Roth 401(k) vs Traditional 401(k): A Real Worked Tax Comparison

Roth or traditional is one of the first choices a 401(k) plan asks a new employee to make, usually on day one, before most people have thought about their own future tax bracket at all. The 401(k) calculator on this site projects a single account balance and does not try to referee that choice for you. This guide does, with actual numbers worked through by hand rather than a general rule of thumb, so you can see exactly where the two paths tie, where one clearly wins, and why a detail buried in how the IRS sets its contribution limit quietly favors one of them in a way most explanations skip entirely in a typical roth 401k vs traditional 401k comparison.

Roth 401(k) vs traditional 401(k): the mechanical difference

A traditional 401(k) contribution reduces your taxable income the year you make it. The money grows without being taxed along the way, and when you eventually withdraw it in retirement, the full amount, contributions and growth together, is taxed as ordinary income at whatever rate applies then.

A Roth 401(k) contribution does the opposite. You pay income tax on that money now, before it goes into the account, exactly as if you had never contributed it at all. It then grows completely untaxed, and qualified withdrawals in retirement, contributions and growth together, come out with no further tax owed at all.

Both share the same combined annual IRS elective deferral limit, which for 2026 is $24,500 for savers under 50 and $32,500 for savers 50 and older, including the catch-up allowance. That one shared limit is the detail this guide keeps coming back to, because it behaves very differently depending on which bucket you put your dollars in.

A clean worked example: the tie case

Start with the simplest possible scenario, to establish a baseline before anything more interesting happens. Assume a saver contributes a single $24,500 amount, growing at an assumed 7% annual return for 20 years, with an identical 22% tax rate both today and at retirement. Treating it as one lump sum rather than 20 separate annual contributions keeps the arithmetic focused purely on the tax mechanics rather than on compounding schedules, which the 401(k) calculator already handles for a real, multi-year contribution stream.

Over 20 years at 7%, $1 grows to roughly $3.8697. So $24,500 grows to about $94,807 before any tax is considered.

Traditional path. The full $24,500 goes in pretax, growing to $94,807. At withdrawal, 22% tax applies to the whole amount: $94,807 x (1 - 0.22) = $73,950 actually spendable.

Roth path, same paycheck impact. To make this a fair comparison, assume the same $24,500 of gross pay is set aside either way. For the Roth path, tax is paid immediately at 22%, leaving $24,500 x (1 - 0.22) = $19,110 actually contributed. That grows to $19,110 x 3.8697 = $73,950, and since it is a qualified Roth withdrawal, nothing further is owed.

Both numbers land on $73,950, to the dollar. This is not a coincidence; it is the mathematical proof of the standard claim that "Roth and traditional are equivalent if your tax rate never changes," and it holds exactly because the two paths are compared on an equal-paycheck-impact basis, meaning the same amount of your current take-home pay is affected either way.

When tax rates actually change: two more worked cases

Real careers rarely hold a flat tax rate for 20 or 30 years, so the more useful question is what happens when today's rate and retirement's rate differ. Using the same $24,500-at-7%-for-20-years setup, and again comparing on an equal-paycheck-impact basis:

Case: taxed at 12% today, 24% in retirement (a plausible pattern for an early-career saver whose income and tax bracket both rise over time). Traditional: $94,807 x (1 - 0.24) = $72,054. Roth: contribute $24,500 x (1 - 0.12) = $21,560 now, which grows to $21,560 x 3.8697 = $83,430. Roth wins by about $11,377.

Case: taxed at 32% today, 12% in retirement (a plausible pattern for a peak-earnings-year saver who expects a much lighter income in retirement). Traditional: $94,807 x (1 - 0.12) = $83,430. Roth: contribute $24,500 x (1 - 0.32) = $16,660 now, which grows to $16,660 x 3.8697 = $64,469. Traditional wins by about $14,961.

The pattern behind all three rows is the same rule stated plainly: pay tax whenever the rate is lower, and defer tax whenever the rate is higher. A traditional 401(k) defers tax from today until retirement, so it wins when today's rate is the higher of the two. A Roth 401(k) pays tax at the rate you face now, so it wins when that rate is the lower of the two. When the two rates happen to match, as the first row shows, neither approach has a mechanical edge, and the decision comes down to which future rate you think is more likely.

The part most explanations leave out: maxing out the same dollar limit

Every roth 401k vs traditional 401k comparison up to this point assumed a fair, equal-paycheck-impact comparison, meaning the same amount of current gross pay was affected by the decision either way. Most people do not actually think about their 401(k) contribution that way in practice; they think in terms of hitting the IRS's dollar limit, $24,500 for 2026, regardless of which bucket it goes into. That shift in framing changes the comparison meaningfully.

