | $256,328.96 | $256,328.96 | $61,518.95 |
| 10 | $295,671.99 | $295,671.99 | $70,961.28 |
| 11 | $337,769.03 | $337,769.03 | $81,064.57 |
| 12 | $382,812.86 | $382,812.86 | $91,875.09 |