Mega Backdoor Roth in a Solo 401(k): CPA Strategy Guide

The mega backdoor Roth strategy lets a self-employed client move up to $46,500 of after-tax dollars into a Roth account annually — far beyond the $7,000 Roth IRA contribution limit — by using the unused §415 space in a Solo 401(k) plan. The strategy requires a plan document that explicitly permits after-tax (non-Roth) employee contributions and either an in-plan Roth conversion or an in-service distribution to a Roth IRA. Most off-the-shelf custodian prototype plans do not allow this. CPAs who identify clients with the right profile and help them source a qualifying plan can unlock the largest Roth accumulation vehicle available to a sole proprietor or S-corp owner with no other employees.

What the §415 Limit Makes Possible

IRC §415(c) caps annual additions to a defined contribution plan at the lesser of 100% of compensation or $70,000 in 2025 ($77,500 for participants age 50 or older; $73,500 for ages 60–63 under SECURE 2.0's enhanced catch-up, effective January 1, 2025). "Annual additions" under §415(c)(2) include employee pre-tax deferrals, Roth employee deferrals, employer contributions, and after-tax employee contributions. They do not include loan repayments, rollover contributions, or catch-up contributions.

For a typical high-income Solo 401(k) participant, the §415 bucket is not fully used:

Example: An S-corp owner pays herself $150,000 W-2 salary. Employee deferral: $23,500. Employer profit-sharing: $37,500 (25% × $150,000). §415 capacity used: $61,000. After-tax contribution room: $9,000. If she is age 55: employee deferral rises to $31,000, profit-sharing stays at $37,500, §415 ceiling rises to $77,500, leaving $9,000 in after-tax room — same in this case because the higher deferral and higher ceiling offset each other. At a higher salary — say, $200,000 — employer profit-sharing maxes out at $50,000, the $23,500 deferral fills the rest to $73,500, leaving only $3,500 in after-tax capacity before the age-50+ catch-up.

The largest after-tax room exists when employer profit-sharing is small relative to compensation, or when the owner's compensation is structured to leave slack under the §415 ceiling. A sole proprietor earning $100,000 net SE income contributes roughly $18,587 in employer profit-sharing (after the SE tax deduction adjustment). Add the $23,500 employee deferral: $42,087 total. After-tax capacity: $27,913 — a meaningful Roth accumulation opportunity.

See SEP IRA vs SIMPLE IRA vs Solo 401(k): Choosing the Right Retirement Plan for Small Business Clients for a comparison of contribution mechanics across plan types.

The Two Execution Pathways

After funding the after-tax (non-Roth) bucket, the client converts those dollars to Roth status through one of two routes:

Pathway 1: In-Plan Roth Conversion

IRC §402A(c)(4), added by ATRA 2012 and broadened by SECURE 2.0 §120, allows plans to offer in-plan Roth conversions — the participant moves after-tax contributions from the non-Roth account to the Roth sub-account within the same plan. The plan document must explicitly permit in-plan Roth conversions. Earnings on the after-tax contributions accumulated prior to conversion are taxable; the after-tax contributions themselves convert tax-free because basis already exists.

The converted amount is reported on Form 1099-R with distribution code "G" (direct rollover) or "H" (in-plan Roth conversion), and the taxable portion is included in gross income in the conversion year. If the client converts promptly after each contribution — before any growth occurs — the taxable portion is essentially zero.

Pathway 2: In-Service Distribution to Roth IRA

Alternatively, if the plan allows in-service withdrawals of after-tax contributions (permissible under IRC §72(d)), the participant can roll the after-tax amount directly to a Roth IRA and the earnings to a Traditional IRA (splitting the check as permitted by IRS Notice 2014-54). The distribution rules require 60-day rollover treatment or a direct trustee-to-trustee transfer to avoid withholding. This pathway converts the after-tax dollars to a Roth IRA — with Roth IRA contribution limits irrelevant because this is a rollover — and separates out any taxable earnings cleanly.

Practical note: In-plan Roth conversion is cleaner for ongoing management. The in-service distribution route is useful when the client wants to consolidate Roth assets in an IRA and exit the plan eventually, or when the plan custodian does not support in-plan Roth conversions but does allow in-service distributions.

