Roth 401(k) vs. Traditional 401(k): Tax Math, RMDs, and NUA Strategy

The Math Has One Variable That Matters

A 40-year-old contributing $23,000 to a traditional 401(k) in the 24% federal bracket defers $5,520 in federal income tax today. Contributing the same $23,000 to a Roth 401(k) generates no current deduction — but all future growth is tax-free. The break-even point between these strategies depends on a single variable: whether the marginal tax rate at withdrawal is higher or lower than the rate at contribution. If the rate is the same, both strategies produce identical after-tax wealth. The Roth wins if the withdrawal rate is higher; the traditional wins if the withdrawal rate is lower. Everything else — the investment return, the account balance, the holding period — cancels out mathematically when tax rates are held constant.

Key Differences Side by Side

SECURE 2.0 and the Roth 401(k) RMD Elimination

The SECURE 2.0 Act of 2022 eliminated required minimum distributions from Roth 401(k) accounts starting January 1, 2024. Before this change, Roth 401(k) assets were subject to the same RMD rules as traditional 401(k) accounts — a significant disadvantage relative to Roth IRAs, which have never had RMDs for the original owner. The RMD elimination makes the Roth 401(k) equivalent to a Roth IRA for accumulation purposes, removing a major reason to roll Roth 401(k) assets into a Roth IRA at retirement. For high earners who exceed the Roth IRA income limits (phase-out begins at $146,000 single / $230,000 married in 2024), the Roth 401(k) is the primary direct-contribution Roth savings vehicle.

No income limit. No RMD. Tax-free growth.

• Roth 401(k) contributions are not subject to the income phase-out limits that restrict Roth IRA contributions.
• The 5-year holding period for Roth 401(k) tax-free withdrawals starts from the first Roth contribution to the plan, not from rollover.
• If the plan allows, in-plan Roth rollovers let participants convert existing traditional 401(k) balances to Roth within the same plan — a taxable event but no need to distribute funds.

Net Unrealized Appreciation: The Employer Stock Exception

Net unrealized appreciation (NUA) is a tax strategy that applies specifically to employer stock held inside a 401(k). When a participant takes a lump-sum distribution of employer stock, NUA — the difference between the stock's cost basis inside the plan and its fair market value at distribution — is taxed at long-term capital gains rates rather than ordinary income rates, regardless of how long the participant actually held the stock or their age. The cost basis of the stock is taxed as ordinary income upon distribution, but the appreciation escapes ordinary income tax treatment entirely.

A participant with $500,000 of employer stock in their 401(k) at a plan cost basis of $80,000 has $420,000 of NUA. If they roll the entire amount into an IRA, all $500,000 eventually faces ordinary income tax. Using NUA treatment instead, only $80,000 is ordinary income at distribution; the $420,000 gain is taxed at capital gains rates (0%, 15%, or 20%) — potentially saving tens of thousands of dollars in federal taxes.

• NUA only applies to a lump-sum distribution — the entire plan balance must be distributed within one tax year.
• A triggering event is required: separation from service, reaching age 59½, disability, or death.
• NUA is most valuable when the employer stock has a low cost basis inside the plan and the participant has a high ordinary income tax rate.
• Once stock is rolled to an IRA, the NUA opportunity is lost permanently — the decision is irreversible.

The Employer Match Caveat

Regardless of whether a participant contributes to a traditional or Roth 401(k), employer matching contributions always go into the pre-tax traditional side of the account. This was universal law before SECURE 2.0; the act gave plans the option starting in 2023 to allow employees to elect that employer match contributions also be deposited into the Roth account — a taxable event for the employee. Few plans implemented this option immediately. Most participants in 2024 still receive employer matches as pre-tax contributions even when all of their own contributions are Roth. This means virtually every 401(k) participant has some pre-tax money to manage, regardless of their Roth election — and some eventual taxable RMD exposure from the match portion.

Practical Allocation Across Tax Brackets

For taxpayers in the 22% and 24% federal brackets, contributing to both traditional and Roth simultaneously provides tax diversification — flexibility to draw from either pre-tax or tax-free accounts in retirement depending on which produces a lower effective rate. Tax diversification is particularly valuable because future tax rates are unknown. Filling the traditional 401(k) up to the employer match, then directing additional contributions to a Roth 401(k), is a common middle-ground approach. High earners in the 32%, 35%, or 37% bracket generally derive more immediate value from the pre-tax traditional deduction, while those in the 10% or 12% bracket rarely benefit from deferring taxes at those low rates when future ordinary income rates seem likely to be similar or higher.

This article is for informational purposes only and does not constitute financial or tax advice.