What Percentage Should Go to Roth vs 401(k)?

Roth vs 401(k): Why the Split Matters More Than the Choice

The roth vs 401k question is not really a binary decision at the $5M+ level. It is a tax-rate arbitrage problem across decades, and the optimal split changes depending on your current bracket, your projected RMDs, your state of residence, and whether your plan document allows after-tax contributions. Getting this wrong by defaulting to maximum pre-tax contributions throughout a high-earning career can cost seven figures in avoidable taxes.

2025 Contribution Limits and Income Thresholds You Need to Know

Before modeling any split, get the numbers straight. According to the IRS, the 2025 elective deferral limit for 401(k) plans is $23,500, with a $7,500 catch-up for participants age 50 and older. Under SECURE 2.0, participants aged 60 to 63 get an enhanced catch-up of $11,250 instead of the standard $7,500.

Direct Roth IRA contributions are capped at $7,000 ($8,000 if you are 50 or older), but phase out between $150,000 and $165,000 MAGI for single filers and between $236,000 and $246,000 for married filing jointly. If you are reading this, you almost certainly cannot contribute directly to a Roth IRA. That does not mean Roth accumulation is off the table.

The total Section 415(c) limit for 2025 is $70,000. That ceiling is the one that matters most for high earners, because it governs the mega backdoor Roth strategy discussed below.

Sources: IRS Notice 2024-80; IRS IRC Section 402(g)

What Is the Optimal Roth vs 401(k) Split for High-Income Earners?

There is no universal answer, but there is a correct analytical framework. The standard advice to maximize pre-tax contributions because you are in a high bracket today ignores the tax cost of the entire future account balance, not just the current-year deduction.

Consider the math. A $23,500 traditional 401(k) contribution at a 37% marginal rate saves $8,695 in taxes today. But every dollar of that account, including 30 years of compounded growth, gets taxed at withdrawal. A Roth contribution foregoes that deduction but allows all compounded growth to be withdrawn tax-free. For a 35-year-old with a 30-plus-year horizon, the tax-free compounding benefit of Roth accounts can outweigh a meaningful current-year rate advantage for traditional accounts.

The breakeven is not simply "will my tax rate be higher or lower in retirement?" It is a present-value analysis of the tax on the entire future balance. High earners with long time horizons should run this calculation explicitly rather than defaulting to the pre-tax option because it feels like the obvious move.

A practical starting framework by current marginal rate:

• 37% bracket now, expecting 37% in retirement: Prioritize Roth. The rate arbitrage is neutral, but Roth wins on RMD avoidance and estate planning flexibility.
• 37% bracket now, expecting 24% in retirement: Traditional 401(k) contributions make more sense for the bulk of deferrals, but maintain some Roth exposure for tax diversification.
• 32% or below now: Lean heavily toward Roth, especially if you are under 45 with decades of compounding ahead.

For Roth deferral strategies that go beyond the basic split, the decision tree gets more nuanced once you layer in state taxes and RMD projections.

The Mega Backdoor Roth: The Most Powerful Tool Most People Ignore

If your employer plan allows after-tax contributions and either in-service withdrawals or in-plan Roth conversions, you can contribute up to $46,500 in after-tax dollars to your 401(k) in 2025. That is the $70,000 Section 415(c) limit minus the $23,500 employee deferral. Roll those after-tax contributions into a Roth IRA or convert them in-plan, and you have effectively created Roth-equivalent savings at a scale that dwarfs the standard $7,000 IRA limit.

This is the single most powerful Roth accumulation tool available to high-income earners who are phased out of direct contributions. Yet most people never ask whether their plan document allows it.

The execution steps are straightforward:

1. Confirm your plan document permits after-tax (non-Roth) contributions above the standard deferral limit.
2. Confirm the plan allows either in-service withdrawals or in-plan Roth conversions.
3. Make after-tax contributions up to the Section 415(c) ceiling after accounting for employer match and your standard deferral.
4. Convert or roll out promptly to minimize taxable earnings accumulating in the after-tax account.

The earnings on after-tax contributions are taxable upon conversion, so speed matters. Letting after-tax dollars sit for months before converting creates a small but real tax drag.

For a detailed comparison of Roth 401(k) vs backdoor Roth options, the choice often comes down to plan quality and whether your employer's fund lineup justifies keeping assets inside the 401(k) versus rolling to an IRA.

The Pro-Rata Rule and Why It Trips Up High Earners

The standard backdoor Roth conversions strategy involves making a non-deductible traditional IRA contribution and immediately converting it to Roth. Simple in concept. The pro-rata rule is where it breaks down for people who also hold pre-tax IRA balances.

The IRS treats all your traditional IRA assets as a single pool when calculating the taxable portion of any conversion. If you have $93,000 in a rollover IRA from a previous employer and make a $7,000 non-deductible contribution, your non-deductible basis represents only 7% of the total $100,000 pool. Converting $7,000 means 93% of that conversion ($6,510) is taxable. The backdoor effectively fails.

