The short version
Compare your marginal tax rate today (the tax on your last dollar) with your best guess at your marginal rate in retirement. Whichever end is lower, pay the tax there: lower rate later → Traditional; lower rate now → Roth. Same on both ends → the two paths leave the same amount, and the tiebreakers below decide. The Roth vs traditional calculator runs your numbers and draws the crossover chart so you can see where the answer flips.
Why the rule works (the math in plain words)
Both paths invest the same pre-tax dollars and grow at the same rate. Traditional taxes the pile once, at withdrawal. Roth taxes the contribution once, up front, and the rest is free. So the two formulas are the same three multiplications in a different order: tax × grow versus grow × tax. Multiplication does not care about order, which is why equal rates on both ends produce identical balances — a fact the calculator will happily demonstrate if you set both rates to 22%.
This kills two common myths. First, "Roth is better because the growth is tax-free" — the growth is effectively untaxed in the Traditional too, because you invested dollars the government had not touched yet. Second, "Traditional is better because of the deduction" — the deduction is just choosing to pay the tax later instead of now. Neither side has magic; both are just a bet on which rate is lower.
The tiebreaker cases
When the rates look close, the fine print decides. Required withdrawals: Traditional 401(k)s force you to start withdrawing at 73; Roth 401(k)s have no required withdrawals at all (a 2024 rule change), so the money can keep compounding untouched. Early access: Roth contributions can be withdrawn anytime without tax or penalty — Traditional money is locked behind penalties until 59½. Today's paycheck: Traditional's deduction lowers this year's tax bill, which is real money in your pocket now. State taxes: moving from a high-tax state to a low-tax one in retirement is a pure win for Traditional; the reverse favors Roth. The match: employer matching dollars always land on the pre-tax side, so a 100% Roth election still leaves you with a taxable Traditional balance at the end — plan for it.
How to calculate it (the 3-step method)
- Find your marginal rate today.
Not your effective rate — the rate your last dollar of income is taxed at, federal plus state. A single filer deep in the 22% federal bracket with 5% state tax is roughly at 27%. - Guess your retirement marginal rate.
Most retirees land lower: no salary, smaller withdrawals, and often a cheaper state. Try three scenarios — lower, same, higher — instead of one confident guess. - Run the crossover.
Type both rates into the calculator and read the crossover chart: the lines cross at your rate today, and whichever zone your retirement rate lands in names the winner. Use the "lower / same / higher" presets to test all three guesses in seconds.
Worked example: $10,000 a year, 30 years, 7% growth
At 7% for 30 years each dollar multiplies by 7.612, so $10,000 a year becomes $76,122.55 before tax:
Roth (22% now): $7,800 goes in after the up-front tax → grows to $59,375.59, zero tax at withdrawal
Answer: Traditional wins by $7,612.26 — entirely because 12% is smaller than 22%. At 22% on both ends the two print the identical $59,375.59.
When splitting is the right answer
If your two rate guesses straddle the line — maybe lower, maybe not — splitting contributions between Roth and traditional is the rational hedge, not a cop-out. It also buys you something the math above does not price: two tax buckets in retirement. With both a taxable and a tax-free pool, you can tune your withdrawals year by year to stay under bracket thresholds, which is worth real money. A common default: Traditional up to the full employer match (free money first), then Roth for the rest while your bracket is low, shifting toward Traditional as raises push your bracket up.
Mistakes that flip the answer the wrong way
The biggest is using your effective tax rate — total tax divided by total income — instead of your marginal rate. The 401(k) decision is about the next dollar, and the next dollar is taxed at the marginal rate, which is always higher. Using the effective rate understates today's tax and quietly biases everything toward Roth. The second is forgetting state taxes: a 5% state rate moves the needle as much as a federal bracket step. The third is treating the choice as permanent — most plans let you change the mix every paycheck, so "Roth while young, Traditional while peak-earning" is a legitimate lifetime strategy, not a contradiction.
Frequently asked questions
How do I calculate whether Roth or traditional is better for me?
Three steps: find your marginal tax rate today (federal + state, on your last dollar), estimate your marginal rate in retirement, and compare. Lower later → Traditional. Lower now → Roth. Run both rates through the calculator — the crossover chart shows exactly where your answer flips.
Should I do Roth or traditional if I am in the 22% bracket?
It depends on where you will be in retirement, not on the 22% itself. Most 22%-bracket workers drop to the 12% bracket in retirement, which favors Traditional. But if you are early-career with big raises ahead, Roth can win. Test the "lower / same / higher" presets rather than guessing once.
Is it smart to split 401(k) contributions between Roth and traditional?
Yes — it is the standard hedge against an unknowable future tax rate, and it gives you two tax buckets to draw from in retirement, letting you tune withdrawals to stay under bracket thresholds. Many plans let you set a percentage split per paycheck.
Does the employer match count as Roth if I choose Roth?
No. Matching contributions are always made pre-tax and land in the traditional side of your account, even when your own contributions are 100% Roth. The match and its growth will be taxed at withdrawal.
Can I change from traditional to Roth mid-year?
Usually yes — most plans let you change your contribution type and amount at any time. The change applies to future paychecks only; money already contributed keeps its original tax character. This makes "Roth while young, traditional at peak earnings" a workable lifetime plan.
What is the difference between a Roth 401(k) and a Roth IRA?
Same tax treatment, different wrappers. The Roth 401(k) sits inside your employer's plan: no income limit on choosing it and much higher contribution limits. The Roth IRA is your own account: income phase-outs limit who can contribute directly, with lower limits. Many people fund both — the 401(k) for volume, the IRA for investment choice. If you are weighing debt payoff against investing instead, see pay off debt or invest.
Educational content, not tax advice. Tax rules change — the Secure 2.0 RMD note above reflects rules in effect for 2026. Consider a tax professional for your situation.