Plan Document Requirements — Why Most Prototype Plans Don't Work

The mega backdoor Roth requires three specific provisions in the plan document:

1. Authorization of after-tax (non-Roth) employee contributions under IRC §401(a)(1). Standard prototype plans from Fidelity, Vanguard, and Schwab's self-directed Solo 401(k) products do not include this feature as of 2025.
2. In-plan Roth conversion permission (if using Pathway 1), or explicit authorization of in-service distributions of after-tax contributions (if using Pathway 2).
3. Separate accounting for after-tax amounts and earnings on those amounts, required by IRC §402(c)(2) and IRS Notice 2014-54 to properly track basis.

CPAs should direct clients to third-party administrators (TPAs) that offer custom Solo 401(k) plan documents — examples include Carry, MySolo401k, and several regional TPAs. The annual cost for a custom plan document with TPA administration is typically $100–$500 per year, a modest cost relative to the Roth accumulation opportunity. Plans with assets exceeding $250,000 must file Form 5500-EZ annually under IRC §6058(a).

Warning: Do not attempt this strategy with a plan that lacks explicit authorization. Contributions that exceed permitted limits or violate plan terms create excess contribution issues under §415 or plan disqualification risk.

Coordination With W-2 Wages and S-Corp Structure

For S-corp owners using a Solo 401(k), the employer profit-sharing contribution is capped at 25% of W-2 compensation paid to the owner — not net income. This creates a planning variable: higher W-2 wages increase employer contribution capacity (and FICA exposure) while lower wages reduce profit-sharing but increase after-tax room under §415 if the plan document permits it.

A CPA optimizing for both SE tax minimization and Roth accumulation should model several salary scenarios:

• At low W-2 wages (e.g., $60,000): profit-sharing = $15,000; total with $23,500 deferral = $38,500; after-tax room = $31,500 (before age-50+ adjustments).
• At high W-2 wages (e.g., $200,000): profit-sharing = $50,000; total with $23,500 deferral = $73,500; after-tax room = $3,500 — or zero at 60+ with the higher $73,500 ceiling already exhausted.

The sweet spot for the mega backdoor Roth is a moderate W-2 salary that satisfies reasonable compensation requirements without pushing employer profit-sharing so high that no §415 capacity remains for after-tax contributions.

How This Interacts With a Day Job 401(k)

If the client has W-2 income from an unrelated employer where they also participate in a 401(k), employee deferral limits are per-person across all plans (IRC §402(g)(1)): the combined pre-tax and Roth deferrals across all plans cannot exceed $23,500 in 2025 ($31,000 at 50+). However, §415 annual additions limits apply per employer. This means:

• The client cannot double-dip on employee deferrals — if they defer $23,500 at their day job, they cannot defer anything in their Solo 401(k).
• But employer profit-sharing and after-tax contributions to the Solo 401(k) are subject to a separate §415 limit for the self-employment entity. The Solo 401(k) can still receive employer profit-sharing and after-tax contributions even if the employee deferral is zero — the §415 ceiling at the sole-prop entity is still $70,000.

In this scenario, the after-tax capacity becomes: $70,000 − employer profit-sharing = total after-tax room. For a sole proprietor with $100,000 net SE income, that's roughly $70,000 − $18,587 = $51,413 in after-tax Roth capacity (subject to the 100%-of-compensation limit and provided the plan document permits it). This is a significant opportunity for high earners who already maximize their day-job 401(k).

For the SECURE 2.0 catch-up rules governing high-earner Roth treatment, see Mandatory Roth Catch-Up Contributions 2026: Employer and CPA Action Guide.

Taxation and Basis Tracking

After-tax contributions establish cost basis in the plan under IRC §72(d)(1). The CPA should maintain records of cumulative after-tax contributions because:

• At in-plan Roth conversion: only earnings accumulated since contribution are taxable; the after-tax principal converts tax-free.
• At eventual distribution (if basis is not converted): the pro-rata rule of IRC §72(e) governs recovery of basis across all non-Roth 401(k) balances, which can complicate basis recovery if pre-tax assets are large.
• For in-service distributions to a Roth IRA under Notice 2014-54: the after-tax amount rolls directly to the Roth IRA (no tax); earnings roll to a Traditional IRA (deferred, not immediately taxable).

The cleanest execution — converting after-tax contributions immediately before earnings accumulate — minimizes recordkeeping complexity and taxable income at conversion. This is sometimes called "rinse and repeat": contribute after-tax dollars, convert the same day, repeat next contribution period.

For how Roth conversions interact with MAGI, IRMAA, and the QBI deduction, see How to Advise Clients on Roth IRA Conversions: A CPA's Step-by-Step Tax Guide.

Who Is the Right Candidate?

