Mega backdoor Roth in a solo 401(k): tax planning math for 1099 and S-corp physicians
Your 401(k) deferrals are maxed and the backdoor Roth IRA is done for the year, yet there is still 1099 income sitting in the practice account with no obvious tax advantaged place to put it.
For a lot of self employed physicians, the next move is the mega backdoor Roth inside a solo 401(k). It can move tens of thousands of dollars a year into Roth money, far more than an IRA allows. It also only works if your plan document is set up for it, and it has tax rules that trip up smart people.
This guide covers how it works for 1099 and S-corp physicians in 2026, starting with the rules your plan has to meet.
In this blog
What is a mega backdoor Roth?
A mega backdoor Roth is a two step move inside a 401(k). You make after tax (non Roth) contributions above the normal employee deferral limit, then move that money into the plan's Roth account. The after tax dollars go in already taxed, so converting them does not create new tax on the contributions themselves.
You only need three terms to follow the rest of this post:
Elective deferrals: the pretax or Roth money you choose to defer from pay. For 2026 the limit is $24,500 (IRS Notice 2025-67).
Annual additions: everything that lands in your account in a year from all sources. The tax code counts employer contributions and employee contributions, including after tax money, toward one cap (26 U.S.C. 415(c)). For 2026 that cap is $72,000 (Notice 2025-67).
After tax contributions: employee money that goes in after tax but is not designated Roth. Growth on it is treated as pretax.
The gap between what you defer, what your business contributes, and that $72,000 cap is the room a mega backdoor Roth fills.
Can you do a mega backdoor Roth in a solo 401(k)?
Yes, if your plan allows it. A plan with a Roth account can permit an in plan Roth rollover of an amount not otherwise distributable under the plan, according to IRS Notice 2013-74. That is the legal hook for converting after tax money while you are still working.
Your solo 401(k) plan document has to permit two things:
After tax employee contributions, as a separate source from pretax and Roth deferrals
In plan Roth rollovers (or in service distributions) of that after tax money
Not every solo 401(k) permits after tax contributions or in plan Roth rollovers. If yours does not, the plan may need to be amended or replaced before you contribute a dollar. We treat this as the first question in any mega backdoor conversation, because nothing else matters until the plan document says yes.
Is the mega backdoor Roth still allowed in 2026?
Under current law as of September 2026, yes. The in plan Roth rollover rule above is still on the books. Tax law changes, though, so a plan built around this move deserves a fresh look every year.
Does nondiscrimination testing get in the way?
After tax contributions are normally subject to a fairness test (the ACP test) that compares highly paid employees to everyone else. In an owner only plan, that test is rarely a problem. The regulations say that if no non-highly compensated employees are eligible for the year, the plan is deemed to satisfy the ACP test (Treas. Reg. 1.401(m)-2).
That changes the moment you hire staff who become eligible. If your practice is growing, tell your tax team before the first hire, not after.
How much can a physician put in? The 2026 math
Start with the cap and work backward. For 2026:
Item | 2026 figure |
Employee deferral limit (402(g)) | $24,500 |
Total annual additions cap (415(c)) | $72,000 |
Catch up, age 50 or older | $8,000 |
Catch up, age 60 to 63 in 2026 | $11,250 |
Compensation limit (401(a)(17)) | $360,000 |
Source: IRS Notice 2025-67.
Catch up contributions sit on top of the $72,000 cap. The IRS puts the total at $80,000 with catch ups, or up to $83,250 for those age 60 to 63 (IRS contribution limits page).
The formula for after tax room is simple:
$72,000 − employee deferrals in this plan − employer contribution = space for after tax contributions
How big that employer contribution is depends on how your practice is taxed.
Sole proprietor or single member LLC
In a solo 401(k), the employer side can be up to 25% of compensation as the plan defines it (IRS, One Participant 401(k) Plans). For a self employed physician, compensation is earned income, which the IRS defines as net earnings from self employment after deducting both one half of your self employment tax and the contributions for yourself.
Because earned income is figured after subtracting half of your self employment tax and the contribution itself, the math is circular. The IRS solves it with a reduced rate: divide the plan rate by 100% plus the plan rate (IRS, self employed contribution calculation).
A 25% plan rate becomes 25% divided by 125%, or 20%, applied to net self employment earnings after the half of SE tax deduction. The full worksheet is in Publication 560.
S-corp physician
If your practice is an S corporation, contributions are based on your W-2 compensation only. The IRS says contributions to a retirement plan can only be made from compensation, and S corporation distributions do not count (IRS S corporation FAQ).
That has a planning consequence. The employer contribution is "up to" 25%, so it is optional: skipping it or making it smaller leaves more of the $72,000 cap open for after tax dollars, without touching salary. Salary itself is set by reasonable compensation rules, not by the retirement plan.
The catch: total annual additions cannot exceed 100% of your compensation (26 U.S.C. 415(c)(1)(B)). Set the salary too low and it caps the total, too.
Your plan administrator sets how after tax contributions run through payroll. Salary level is its own judgment call, tied to reasonable compensation rules we cover in our guide to S-corp tax structure for physicians.
