Traditional vs. Roth 401(k): Same Plan, Two Tax Buckets

The enrollment form has two boxes, traditional and Roth, and most people tick whichever one a coworker ticked. It is the same plan, the same match, the same fund menu, the same annual limit. The only thing the box decides is when you settle up with the IRS: on the way in, or on the way out. That timing question has a cleaner answer than the forum debates suggest, once you write the math down.

Same plan, two buckets

A traditional 401(k) contribution comes out of your paycheck before federal income tax, so it lowers this year's taxable wages, and every dollar withdrawn in retirement is taxed as ordinary income. A Roth 401(k) contribution comes out after tax, so it does nothing for this year's bill, and qualified withdrawals in retirement, contributions and growth alike, are tax-free. Both buckets grow without annual tax drag. Both share one combined contribution limit, so filling one reduces what you can put in the other. Both hold the same investments and follow the same loan and vesting rules. And unlike a Roth IRA, the Roth side of a 401(k) has no income limit, which is why high earners who are shut out of a Roth IRA can still build a Roth balance here.

The math is symmetric, so the rate is everything

Take $10,000 of salary you intend to save, and assume it triples by retirement. Suppose your marginal rate today is 25% and consider three possible rates at withdrawal (all illustrative):

When the two rates match, the outcomes match exactly; the order of multiplication does not matter. Traditional wins when your rate at withdrawal is lower than your rate today. Roth wins when it is higher. That is the entire decision, and the Roth vs. traditional calculator runs it on your own rates, contribution, and horizon.

One subtlety tilts the tie toward Roth for people who max out: the annual limit is the same dollar figure for both buckets, but a Roth dollar is worth more because the tax is already paid. Someone contributing the full limit into the Roth side is sheltering more real spending power than someone contributing the full limit pre-tax.

Guessing your future rate honestly

Nobody knows their retirement bracket, but the inputs are not random. Your rate at withdrawal is set by the income you will have then: Social Security, any pension, required distributions from pre-tax accounts, rental or part-time income, and the standard deduction and low brackets that retirement withdrawals fill from the bottom up. Many retirees pay a lower average rate on withdrawals than their working marginal rate, because those withdrawals fill the empty low brackets first. Against that, a large pre-tax balance can force big required withdrawals, and tax law itself can change. The rough heuristics that follow from all this:

• Early career or a low-income year: the tax you would save today is small, so Roth costs little and locks in tax-free growth for the longest horizon.
• Peak earning years: the deduction is worth the most it will ever be, so traditional usually wins, especially if retirement income will be modest.
• A move to a no-income-tax state in retirement favors traditional; a move the other way favors Roth.
• A very large pre-tax balance already pushes new money toward Roth, since the required withdrawals from what you have will already fill the low brackets.

The match lands pre-tax, whichever box you tick

Whatever you choose for your own contributions, the employer contribution goes into the traditional bucket in nearly every plan, untaxed on the way in and taxed on the way out. Contribute 100% Roth for thirty years and you will still retire with a pre-tax balance made of match and its growth. That is worth knowing for two reasons: it means a Roth contributor is automatically split, and it means the match is not a reason to choose either box. (Plans are now permitted to offer a Roth match; where one does, the match is taxable income to you in the year it is deposited.) How matches work and why they come first is covered in the 401(k) match guide.

Splitting as a hedge

Because the decision rests on a rate nobody can know, many people split contributions between the two buckets and treat the split as insurance. The payoff comes in retirement, when holding both kinds of money lets you decide each year how much taxable income to create: pre-tax withdrawals to fill the low brackets, Roth withdrawals on top without pushing into higher ones or triggering the income-based surcharges on Medicare premiums. A 50/50 split is a reasonable default when you have no strong view; tilting toward traditional in high-income years and Roth in low ones is the refinement. The same logic across IRAs is worked through in Roth vs. traditional IRA.

Required distributions, in one paragraph

A traditional 401(k) requires you to start withdrawing, and paying tax on, a minimum amount each year once you reach the required beginning age set in law (currently in the early seventies, and scheduled to rise), unless you are still working for that employer. Roth 401(k) balances no longer have lifetime required distributions as of 2024, matching Roth IRAs. Inherited accounts of both kinds follow their own withdrawal timelines. The practical effect is that a Roth balance can sit untouched as long as you like, while a pre-tax balance will eventually generate taxable income whether you need it or not; the 401(k) calculator shows how large that balance is likely to be when the clock starts.

A short decision list

1. Capture the full match first, in either box; it is pre-tax regardless.
2. Estimate whether your rate today is higher or lower than your likely rate at withdrawal.
3. Lean traditional if today is higher, Roth if today is lower, and split if you cannot tell.
4. Revisit the box whenever your income changes materially; the choice is not permanent and applies only to future contributions.

Whichever bucket you fill, the two balances are one retirement. Seeing the pre-tax and Roth totals side by side, along with everything outside the plan, is what net worth tracking is for.