The fix is to roll your pre-tax IRA balances into your current employer's 401(k) before executing the backdoor strategy. Most plans accept incoming rollovers. Once the pre-tax IRA balance is zero, the non-deductible contribution converts cleanly.

If you cannot roll pre-tax IRA assets into a 401(k), the backdoor Roth becomes inefficient and you need to model whether the tax cost of the pro-rata calculation is worth it. In many cases for high earners with large rollover IRAs, it is not.

The RMD Problem: Why a $3M Pre-Tax Balance Is a Tax Time Bomb

This is where the Roth vs 401(k) decision intersects directly with wealth preservation. A $3 million traditional 401(k) balance at age 73 generates an RMD of approximately $116,000 in year one, using the IRS Uniform Lifetime Table divisor of 26.5. That $116,000 stacks on top of Social Security, investment income, and any other distributions. For someone with a $5M+ portfolio generating meaningful taxable income, this can push effective federal rates well above 30% and trigger Medicare IRMAA surcharges.

IRMAA surcharges for 2025 add up to $628.90 per month per person at the highest income tier, which is an additional $7,547 annually per person, purely because of income level. A large pre-tax 401(k) balance mechanically forces income recognition you may not need and cannot control.

Note: Federal tax estimates are illustrative, assuming the full RMD falls in the 37% bracket. Actual liability depends on total income stack.

Roth 401(k) accounts, following SECURE 2.0 changes effective 2024, no longer require RMDs during the owner's lifetime. Roth IRAs have never required them. This structural advantage alone justifies a meaningful Roth allocation for anyone building a portfolio they expect to exceed $3M in pre-tax accounts.

Should I Max Out My 401(k) or Roth IRA First If I Earn Over $300,000?

The sequencing question has a clear answer for most high earners: capture the full employer match first, always. That is an immediate 50% to 100% return on capital, which no tax treatment can match.

After the match, the optimal sequence depends on your plan quality and whether the mega backdoor Roth is available:

1. Employer match threshold in traditional or Roth 401(k): Capture 100% of available match.
2. Backdoor Roth IRA ($7,000 per person, $14,000 for couples): Low friction, clean execution if you have no pre-tax IRA balances.
3. Mega backdoor Roth (up to $46,500 after-tax): If your plan permits, this is the highest-priority Roth accumulation vehicle by dollar volume.
4. Remaining 401(k) space in traditional or Roth designation: Allocate based on your current bracket and RMD projections.
5. Taxable brokerage: After exhausting tax-advantaged space, taxable accounts with tax-loss harvesting and long-term capital gains treatment are the next tier.

The standard advice to max your 401(k) before anything else ignores the fact that plan quality varies enormously. If your 401(k) has high-expense-ratio funds and no mega backdoor option, the backdoor Roth IRA may be a better second priority than filling the remaining pre-tax 401(k) space.

State Tax Implications That Can Flip the Entire Analysis

State tax treatment of retirement distributions varies enough to materially change the Roth vs. traditional calculus. Illinois, Pennsylvania, and Mississippi exempt all retirement income from state tax. A high earner in Illinois who plans to retire in-state faces zero state tax on traditional 401(k) withdrawals, which makes pre-tax contributions more attractive than the federal-only analysis suggests.

California taxes all retirement income at rates up to 13.3%. For a California resident who will remain in-state during retirement, Roth accounts are significantly more valuable because that 13.3% applies to every traditional withdrawal. At a $5M portfolio, the lifetime state tax differential between Roth and traditional can easily exceed $500,000.

The relocation angle matters here. High-net-worth individuals are disproportionately likely to consider moving to a no-income-tax state as part of their FIRE strategy. If you are currently in California or New York and plan to retire in Florida or Texas, the state tax benefit of Roth accounts largely disappears at the retirement end. In that case, pre-tax contributions during high-earning California years, followed by converting a 401(k) to a Roth IRA after establishing residency in a no-tax state, can be the most efficient sequence.

Model your specific state transition explicitly. The difference between "I'll figure out the state later" and a deliberate relocation-timed conversion strategy can be six figures.

Roth Conversion Ladders and the Early Retirement Window

For FatFIRE individuals who retire before 59½, the years between retirement and age 73 (when RMDs begin) represent the single most valuable conversion window available. Income often drops sharply, creating room to convert pre-tax balances at lower marginal rates before Social Security, RMDs, and other income sources stack up.

According to Fidelity's analysis, Roth conversions are most advantageous when executed during years when taxable income is temporarily lower, precisely the gap years that early retirees experience. A person retiring at 50 with $4M in pre-tax accounts and $2M in taxable assets has roughly 23 years to systematically convert pre-tax balances into Roth accounts before RMDs force the issue.

The conversion ladder strategy works as follows: each year, convert enough pre-tax assets to fill your current tax bracket without spilling into the next. At a 22% or 24% effective rate, you are locking in a known tax cost on assets that would otherwise face 37% plus IRMAA surcharges at 73. The Roth conversion strategies after 60 framework applies similar logic for those closer to traditional retirement age.