The mega backdoor Roth Solo 401(k) strategy delivers the most value for clients who:

• Are self-employed with no non-owner employees (or a business with only their spouse as employee)
• Have maxed out employee deferrals and employer profit-sharing but still have §415 room
• Are in a high current-year tax bracket and project lower (or equal) rates in retirement — if the conversion generates taxable earnings, the bracket math must support it
• Have a long investment horizon to let Roth growth compound tax-free
• Do not need the Roth IRA for emergency liquidity (assets in the plan have different access rules than a Roth IRA's contribution basis)

The strategy is not appropriate when the plan document doesn't support it, when the client has significant after-tax contribution earnings that would make the conversion taxable at a high rate, or when IRMAA exposure is a concern and the added income from conversion earnings would push the client into a higher Medicare premium tier.

Frequently Asked Questions

Can a married couple use the mega backdoor Roth in a Solo 401(k)?

Yes, if both spouses earn compensation from the business. A Solo 401(k) can cover a sole proprietor and their spouse who works in the business. Each spouse maintains a separate participant account with their own §415 limit. Both can make after-tax contributions up to their respective §415 ceilings, provided the plan document permits it. The contribution limits are not shared between spouses.

Does the after-tax contribution reduce the employer profit-sharing deduction?

No. After-tax employee contributions are not employer contributions under IRC §404 and do not count against the employer's deductible profit-sharing limit (25% of W-2 compensation or the net SE income equivalent). They are employee contributions funded with already-taxed dollars, with no deduction to the employee or employer.

What happens to the Solo 401(k) if the business later hires employees?

If the business adds an employee who is not the owner's spouse and who meets the plan's eligibility requirements, the plan loses its "one-participant plan" status and must satisfy the nondiscrimination requirements of IRC §401(a)(4) and §410(b). The mega backdoor Roth strategy may still work if the plan passes nondiscrimination testing — but the plan sponsor (the business owner) would need a TPA to perform annual testing. Most Solo 401(k) prototype plans terminate when employees are added; a custom TPA-administered plan can continue.

Is there an income limit for after-tax contributions to a Solo 401(k)?

No. Unlike Roth IRA direct contributions — which phase out at $150,000–$165,000 MAGI for single filers and $236,000–$246,000 for married filing jointly in 2026 — after-tax contributions to a qualified plan like a Solo 401(k) have no income limit. The §415 ceiling is the only cap. High-income earners who cannot directly fund a Roth IRA can execute the mega backdoor Roth entirely through the plan. See Roth IRA: Definition and How It Works for Roth IRA income phase-out details.

What custodians support the mega backdoor Roth in a Solo 401(k)?

As of 2025, most major discount custodians (Vanguard, Fidelity's self-directed Solo 401(k), Schwab) do not support after-tax contributions in their prototype plans. Custodians and TPAs that do include Carry, MySolo401k, Nabers Group, and several regional TPA firms. Costs range from $100–$500 per year for plan administration. Some custodians require the client to direct their own investment elections.

Can the after-tax dollars be invested in the same funds as the pre-tax 401(k) balance?

Yes, if the plan custodian supports it. After-tax contributions and their earnings are tracked in a separate sub-account for basis purposes, but they are typically invested in the same investment menu as the rest of the plan. The separate accounting requirement under IRC §402(c)(2) is an accounting distinction, not a physical separation of investment accounts.

How does the mega backdoor Roth interact with SECURE 2.0 changes?

SECURE 2.0 (Pub. L. 117-328) broadened in-plan Roth conversion availability (§120) and expanded it to any plan type. The enhanced catch-up contribution limits for ages 60–63 ($11,250 additional in 2025, i.e., total catch-up of $11,250, bringing the deferral to $34,750) increase the employee deferral but also raise the §415 ceiling to $77,500 — the after-tax room is not necessarily reduced. However, the mandatory Roth treatment of catch-up contributions for employees earning over $145,000 in prior-year FICA wages (effective 2026) means some catch-up dollars must be Roth rather than pre-tax, a distinction that does not affect after-tax contribution mechanics. See SECURE 2.0 Act Retirement Plan Changes Every CPA Must Know for 2025–2026 for the full provision timeline.

How Arvori Can Help

Arvori connects CPAs with insurance and financial service professionals who work with the same high-income self-employed clients who benefit most from the mega backdoor Roth strategy. If your clients need retirement plan services alongside their tax planning, Arvori's referral platform can match them with qualified professionals — and strengthen your practice's cross-service relationships. Learn more at arvori.app.