Roth vs. traditional 401(k): the core difference
A traditional, pre-tax 401(k) contribution gives you an immediate income-tax deduction in the year you make it, and the money grows tax-deferred. When you withdraw it in retirement, every dollar — contribution and growth — is taxed as ordinary income.
A Roth contribution (including amounts moved into Roth through the mega backdoor Roth) gets the opposite treatment: no deduction now, but qualified withdrawals in retirement — contributions and growth — are tax-free. You're paying tax on a smaller seed today instead of a much larger harvest later.
The decision hinges largely on one question: do you expect your tax rate in retirement to be higher than it is today, lower, or about the same? Higher expected future rates favor Roth. Lower expected future rates favor traditional. The honest answer for most people is "uncertain," which is why tax diversification often beats picking a single bucket.
How after-tax retirement value compares
A fair comparison between Roth and traditional has to account for the different tax treatment of each. Comparing nominal balances at retirement is misleading — a $1 million traditional account isn't equivalent to a $1 million Roth account, because the traditional balance still owes income tax on every withdrawal. The right comparison is after-tax retirement value: what each pot is actually worth to you net of taxes once the money comes out.
When you fund both with the same contribution amount, pre-tax dollars grow on a larger base (because no tax was taken out first), but the entire balance is reduced by your retirement tax rate at withdrawal. Roth dollars start from a smaller base (because you paid income tax first), but the entire balance is yours to keep. The calculator above projects both side by side for your inputs, so you can see how the after-tax value lands under different return and tax-rate assumptions. Pair it with the compound interest projection to stress-test how long-horizon growth changes the picture.
Why the mega backdoor Roth changes the math
In a side-by-side at equal contribution amounts, Roth versus traditional often comes out close, with the tiebreaker being your assumed future tax rate. The mega backdoor changes the framing because it lets you put far more into Roth than the standard limits allow — up to $47,500 of after-tax contributions in 2026 on top of your regular $24,500 deferral, instead of capping your Roth at the elective deferral limit alone.
That matters if you expect higher future tax rates, want tax-free growth on a much larger pot, or value the fact that Roth IRAs aren't subject to required minimum distributions in your lifetime. Note again that mega backdoor is plan-dependent — it requires after-tax contributions plus in-plan Roth conversion or in-service withdrawal, which many plans don't offer.
Choosing for your situation
Lean Roth (and mega backdoor where available) when you expect higher tax rates in retirement than today, when you want tax-free growth on as large a balance as possible, or when you value tax diversification — having both pre-tax and Roth pots gives you flexibility to manage your tax bracket in retirement year by year. Lean traditional when you expect a lower retirement tax rate, when you need the deduction now to free up cash, or when you're early-career and the immediate tax break compounds longer.
In practice, many high earners use a deliberate mix: pre-tax for the regular 401(k) deferral to capture the deduction now, plus Roth via the mega backdoor for additional space. Coordinating with your after-tax income shows whether the cash flow supports both at once.
Frequently asked questions
Is a Roth or traditional 401(k) better?
There's no universal answer. The math depends on your current marginal tax rate versus the rate you expect to pay in retirement. If you expect to be in a higher bracket later, Roth tends to win; if lower, traditional tends to win. Since retirement tax rates aren't knowable in advance, many people split the difference.
Should I do mega backdoor Roth or traditional pre-tax?
They're not mutually exclusive — most people who can do the mega backdoor are also maxing their regular pre-tax or Roth deferral first. The mega backdoor is a layer on top. Tax diversification — having meaningful balances in both pre-tax and Roth — gives you more flexibility in retirement than going all-in on either side. This is general framing, not personalized advice.
Do Roth 401(k)s have required minimum distributions?
Roth IRAs have no RMDs during the original owner's lifetime. Roth 401(k)s historically did, but under SECURE 2.0, Roth 401(k) accounts are no longer subject to RMDs starting in 2024. Traditional 401(k) and traditional IRA balances are still subject to RMDs starting at age 73 under current rules.
What tax rate should I assume in retirement?
Honestly, it's unknowable — future tax rates depend on future law and your future income. A reasonable approach is to model a range: a conservative case where rates are higher than today, a base case where they're similar, and an optimistic case where they're lower. The calculator above lets you set the federal rate at withdrawal and the state you'll retire in, so you can see how sensitive the comparison is to that assumption.
Can I contribute to both?
Yes — your elective deferral can be split between pre-tax and Roth 401(k) in any combination, within the combined deferral limit ($24,500 in 2026, plus catch-ups if eligible). Mega backdoor after-tax contributions are separate and stack on top, within the overall $72,000 Section 415(c) cap.
Does tax-free growth really beat the upfront deduction?
It depends on your rate assumptions, time horizon, and what you'd do with the cash freed up by the upfront deduction. If you'd invest the deduction-funded savings in a taxable account, traditional gains some ground; if you'd spend it, Roth pulls further ahead. The calculator shows the after-tax comparison for your specific assumptions.
This calculator and its explanations are educational and give estimates only — not investment, tax or legal advice. Real returns, inflation and tax outcomes vary; talk to a qualified professional about your own situation.