Roth or Traditional: The Decision Rule

Put $7,500 of gross pay into a traditional account at a 12% marginal rate, leave it 25 years at 7%, and withdraw it at 12%: you end with $35,821. Do the same into a Roth: you end with $35,821. To the dollar.

That identity is the whole subject. The two accounts differ only in when the tax is taken, so the decision is one comparison and everything else is a tiebreak.

The one comparison

Your marginal rate today, against the marginal rate you expect when you withdraw.

Traditional deducts the contribution now and taxes the withdrawal. Roth taxes the contribution now and the withdrawal is free. If the rate is the same at both ends, the order of operations does not matter, which is why the two numbers above are equal.

Which gives the rule:

• Expect a lower rate in retirement: take the deduction now. Traditional.
• Expect a higher rate in retirement: pay the tax now. Roth.
• Expect the same: it does not matter, so pick on a tiebreak.

Two things about that word "marginal", because it is where most of the confusion comes from. It is the rate on the last dollar, not your average rate. And you are comparing a marginal rate now with a marginal rate later, which means asking what bracket your withdrawals will sit in, not what bracket your salary sits in today.

Three earners, worked

$7,500 of gross pay contributed, 25 years, 7% a year. Traditional puts the whole $7,500 in and taxes the result; Roth puts in what survives tax today and the result is free.

Read the first row and then the symmetry of the rest. The advantage is exactly proportional to the gap between the two rates, in whichever direction the gap runs. There is no hidden edge to either account.

One note on the framing, because it is the most common way this comparison is done wrong. We held gross pay constant, not the amount landing in the account. Comparing "$7,500 into a traditional" with "$7,500 into a Roth" is comparing two different sacrifices: the second costs you $9,615 of gross pay at a 22% rate. Done that way, Roth always looks better, and the difference is an accounting artefact.

Why "tax rates will be higher" is not an argument

It is the most repeated reason to choose Roth and it does not follow, for two reasons.

What matters is your rate, not the rates. Most people's income falls when the salary stops, and a lower income sits in lower brackets even if every bracket has moved up. Someone at a 24% marginal rate on a $150,000 salary is very often withdrawing at 12% or 22% in retirement, because they are no longer earning $150,000.

Withdrawals fill the brackets from the bottom. Your salary is stacked on top of everything else, so the deduction you take today is at your top rate. Your withdrawals in retirement start at the bottom of the schedule, so the first tranche is taxed at the lowest rates. That asymmetry quietly favours traditional for most middle and high earners, and almost nobody accounts for it.

None of which means rates will not rise. It means "rates might rise" is a reason to hold some of each rather than a reason to go all Roth.

The tiebreaks, ranked

When the rates look close, these decide it. In our order of weight.

1. No required minimum distributions on a Roth IRA. The strongest argument for Roth that has nothing to do with rates. A traditional IRA must start distributing at a set age whether you need the money or not, and those distributions are income that can affect other things. A Roth IRA has no such requirement during the owner's lifetime, so it can be left alone.

2. Roth 401(k) has no income limit. Roth IRA contributions phase out between $153,000 and $168,000 for single filers in 2026, and between $242,000 and $252,000 for married couples filing jointly. A Roth 401(k) has no income limit at all, which is the route for high earners who want Roth money.

3. Roth contributions are more flexible in an emergency. Contributions to a Roth IRA, as distinct from earnings, can generally be withdrawn without tax or penalty. That is not a reason to treat it as savings, and it is a genuine difference when the alternative is a card at 22.15%.

4. Traditional lowers this year's income. Which can matter beyond the tax itself, because income thresholds govern eligibility for several credits and phase-outs. A deduction that keeps you under a threshold can be worth more than its face value.

5. Roth removes future uncertainty. Paying tax now means the balance is genuinely yours, with no future rate to forecast. That has a psychological value we would not dismiss.

Splitting the difference, and when that is sensible

You can contribute to both, subject to a shared limit: $24,500 across Roth and traditional elective deferrals in 2026, and $7,500 across IRAs.

Splitting is not indecision when the reason is stated. Two good reasons:

You cannot predict your retirement rate, honestly. Someone 30 years out with a variable career is guessing. Holding both buckets means that in any given retirement year you can choose which to draw from, and that optionality is worth something real.

You want low-tax years available later. Retiring before other income starts creates years at a very low rate, which are the cheapest years to withdraw from a traditional account or to convert. Having a traditional balance to draw from in those years, and a Roth for the expensive ones, is a better position than having only one.

What splitting is not is a way to avoid the question. If your marginal rate is 32% today, most of the money should be traditional, and a Roth slice on top is a hedge rather than a compromise.

When this does not apply

If you are not yet getting the full employer match. The Roth question is a rounding error beside a 50% match. Capture the match first, in whatever the default bucket is: what the 401(k) match is actually worth.

If your income is unusually low this year. A year of low income is the best time to hold Roth money, and also the best time to convert a traditional balance. If you are between jobs, on parental leave or starting a business, this year is different from your normal year and should be treated as such.

If your income is above the Roth IRA phase-out. Direct Roth IRA contributions are limited or unavailable. A Roth 401(k) is not, and there are other routes with their own rules that need care. For 2026 the phase-out runs from $153,000 to $168,000 for single filers and $242,000 to $252,000 for married filing jointly, and a pre-tax IRA balance complicates the alternatives: see what to do with an old 401(k).

If you expect a large traditional balance already. Someone with $900,000 in a traditional 401(k) is looking at sizeable required distributions later, which pushes towards Roth for new money regardless of the current rate comparison.

If you are choosing between contributing and not. Then stop reading this and contribute. The gap between the two accounts is a few thousand dollars over 25 years. The gap between contributing and not is the whole balance.

Next steps

1. Find your current marginal rate, which is the bracket your last dollar of income falls in, not your effective rate.
2. Estimate your retirement income and the bracket it would sit in. Rough is fine; the direction is what matters.
3. If your current rate is clearly higher, go traditional. Clearly lower, go Roth. Within a bracket of each other, split.
4. Check whether your plan offers a Roth option at all, and whether the employer match goes into a traditional bucket.
5. Then stop optimising and look at what the money is invested in, which matters more over 25 years: what 0.85% actually costs you.