Any 401k max contribution calculator starts from the same fact: the annual limit is a dollar cap, not a percentage, and it is shared between the Roth and traditional sides.

The IRS limit is a nominal dollar figure. It does not adjust for the fact that a traditional dollar is pretax and a Roth dollar is already taxed. That means contributing the full $24,500 to a Roth 401(k) actually shelters more real, inflation-adjusted purchasing power from future tax than contributing the full $24,500 to a traditional 401(k), because the traditional dollar still owes tax later while the Roth dollar has already settled its tax bill.

Here is the same $94,807-after-20-years figure, but now comparing two savers who both simply max out the $24,500 limit, one traditional and one Roth, at a 22% tax rate in both cases:

Traditional, maxed out. $24,500 pretax grows to $94,807, taxed at 22% on withdrawal: $94,807 x 0.78 = $73,950 spendable.

Roth, maxed out. $24,500 of already-taxed dollars grows to $94,807, with nothing further owed: $94,807 spendable.

The Roth saver ends up with $20,857 more. That number looks like free money, but it is not; it reflects the fact that actually getting $24,500 of after-tax dollars into a Roth account requires more pretax income than getting $24,500 of pretax dollars into a traditional account. At a 22% tax rate, having $24,500 left after tax requires earning $24,500 / (1 - 0.22) = $31,410 pretax, about $6,910 more pretax income diverted toward the Roth contribution than the traditional path requires for the same nominal $24,500 figure. The "extra" $20,857 at the end is the payoff for using more of today's paycheck now, not a loophole with no cost attached.

This is the real, verifiable version of the claim that "a Roth 401(k) effectively lets you save more." It is true, but only for someone who has the additional take-home pay available to actually max it out; for someone whose contribution is limited by what they can currently afford to set aside, the equal-paycheck-impact comparison earlier in this guide is the more honest way to frame the decision.

What the choice actually does to a real paycheck

The worked examples above compare final retirement balances, but the decision is felt immediately, every pay period, in a way that is worth making concrete. Take a saver earning $70,000 a year, paid biweekly (26 paychecks), contributing 10% of salary, or $7,000 a year, at a 22% marginal tax rate.

Two questions get mixed together here: how much to put in 401k overall, and how much of it to send to the Roth side. The first is a budget decision and the second is a tax decision. Your 401k balance at retirement depends on the first far more than on the second.

Traditional contribution. $7,000 a year is $269.23 per paycheck, taken out before tax. The paycheck's taxable income drops by that amount, so the actual reduction in take-home pay is smaller than $269.23, roughly $269.23 x (1 - 0.22) = $210.00 per paycheck, since the contribution also lowers the tax withheld that period.

Roth contribution. $7,000 a year is again $269.23 per paycheck, but this time it comes out of already-taxed pay, so take-home pay drops by the full $269.23 per paycheck, since there is no offsetting reduction in tax withheld.

Both savers end up with the same $7,000 a year sitting in their 401(k), but the Roth saver feels a bigger hit to their take-home pay along the way, about $59.23 more per paycheck in this example, or roughly $1,540 a year. That gap is exactly the tax being paid now instead of later, made concrete in a way the abstract percentages earlier in this guide do not show on their own. Anyone finding a Roth contribution surprisingly tight against their monthly budget is not imagining it; contributing the same nominal dollar amount to Roth genuinely costs more out of today's paycheck, which is the flip side of the "shelters more" advantage covered above.

Two retirement-specific effects worth knowing about

Beyond the headline tax-now-versus-tax-later trade-off, two mechanical details tied specifically to retirement income can tip the decision further, and both are frequently left out of a basic Roth-versus-traditional comparison.

To calculate taxes on 401k withdrawal amounts in retirement, apply your marginal bracket to each traditional dollar taken out; a qualified Roth withdrawal adds nothing to taxable income. That is the math behind any taxes on 401k withdrawal calculator. Before age 59½ the picture changes: the early withdrawal card in the 401k calculator works as a 401k withdrawal tax calculator and 401k penalty calculator in one, and as a 401k cash out calculator or 401k early payout calculator you can use to estimate 401k cash out proceeds after tax and penalty.

Social Security's taxability. Up to 85% of Social Security benefits can become taxable income, depending on a figure the IRS calls "combined income": your adjusted gross income, plus any tax-exempt interest, plus half of your Social Security benefit itself. Traditional 401(k) withdrawals count as ordinary income and raise adjusted gross income directly, which can push more of a retiree's Social Security benefit into taxable territory. Qualified Roth 401(k) withdrawals are not included in adjusted gross income at all, so drawing from a Roth account instead, or blending withdrawals from both a traditional and a Roth bucket in a given year, is a real, commonly used lever retirees use to manage how much of their own Social Security ends up taxed.

