Key takeaway
The decision is a guess about your future tax rate plus a preference for certainty now versus flexibility later.
The core tradeoff
A Traditional 401(k) lowers your taxable income today—contributions go in pre-tax, and you pay ordinary income tax when you withdraw in retirement. A Roth 401(k) uses after-tax money—no deduction now, but qualified withdrawals, including all the growth, come out completely tax-free.
Unlike IRAs, both 401(k) types share the same high annual contribution ceiling and are offered through an employer. The decision is again about when you pay the tax: now (Roth) or later (Traditional).
When Roth 401(k) tends to look stronger
Roth usually wins when:
- You are early career and your current bracket is low, so paying tax now is cheap.
- You expect a higher bracket later, from career growth, required withdrawals, or other retirement income.
- You want tax-free flexibility in retirement, pulling from a pool that adds nothing to your taxable income.
- Your employer offers a match—note the match always goes into a pre-tax Traditional portion regardless of which you choose.
Because the contribution limit is the same, a Roth 401(k) is effectively a bigger gift: you pay tax on the contribution but shield all future growth from tax forever.
When Traditional 401(k) can be compelling
Traditional shines when the deduction helps this year:
- You are in a high bracket now and the deduction drops you into a lower one or under a threshold that protects other tax breaks.
- You expect to spend down to a lower bracket in retirement, so deferring tax is the better math.
- You want to lower current taxable income to qualify for credits or deductions phased out by income.
The tradeoff is that every withdrawal is taxed, and required minimum distributions start in your early seventies, forcing taxable income whether you need the cash or not.
A quick example with real numbers
Suppose you contribute $10,000 a year and it grows to $80,000 by retirement. In a Roth 401(k), that $80,000 comes out tax-free. In a Traditional 401(k), the same $80,000 is taxed as ordinary income on withdrawal. At a 22% retirement bracket, the Traditional leaves you about $62,400 of spending power from that pool, while the Roth leaves the full $80,000.
The flip side is real too. If you are in the 24% bracket today and drop to 12% in retirement, the Traditional deduction saved you 24% up front and you pay only 12% later—a clear win for Traditional. The choice is a bet on two tax rates separated by decades.
State tax adds another layer
Federal rules get attention, but state tax can change the math. Some states do not tax Roth withdrawals, strengthening Roth for residents there. Others tax Traditional withdrawals but gave no deduction when you contributed. If you might retire in a different state, that uncertainty favors spreading across both.
Common mistakes to avoid
- Assuming Roth is always better because "tax-free" sounds nice, without checking your current bracket.
- Forgetting the match is pre-tax. Even if you contribute Roth, the employer match is Traditional and taxed later.
- Ignoring RMDs on the Traditional portion starting in your early seventies.
- Picking one forever. You can often change the split each year as your income changes, and many people use both.
The real question
The comparison is not "which account is better" but a guess about your future tax rate plus a preference for certainty now versus flexibility later. If you cannot predict your retirement income with confidence, splitting between the two—or shifting toward Roth while young—is a reasonable hedge.
For the IRA version of this same decision, see Roth vs Traditional IRA.
Related next steps
Model growth with the 401k Calculator, compare the two side by side with the Roth IRA Calculator, and read how a 401(k) works for the contribution mechanics.