Answer
Can I do a mega backdoor Roth with a Solo 401(k)?
Short answer
Yes, but only if your Solo 401(k) plan document explicitly allows after-tax contributions and in-plan Roth conversions. Most free prototype plans from Fidelity, Schwab, and Vanguard do not include this feature. A custom plan document from a third-party administrator (TPA) typically does, at a cost of $500 to $1,500 per year. When available, the strategy allows contributions above the standard employee deferral, up to the $70,000 combined limit, converted immediately to Roth with no income limit.
Solo 401(k) contribution layers — $70K combined limit (2026)
Pre-tax employee deferral
$23,500
Employer profit-sharing
$20,000
After-tax (Mega Backdoor Roth)
$26,500
Total 2026 combined limit
$70,000
The $26,500 after-tax portion converts to Roth immediately. Tax-free growth, no income limit, no $7,000 IRA cap.
How it works: the 2026 Solo 401(k) combined limit is $70,000. You max the pre-tax employee deferral ($23,500), make the employer profit-sharing contribution (say $20,000), and have $26,500 of remaining room. Under a plan that allows after-tax contributions, you deposit that $26,500 in after-tax (non-deductible) dollars. You then immediately convert those after-tax contributions to Roth within the plan. The converted amount grows tax-free, and future qualified distributions are tax-free.
Why not all plans support it: the plan document must specifically authorize both after-tax employee contributions and in-plan Roth rollovers (or in-service withdrawals to a Roth IRA). IRS Notice 2014-54 clarified the mechanics, but plan providers must choose to include it. Free brokerage prototype plans are designed for simplicity and exclude this feature.
Who it makes sense for: high earners who have already maxed the pre-tax deferral and employer contribution, earn too much for a direct Roth IRA (phase-out begins at $150,000 single in 2026), have a long investment horizon where tax-free growth compounds meaningfully, and can justify the TPA cost against the tax benefit. At 37% marginal rate, $26,500 in after-tax contributions that grows to $100,000 over 15 years produces $73,500 in tax-free growth. The annual TPA cost of $1,200 pays for itself quickly.
The simpler alternative: the standard Roth Solo 401(k) employee deferral of $23,500 (no income limit, available from most major free brokerage plans). For most self-employed people, this provides sufficient Roth access without the TPA cost or complexity. Evaluate the mega backdoor only after maxing the standard Roth deferral.
Interaction with the backdoor Roth IRA: the mega backdoor Roth is separate from the traditional backdoor Roth IRA ($7,000 nondeductible IRA contribution plus immediate Roth conversion). Both strategies can be executed in the same year. The Solo 401(k) does not create pro-rata issues for the backdoor IRA if the 401(k) is not an IRA (it is not). The two are independent.
Roth vs taxable growth on $26,500
At 37% marginal rate, Roth compounding pulls far ahead over time.
Same $26,500 after-tax starting amount. Roth grows at 7% tax-free. Taxable account grows at 7% but distributions are taxed at 37%.
10 years
Roth 401(k)
Taxable
15 years
Roth 401(k)
Taxable
20 years
Roth 401(k)
Taxable
25 years
Roth 401(k)
Taxable
Illustrative. 7% gross return assumed. Taxable account return net of 37% tax drag. Actual results vary.
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