At a glance
- What it is
- Making after-tax (non-Roth) 401(k) contributions on top of your regular contributions, then converting them to Roth inside the plan.
- What it saves
- Future tax. The converted money and its growth come out tax-free in retirement if the Roth rules are met.
- Best fit
- High earners with savings beyond the basics.
- Main catch
- Your plan document has to allow both after-tax contributions and in-plan conversions. Many off-the-shelf plans don’t.
- 2026 total
- $72,000 across all contributions to the plan, not counting catch-up.
Key takeaways
- The mega backdoor Roth means making after-tax 401(k) contributions above your regular ones, then converting them to Roth inside the plan.
- It fits high earners who already max out regular contributions, especially owners with no employees using a Solo 401(k).
- In the illustrative example, an S corp owner with a $100,000 salary reaches the full $72,000 2026 limit, including $22,500 after-tax.
- The catch: your plan document must allow both after-tax contributions and in-plan Roth conversions, and many off-the-shelf plans don’t.
The numbers
Picture an S corp owner, age 45, with no employees, a $100,000 W-2 salary, and a Solo 401(k) whose plan document allows after-tax contributions and in-plan conversions.
For illustration purposes. Tax law may change, which may change these calculations slightly.
| Employee deferral (2026 limit) | $24,500 |
| Employer profit share: 25% × $100,000 | $25,000 |
| After-tax contribution: $72,000 − $24,500 − $25,000 | $22,500 |
| Total 2026 contributions | $72,000 |
The $22,500 after-tax contribution gets no deduction; it comes from money that has already been taxed. Converted to Roth right away, before it earns anything, the conversion adds $0 to taxable income. Any earnings between the contribution and the conversion are taxable when converted, which is why timing matters.
Add up all three pieces and the full $72,000 can end up Roth: the deferral can be made as Roth, the employer contribution can be designated Roth if the plan allows (it’s taxable to you in the year it’s made), and the after-tax piece is converted. Whether to make the first two pieces pre-tax or Roth is a separate decision to model.
This only works if your plan document says it does. Many standard Solo 401(k) plans from big brokerages don’t allow after-tax contributions, so the first step is reading the plan, or switching to one that does.
Could this strategy work for your business?
In a free tax analysis, your tax strategist checks whether it fits and what it could save, using your real numbers.
How it works
- Max your regular contributions. Employee deferral and employer profit share first.
- Add after-tax contributions. Contribute after-tax dollars up to the $72,000 total for 2026, or 100% of your compensation if that is lower.
- Convert in-plan to Roth. Convert the after-tax money to the Roth account inside the plan, ideally soon after each contribution.
- Report it correctly. The plan issues a Form 1099-R for the conversion, and only the earnings, if any, are taxable.
Who it fits
- High earners who already max out their regular contributions and want more money growing tax-free.
- Owners with no employees, where a Solo 401(k) can be written to allow it.
- Owners with cash flow to save beyond the basics without needing the money for years.
When it doesn’t fit
- Plans that don’t allow after-tax contributions or in-plan conversions.
- Businesses with employees, where after-tax contributions must pass nondiscrimination testing that owner-heavy plans often fail.
- Owners who would be better served by a pre-tax deduction now, or who don’t have the cash to set aside.
The rules
- The total limit. All contributions to the plan, from every source, can’t exceed $72,000 for 2026 or 100% of compensation. Catch-up contributions are on top of that.
- The plan document controls. After-tax contributions and in-plan Roth conversions are both optional plan features.
- Earnings are taxed on conversion. The after-tax contributions themselves aren’t taxed again; any growth before conversion is.
- Roth distribution rules. Tax-free withdrawals generally require age 59½ and a Roth account that is at least five years old.
Risks to weigh
- Waiting to convert. Leaving after-tax money unconverted lets taxable earnings build up.
- Plan errors. Contributions the plan doesn’t permit, or totals over the limit, have to be corrected.
- Locking up cash. Money in a 401(k) is harder to reach than money in a brokerage account.
- Rules change. Congress has proposed limiting this strategy before; it remains allowed under current law.
How we handle it for clients
With a Cannon & Brown tax plan, we review your plan document, figure how much after-tax room you have each year, coordinate the contributions and conversions with payroll and your provider, and make sure the conversion is reported correctly on your return. Because our technology keeps your numbers current all year, we can test this strategy against your real numbers and adjust it as your business changes.
Illustrative example. The business owner and figures on this page are hypothetical, used to explain how the strategy works. They are not a client and not a promise of results. The example assumes a $100,000 W-2 salary, no employees, no other 401(k) participation, and a plan that allows after-tax contributions and in-plan Roth conversions. This page is educational and is not tax advice for your situation; tax rules and limits change.
Brooks leads Cannon & Brown Tax Advisors, a forward-thinking tax firm serving business owners in Oklahoma and beyond. He builds year-round tax strategies that help business owners keep more of what they earn and reach their goals. More about Brooks
Common questions about the mega backdoor Roth
Does my Solo 401(k) allow a mega backdoor Roth?
Only if the plan document allows after-tax contributions and in-plan Roth conversions. Many standard Solo 401(k) plans from big brokerages don’t, so the first step is reading the plan or switching to one that does.
Do I pay tax when I convert after-tax 401(k) money to Roth?
The after-tax contributions themselves aren’t taxed again, but any earnings before the conversion are taxable. That is why converting soon after each contribution matters.
Can a business with employees do a mega backdoor Roth?
It’s harder. After-tax contributions must pass nondiscrimination testing, which owner-heavy plans often fail.
When can I take the Roth money out tax-free?
Tax-free withdrawals generally require age 59½ and a Roth account that is at least five years old.
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