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The mega backdoor Roth: filling the rest of the $72,000
The short answer
The 2026 combined 401(k) limit is $72,000 — but deferrals ($24,500) plus a typical match leave tens of thousands of that ceiling unused. Plans that allow after-tax contributions let you fill the gap, then convert those dollars to Roth. That is the mega backdoor: potentially $20,000–$40,000+ of new Roth money per year — if your plan (or your own Solo 401(k)) has the right two features.
How it works
- Max the normal layers: $24,500 deferral + employer contributions.
- Contribute after-tax up to the remaining room under $72,000.
- Convert promptly — in-plan Roth conversion, or roll after-tax dollars to a Roth IRA (earnings, if any, to a traditional IRA under Notice 2014-54).
- Automate per paycheck if the plan allows; the strategy compounds on consistency.
Physician with employer plan allowing after-tax
Where it goes wrong
- Assuming your plan allows it — read the SPD or ask HR before contributing
- Letting after-tax money sit unconverted for years, building a taxable earnings layer
- Solo 401(k) owners using a template document that silently lacks the feature
Frequently asked questions
What does my 401(k) plan need to allow?
Two features: after-tax (non-Roth) employee contributions, and either in-plan Roth conversions or in-service distributions to a Roth IRA. If you control a Solo 401(k), you can design them in.
How much can the mega backdoor add?
Up to the overall limit — $72,000 in 2026 — minus your deferrals and employer contributions. For many high earners that is $20,000–$40,000 of extra Roth space per year.
Why convert the after-tax money quickly?
Earnings on after-tax contributions are pre-tax until converted; converting promptly keeps the taxable slice near zero.
Does this work in a Solo 401(k)?
Yes — with a plan document that permits after-tax contributions and conversions. Off-the-shelf free plans often do not; a custom document does.
Does your plan have the door?
Send us your plan documents — we will tell you whether the mega backdoor is open, and design it into your Solo 401(k) if you are your own plan sponsor.
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