Note: Simplified illustration. Actual tax depends on total income stack, deductions, and filing status. Bracket thresholds adjust annually.

The $240,000 in taxes on a $1M conversion over 10 years at 24% compares favorably to the alternative: $370,000 in taxes at 37% on the same balance at RMD age, before accounting for the compounded growth that also gets taxed.

Estate Planning: How Roth vs 401(k) Splits Affect Your Heirs

Under SECURE 2.0, non-spouse beneficiaries who inherit traditional 401(k) or IRA accounts must fully distribute the account within 10 years. If the original owner had already begun RMDs, beneficiaries must also take annual distributions during that window. For a beneficiary in a 37% bracket receiving a $2M inherited traditional IRA, the tax cost over the 10-year distribution period can approach $740,000.

Inherited Roth accounts are also subject to the 10-year rule, but distributions remain tax-free. The after-tax inheritance differential between a $2M Roth and a $2M traditional account, for a beneficiary in a high bracket, can be 22% to 37% of the entire account value.

For $5M+ estates, this is not a peripheral consideration. If your estate plan includes passing retirement assets to children or other taxable beneficiaries, the Roth vs. traditional allocation decision is simultaneously a wealth transfer decision. Roth accounts are among the most tax-efficient assets you can leave to heirs who will be in high brackets.

Research published in the Journal of Financial Planning confirms that maintaining accounts across multiple tax treatments, including pre-tax, Roth, and taxable, provides measurable flexibility in managing effective tax rates throughout retirement and into the estate transfer phase. Tax diversification is not just about hedging uncertainty. It is about preserving optionality.

For context on how this interacts with broader withdrawal planning, optimal withdrawal strategies at the $5M+ level typically involve coordinating Roth, pre-tax, and taxable accounts to manage both lifetime income taxes and the estate tax picture simultaneously.

How a $5 Million Net Worth Individual Should Allocate Between Roth and Pre-Tax Accounts

Pulling the analysis together, here is a practical framework for someone with $5M+ in net worth who is still accumulating:

If you are under 50, earning $500K+, and have 15+ years to retirement: Prioritize Roth accumulation aggressively. Use the Roth 401(k) designation for your standard deferral, execute the mega backdoor Roth if your plan permits, and run the backdoor Roth IRA annually. The compounding runway and RMD avoidance justify accepting the current-year tax cost. Consider deferred compensation alternatives for any additional pre-tax capacity.

If you are 50 to 60, in the 37% bracket, with a large existing pre-tax balance: The priority shifts toward limiting additional pre-tax accumulation and beginning to model your conversion window. Max the Roth 401(k) designation, execute the mega backdoor if available, and start projecting your RMD trajectory at 73. If your pre-tax balance is already above $3M, every additional dollar going into a traditional 401(k) is adding to a future tax problem.

If you are self-employed: The solo Roth 401(k) options for self-employed individuals allow you to make both employee Roth deferrals and employer profit-sharing contributions up to the Section 415(c) limit, with no plan document restrictions on after-tax contributions in some plan designs. This is worth structuring carefully with a plan administrator who specializes in high-income self-employed clients.

Vanguard's 2024 "How America Saves" data shows only 14% of participants contributed the maximum allowed to their 401(k). At the FatFIRE level, you are almost certainly in that 14%, which means the optimization question is not whether to max out but how to allocate the maximum across pre-tax, Roth, and after-tax buckets.

For those modeling long-term portfolio sustainability, the 4% rule for retirement planning provides a baseline, but the tax treatment of your withdrawal stack materially affects how much you actually net from any given withdrawal rate. A 4% withdrawal from a $5M all-Roth portfolio is categorically different from a 4% withdrawal from a $5M all-traditional portfolio.

The age-based asset allocation question and the Roth vs. traditional question are related but distinct. Asset allocation determines your return profile. Tax location determines how much of that return you keep.

References

• Internal Revenue Service -- "Publication 590-A: Contributions to Individual Retirement Arrangements (IRAs)" (2024)
• Internal Revenue Service -- "IRC Section 402(g): Retirement Topics -- 401(k) and Profit-Sharing Plan Contribution Limits" (2024)
• Internal Revenue Service -- "Notice 2024-80: 2025 Retirement Plan Contribution Limits" (2024)
• Internal Revenue Service -- "Publication 590-B: Distributions from Individual Retirement Arrangements (IRAs)" (2024)
• Vanguard -- "How America Saves 2024" (2024)
• Journal of Financial Planning -- "Tax Diversification and Retirement Income Optimization" (2022)
• Congressional Research Service -- "SECURE 2.0 Act of 2022 (P.L. 117-328): Summary of Key Retirement Provisions" (2023)
• Fidelity Investments -- "Roth Conversion: Is It Right for You?" (2024)