Roth vs Traditional 401(k): When You Pay Taxes Changes

The core difference is tax timing

Traditional and Roth 401(k) contributions can hold similar investments. The major difference is when income tax is paid.

• Traditional 401(k): contributions generally reduce current federal taxable income; distributions are generally taxable later.
• Roth 401(k): contributions are made after income tax; qualified withdrawals are generally tax-free.

Neither label tells you which one is automatically better.

Why the comparison is easy to oversimplify

A common rule of thumb says “Roth when you are young, Traditional when you earn more.” That can be a useful starting question, but it is not a complete decision rule.

The comparison can depend on:

• Your current marginal tax rate
• Your expected future tax rate
• State taxes now and later
• How much you can actually afford to save
• Whether you compare the same account contribution or the same take-home-pay cost
• Other retirement income and tax diversification
• Current law and future law changes

A simple example

Suppose you have $10,000 of pre-tax income available to save and your assumed current marginal income-tax rate is 30%.

With a Traditional contribution, the full $10,000 can generally go into the account before current income tax.

If instead you want the same current take-home-pay cost using Roth, roughly $7,000 would remain after a simplified 30% tax assumption.

That is a different comparison from putting $10,000 into both accounts. A $10,000 Roth contribution costs more current take-home pay than a $10,000 Traditional contribution.

This is why MFA's Roth vs Traditional calculator offers different comparison bases rather than pretending there is one universal answer.

Current limits still apply

For 2026, the general employee 401(k) deferral limit in MFA's annual-rules engine is $24,500.

Traditional and Roth 401(k) employee contributions share that limit. They are not two separate $24,500 buckets.

Catch-up rules can allow additional contributions for eligible participants, and plan-specific rules still apply.

When Traditional may be more attractive

Traditional can look more attractive when the tax deduction today is especially valuable and you expect withdrawals to face a lower effective or marginal tax rate later.

It can also help preserve current cash flow because each dollar contributed generally costs less after current income-tax effects than the same nominal Roth contribution.

But “my tax bracket will definitely be lower in retirement” is still an assumption, not a fact.

When Roth may be more attractive

Roth can look more attractive when the current tax cost is relatively low compared with the tax rate you expect later, or when tax-free qualified withdrawals would add useful flexibility to your retirement income mix.

Roth also changes the economics when you are already saving near the employee contribution limit. Putting the same nominal maximum into Roth means more after-tax wealth is effectively sheltered inside the account, but it also requires more current cash flow.

Tax diversification is a real consideration

Some households intentionally build both Traditional and Roth balances. The point is not to predict future tax law perfectly. It is to have more than one tax bucket available when deciding where retirement cash flow should come from.

That does not mean a 50/50 split is automatically optimal. It means the choice can be managed as part of a broader plan rather than as an all-or-nothing identity.

Common mistakes

Comparing equal deposits without acknowledging unequal cost

A $10,000 Traditional contribution and a $10,000 Roth contribution do not generally have the same effect on today's paycheck.

Assuming “Roth is tax-free” without the word qualified

Roth tax treatment has rules. Qualified-distribution requirements matter.

Ignoring state taxes

Moving between states can change the comparison materially.

Treating future tax rates as known

Any long-term comparison requires assumptions. Change them and see whether the conclusion changes.

A better way to decide

1. Estimate the current tax cost of each option.
2. Compare both equal-contribution and equal-budget scenarios.
3. Test more than one future tax-rate assumption.
4. Consider the rest of your retirement income and account mix.
5. Revisit the choice when income, state, employer plan or tax law changes.

Bottom line

Traditional vs Roth is not a personality test. It is a tax-timing trade-off with cash-flow consequences. Run both cases, make the assumptions visible and avoid pretending anyone knows future tax rates with certainty.