Roth 401(k) vs traditional 401(k): how to decide which bucket to fund

A traditional 401(k) contribution reduces your taxable income now and is taxed when you withdraw it. A Roth 401(k) contribution gives you no deduction now and, if the distribution is qualified, is never taxed again. Everything else being equal, the right choice is whichever side of that trade puts you in the lower marginal rate — traditional if your rate is higher now than it will be in retirement, Roth if the reverse.

Everything else is rarely equal. Four other rules move the answer, and for higher earners one of them stopped being optional this year.

The arithmetic, and why "equal rates" is a genuine tie

If your marginal rate is identical when you contribute and when you withdraw, the two accounts produce exactly the same after-tax result. That is not an approximation; it falls out of the commutative property of multiplication. A dollar taxed at 22% before it goes in and then grown, and a dollar grown and then taxed at 22% on the way out, end at the same place.

This strips away most of the arguments people make. "Tax-free growth" is not an advantage of the Roth when rates match. The traditional account's growth is untaxed too; the government simply keeps a share of the whole balance at the end. The genuine differences are the rate you pay and a handful of structural rules.

There is one real asymmetry worth naming. The contribution limit is shared between the two, and it is expressed in gross dollars. Putting the maximum into a Roth 401(k) costs you more take-home pay than putting the maximum into a traditional one, because you are also paying the tax on it out of pocket. So for someone who genuinely maxes out, the Roth shelters more economic value inside the plan. For someone contributing a fixed percentage of salary well below the cap, that advantage does not exist.

The rules that actually differ

	Traditional 401(k)	Roth 401(k)
Contribution	Pre-tax; reduces current taxable income	After-tax; no current deduction
Qualified withdrawal	Fully taxable as ordinary income	Tax-free, contributions and earnings
Income limit to contribute	None	None
Annual contribution limit	Shared across both; set by the IRS and changes each year	Same shared limit
Employer contributions	Pre-tax by default	Only Roth if the plan offers it and you elect it
Lifetime RMDs from the account	Yes, from the required beginning age	No, since the 2024 tax year
Five-year rule	Not applicable	Applies before a distribution is qualified
Effect on current-year AGI	Lowers it	No effect

The "no income limit" line is the one most often missed. A Roth IRA phases out for higher earners, which is why the backdoor Roth IRA exists. A Roth 401(k) has no such phase-out. A high earner locked out of direct Roth IRA contributions can still put the full elective deferral into a Roth 401(k) with no workaround at all.

The employer match is not automatically Roth

This is the most common misunderstanding about Roth 401(k)s. Historically, employer matching and non-elective contributions always went into a pre-tax account, even when every dollar of your own money went to the Roth side. Section 604 of the SECURE 2.0 Act changed that for contributions made after 29 December 2022, allowing plans to offer Roth treatment of employer contributions.

Three conditions attach. The plan has to offer it, and many still do not. You have to elect it. And the amount is includible in your income for the year it is contributed, reported on a Form 1099-R with no withholding taken from it, so the tax has to come from somewhere else.

If your plan does not offer it, or you do not elect it, you end up with a split account: Roth deferrals on one side, pre-tax match on the other. That is arguably a feature rather than a problem, but it means "I contribute to the Roth 401(k)" does not mean the whole balance is Roth.

The five-year rule people get wrong

A distribution from a designated Roth account is qualified — meaning the earnings come out tax-free — only if two things are true: a five-taxable-year participation period has elapsed, and you are at least 59½, disabled, or deceased.

The clock starts on the first day of the taxable year for which you first made a designated Roth contribution to that plan. Three details follow that regularly cost people money:

• Plan-to-plan rollovers carry the clock over. Roll a designated Roth account into a new employer's designated Roth account and the original start date follows it.
• Rolling to a Roth IRA does not carry the clock over. The Roth IRA's own five-year clock governs instead. If you have never held a Roth IRA, rolling a fifteen-year-old Roth 401(k) into a brand-new Roth IRA starts a fresh five-year period. Opening a Roth IRA with a token amount years before you need it is a cheap way to have that clock already running. The IRA-side mechanics are in the Roth IRA five-year rule.
• Non-qualified distributions are pro-rated. Take money out of a designated Roth account before it qualifies and each dollar is part contribution and part earnings, in proportion to the account. This is not the Roth IRA ordering rule, where contributions come out first. In a plan there is no contributions-first order to rely on.

RMDs, and what changed in 2024

Designated Roth accounts used to be subject to required minimum distributions during the owner's lifetime, which made no sense — the money was already taxed, and the rule mostly served to push people into rolling the balance to a Roth IRA purely to escape it.

Section 325 of SECURE 2.0 removed the pre-death RMD requirement for designated Roth accounts in 401(k), 403(b) and governmental 457(b) plans, effective from the 2024 taxable year. Roth balances in a workplace plan now behave like a Roth IRA on this point: nothing has to come out during your lifetime.