Medicare premium surcharges (IRMAA). Medicare Part B and Part D premiums include an income-related surcharge for higher earners, based on a version of adjusted gross income from two years prior, applied in tiers rather than smoothly. A large traditional 401(k) withdrawal, for a one-time expense or a Roth conversion, can push a retiree's income into a higher surcharge tier for that measurement year, raising Medicare premiums well after the withdrawal itself. Because qualified Roth withdrawals do not count toward this income measure, a retiree with a mixed Roth-and-traditional balance has more flexibility to control which tier they land in during a given year, simply by choosing which account to draw from.

Neither of these effects changes the core arithmetic worked through earlier in this guide, but both are genuine, IRS-documented reasons that having at least some Roth savings available in retirement is useful even for someone who mechanically comes out ahead with more traditional savings on the numbers alone. Flexibility in which bucket a withdrawal comes from has a value of its own that a single balance projection cannot fully capture.

Where the employer match fits into all of this

An employer match complicates the picture slightly, because historically it has always landed in the traditional, pretax side of the plan, even when your own contribution goes entirely into a Roth 401(k). A 2022 law change lets employers optionally allow the match itself to be received as Roth, but adoption of that option is inconsistent across plans, so it is worth checking your own plan's summary description rather than assuming either way. Either way, the match itself does not change the Roth-versus-traditional decision for your own contribution; it simply means a Roth 401(k) saver often ends up with both a Roth bucket and a smaller traditional bucket inside the same account, funded by the employer, which is a reasonable outcome rather than something to try to avoid.

A 401k matching calculator or 401k with match calculator has to keep the match on the pre-tax side of the tax comparison unless the plan lets you take it as Roth.

State taxes add a second variable most comparisons ignore

Everything above focused on federal income tax, but state income tax runs through the exact same logic and can move the answer on its own. A traditional 401(k) contribution typically reduces state taxable income in the year you contribute, in states that tax income at all, and a traditional withdrawal is typically taxed by whichever state you live in at the time you withdraw it, not the state you lived in when you contributed. A saver who contributes while living in a state with income tax, then retires to one of the handful of states with no income tax at all, effectively gets an extra, state-level version of the "rate falls later" case worked through earlier, favoring traditional contributions further. The reverse is also possible: someone contributing in a no-income-tax state who later retires somewhere with meaningful state income tax has effectively moved into a higher future rate, strengthening the case for Roth. This is genuinely difficult to plan around with any precision, since relocation decades in the future is rarely known in advance, but it is worth naming as a real variable sitting alongside the federal comparison, not folded silently into it.

A practical way to decide without predicting the future

Nobody can know their exact tax bracket several decades from now with real confidence, which is why the flat-rate tie case earlier in this guide matters as a baseline: it shows that being wrong about the direction of the change is the only way either choice meaningfully underperforms the other. A few practical starting points, none of them guarantees:

The 401k calculator covers the projection side, working as a 401k contribution calculator and a 401k match calculator for the balance, while this guide covers the tax side that a roth 401k calculator would need.

Any roth or traditional 401k calculator reduces to the comparison in the tables above. A roth or 401k calculator that ignores the shared contribution limit will understate what the Roth side shelters, and a 401k roth calculator that splits contributions is best run twice, once at each assumed retirement tax rate.

Early-career savers are frequently, though not universally, in one of the lower tax brackets they will see across their whole career, with income and rates typically rising through their thirties and forties. That pattern leans toward Roth.

Savers in their peak earning years, often their late career, are frequently in the highest bracket they will ever see, with retirement income from Social Security and modest withdrawals landing in a lower bracket. That pattern leans toward traditional.

Savers who are genuinely unsure, or who expect a career with unusual income swings, often split contributions between both types within the same plan, hedging against being wrong about which direction future tax rates move, rather than betting the entire balance on one prediction.

None of these are formulas the way the worked examples above are; they are general tendencies, and an individual's actual career path, state tax situation, and future tax legislation can all move the real answer away from the typical pattern. What the worked math in this guide does establish with certainty is the mechanism: the decision is entirely about the tax rate today relative to the tax rate at withdrawal, plus the smaller, separate effect of the IRS's dollar-limit not adjusting for pretax versus after-tax dollars, plus the two retirement-specific wrinkles, Social Security taxability and Medicare premium surcharges, covered above. Everything else is a forecast, not an equation, and the honest answer for most people is that a reasonable, defensible choice made today, revisited every few years as income and tax law actually change, beats waiting for certainty that decades of career and legislation will never fully provide.

Run your own current balance, contribution rate, and years to retirement through the 401(k) calculator to see the projected total first; then apply the roth 401k vs traditional 401k tax-rate comparison in this guide to decide how much of that total to route through the Roth side of the plan instead.