What is the Mega Backdoor Roth?
The Mega Backdoor Roth is a way to get significantly more money into Roth accounts beyond the standard limits. It works by making after-tax (non-Roth) contributions to a 401(k) and then immediately converting those funds to Roth — either within the plan or by rolling them to a Roth IRA.
For self-employed individuals with a Solo 401(k), this strategy can put up to $46,500 of after-tax dollars into Roth treatment in 2026 — in addition to the $23,500 employee deferral (traditional or Roth). Total Roth contributions this way: up to $70,000.
How it works step by step
- Make after-tax contributions: After maxing your pre-tax or Roth employee deferral ($23,500), make additional after-tax contributions up to the $70,000 total limit. At $100,000 net income, this might be $14,776 after the employer contribution and $23,500 deferral, leaving $46,500 - $14,776 - $23,500 = $8,224 of remaining room.
- Convert to Roth: Immediately convert the after-tax contributions to Roth within the plan (in-plan Roth conversion) or roll them to a Roth IRA. Converting quickly limits the taxable gains that accumulate in the after-tax account.
- Tax on conversion: You owe income tax only on any earnings that accumulated in the after-tax account since the contribution — typically minimal if you convert promptly (the 'backdoor' happens quickly).
Which Solo 401(k) providers support this?
Not all providers allow after-tax contributions or in-plan Roth conversions. As of 2026:
Supports after-tax + in-plan Roth conversion: - Fidelity (most flexible, no fees) - E*Trade - TD Ameritrade (now Schwab — check current status)
Does NOT support after-tax contributions: - Vanguard Solo 401(k) - Many prototype plan documents from smaller providers
If you want to use the Mega Backdoor Roth strategy, open your Solo 401(k) at Fidelity and confirm their current plan document supports both after-tax contributions and in-plan conversion before contributing.
Mega Backdoor Roth vs. regular Roth Solo 401(k)
Regular Roth Solo 401(k): Employee deferral of up to $23,500 contributed after-tax into a Roth account. No conversion needed — the money is Roth from the start.
Mega Backdoor Roth: After-tax contributions beyond the $23,500 Roth limit, converted to Roth. This only adds value if you've already maxed the $23,500 Roth deferral and want to push more funds into tax-free growth.
For most self-employed workers, the regular Roth Solo 401(k) is simpler and sufficient. The Mega Backdoor Roth is a power move for high earners (physicians, high-income consultants, software developers) who want to shelter every possible dollar from future taxes.
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Frequently Asked Questions
Is the Mega Backdoor Roth legal?
Yes. The IRS permits after-tax contributions to 401(k) plans and in-plan Roth conversions under specific plan documents. Congress has considered eliminating this strategy but as of 2026 it remains legal. Use it while it's available.
How much can I put into the Mega Backdoor Roth in 2026?
The total Solo 401(k) limit is $70,000. Subtract your employee deferral and employer contribution — the remainder is your maximum after-tax contribution space. At $80,000 net income: $70,000 - $23,500 deferral - $14,776 employer contribution = $31,724 of after-tax Mega Backdoor room.
Which provider should I use for Mega Backdoor Roth in a Solo 401(k)?
Fidelity is the most commonly recommended provider for Mega Backdoor Roth in a Solo 401(k). It supports after-tax contributions and in-plan Roth conversions with no fees. Open the account online in about 20 minutes and confirm the plan document language allows both features before contributing.