The traditional side is unchanged. Pre-tax balances still generate RMDs from the required beginning age, calculated on the whole balance, whether or not you need the money. That distinction is one of the strongest structural arguments for holding at least some Roth: it is the only large retirement bucket you fully control the timing of. See required minimum distribution age for how the calculation works.

Catch-up contributions must now be Roth for higher earners

If you are old enough to make catch-up contributions and you earned above a threshold at the employer sponsoring your plan, you no longer get to choose.

Section 603 of SECURE 2.0 requires catch-up contributions to be made on a Roth basis where the participant's prior-year Social Security wages from that employer exceed a set threshold. Treasury and the IRS issued final regulations on 16 September 2025 (T.D. 10033), and compliance is generally required from 1 January 2026, with limited good-faith relief in the regulations running into 2027.

The details that matter in practice:

• The test uses FICA wages reported by the plan-sponsoring employer for the prior year, not your total household income and not your adjusted gross income. Someone self-employed with no FICA wages from the sponsoring employer is outside the test.
• The threshold was set at $145,000 in the statute and is indexed in $5,000 increments, so the figure that applies to a given year needs confirming rather than assuming.
• It applies to 401(k), 403(b) and governmental 457(b) plans. It does not apply to SEP or SIMPLE plans.
• If the plan offers no Roth option at all, affected participants may not be able to make catch-up contributions.

For anyone in that band, part of the decision has already been made. The mechanics of the catch-up itself are covered in catch-up contributions.

The second-order effects that usually decide it

Comparing marginal rates today against a guessed rate thirty years out is not a satisfying way to make a decision. These considerations are more tractable, because they are about structure rather than forecasting.

Traditional contributions lower this year's AGI; Roth contributions do not. That single dollar of AGI reduction can matter more than the rate comparison if you are sitting just above a threshold — a phase-out, a credit, or a deduction that disappears.

Roth withdrawals stay out of the measures that trigger other costs. Qualified Roth distributions do not enter provisional income for taxing Social Security benefits, and they do not enter the MAGI that sets the Medicare surcharge two years later. Traditional withdrawals do both. That is a second layer of cost on the traditional side that a headline bracket comparison misses entirely. See Medicare IRMAA and tax-free retirement income.

The surviving spouse files single. A married couple splitting withdrawals across joint brackets can find the survivor pushed into narrower single brackets on income that barely fell. Roth balances blunt that. The arithmetic is set out in the widow's penalty.

State income tax may differ between now and later. You get the traditional deduction against the state you live in today and pay the tax to the state you live in at withdrawal. States that don't tax retirement income covers the shape of that.

Rate uncertainty runs both ways. Holding both is not a fudge. It is the only position that lets you fill low brackets from the traditional side in a given year and top up from Roth without adding to taxable income.

An illustrative comparison

The following is a simplified illustration to show the shape of the trade, not a projection. Assume $10,000 of gross salary directed to the plan, a 7% annual return over 20 years, and a single lump withdrawal at the end.

	Traditional	Roth
Amount into the plan	$10,000	$10,000
Out-of-pocket cost at a 24% current rate	$7,600	$10,000
Balance after 20 years at 7%	~$38,700	~$38,700
Tax on withdrawal at a 12% retirement rate	~$4,640	nil
After-tax result	~$34,060	~$38,700
Same, but at a 24% retirement rate	~$29,410	~$38,700

Read the second-to-last row carefully. Even at the lower retirement rate the Roth ends ahead — but only because the Roth column quietly cost $2,400 more out of pocket at the start. Invest that $2,400 outside the plan and the traditional column catches up. This is the trap in most online comparisons, and it is why the equal-rate case really is a tie unless you are contributing at the cap.

Figures are illustrative and rounded, ignore state tax, and assume no employer contributions. Returns, tax rates and contribution limits change; run your own numbers before acting.

Frequently asked questions

Can I contribute to both a Roth and a traditional 401(k) in the same year?

Yes, if your plan offers both. The annual elective deferral limit is shared between them, so you are splitting one allowance rather than getting two. Many plans let you set a percentage split and change it during the year.

Is a Roth 401(k) better than a Roth IRA?

They do different jobs. The Roth 401(k) has a much higher contribution limit and no income phase-out, but the investment menu is whatever the plan offers. The Roth IRA has broader investment choice and friendlier rules on withdrawing contributions. Many people use both, and tax-advantaged retirement accounts sets out how they fit together.

Should I switch my existing pre-tax balance to Roth?

That is a conversion, and it is a separate decision with its own tax bill in the year you do it. Some plans allow in-plan Roth conversions; otherwise it happens after a rollover. The considerations are covered in the Roth conversion guide.

What happens to my Roth 401(k) when I leave the job?

You can generally leave it, roll it to a new employer's designated Roth account, or roll it to a Roth IRA. Only the plan-to-plan route preserves the original five-year clock. See the 401(k) rollover guide.

This article is general education, not personal financial or tax advice. Contribution limits, thresholds and the rules described here change, and several provisions are recent; verify current figures with the IRS and your plan administrator, and discuss your own position with a qualified adviser before acting.