The tradeoff you are making
Every dollar of after tax contribution is a dollar you did not deduct this year.
Employer contributions and pretax deferrals reduce taxable income now.
After tax contributions give no deduction now, but they become Roth money after conversion.
Neither is automatically better. The answer turns on today's bracket versus the one you expect later, and on whether you can spare the cash. That is a tax planning question worth running both ways before you pick.
Dr. Chen: 1099 anesthesiologist, sole proprietor
Dr. Maya Chen is 44, works as a 1099 anesthesiologist, and has net self employment earnings of $300,000. Her solo 401(k) allows after tax contributions and in plan Roth rollovers. This hypothetical is for illustration only.
If she skips the employer contribution this year:
Employee deferral: $24,500
Employer contribution: $0
After tax room: $72,000 minus $24,500 = $47,500
Now the other path. With a full employer contribution, the IRS method gives her 20% of her net earnings after the half of SE tax deduction. Self employment tax is 15.3% (IRS self employment tax rate), so half of it can never be more than 7.65% of her earnings.
Net earnings after the deduction: ~ $277,050 ($300,000 minus $22,950)
Maximum employer contribution: ~ $55,410 (20% of $277,050)
Room left under the cap after her deferral: $47,500
Result: the employer contribution alone fills the rest of the cap, and after tax room drops to $0
So at her income, the choice is stark. Every employer dollar is deductible now and comes straight out of the after tax bucket; every after tax dollar skips the deduction but can become Roth money. The exact employer figure comes from the Publication 560 worksheet.
What if you also have a W-2 hospital 401(k)?
Maxing deferrals at the hospital closes the door on a solo 401(k). It does not, because the two limits below work on different scales.
The employee deferral limit is per person. The IRS says limits on elective deferrals are by person, not by plan (IRS, One Participant 401(k) Plans). If you defer $24,500 at the hospital, you have no regular deferral room left in your solo 401(k). If you are 50 or older and both plans allow catch up contributions, your one catch up limit ($8,000 for 2026, or $11,250 if you are 60 to 63) can be split between the plans (IRS contribution limits).
The $72,000 total cap is per employer. The IRS limits total annual additions to plans maintained by one employer and any related employer (IRS contribution limits). An unrelated hospital and your own practice are measured separately.
So a physician who maxes deferrals at the hospital can still fund a solo 401(k) through employer contributions and after tax money, up to its own cap. If you own part of the group that employs you, the related employer rules can combine the two. Confirm ownership before you count on a second cap.
Dr. Brooks: S-corp hospitalist with a hospital job
Dr. Andre Brooks is 52. He earns W-2 wages at a hospital, where he defers the full $24,500. He also runs a side practice as an S corporation that pays him a $180,000 W-2 salary. This hypothetical is for illustration only.
Regular employee deferral in the solo 401(k): $0 (already used at the hospital)
Employer contribution: up to 25% of $180,000 = $45,000
After tax room: $72,000 minus $45,000 = $27,000
That $27,000 of after tax money can then move to Roth through an in plan rollover, if his plan allows it. At 52 he may also have up to $8,000 of catch up room, which may have to be Roth under the rule below.
This math assumes the hospital and his practice are unrelated employers; if he owns part of the hospital group, the two plans may share one cap.
The Roth catch up rule for high earners
Starting in 2026, catch up contributions for higher earners have to be Roth. For 2026, the rule applies to people whose 2025 wages from the employer sponsoring the plan topped $150,000 (Notice 2025-67). The final regulations generally apply to taxable years beginning after December 31, 2026, with a good faith standard before then (TD 10033).
The details worth knowing:
The test looks at W-2 (FICA) wages from that specific employer, not total income.
A physician with no FICA wages from the plan sponsor, such as a partner with only self employment income, is not subject to it. A 1099 sole proprietor with no W-2 wages from their own practice generally falls outside the rule for that plan.
How is the mega backdoor Roth taxed?
The contributions go in after tax, so converting them is not taxed again. Growth is different. The IRS treats earnings on after tax contributions as pretax amounts in your account (IRS, rollovers of after tax contributions).
For an in plan Roth rollover, the amount included in income is the amount rolled over, less any basis in the amount transferred (IRS, designated Roth accounts). In plain terms:
After tax contributions converted: not taxed again
Earnings converted with them: taxable income in the year of the conversion
So timing matters: the longer after tax money sits before conversion, the more earnings can build up, and those earnings are what get taxed on the way into Roth.
Does the pro rata rule apply?
You may know the pro rata rule from the backdoor Roth IRA. We cover that version of the rule, and how it catches physicians with existing IRA balances, in the SEP-IRA trap every self employed physician should know about.
Inside a 401(k), the question works differently. IRS guidance says a partial distribution from a plan has to include some pretax amounts (IRS, rollovers of after tax contributions).
The tax code lets a plan account for employee contributions and their earnings as a separate contract (26 U.S.C. 72(d)(2)). If your plan does, a conversion from that after tax subaccount carries a proportional share of its earnings (26 U.S.C. 72(e)(8)). The contributions come out tax free; the earnings riding along are taxable.
So the conversion is rarely 100% tax free. Before you convert, ask your plan administrator how the after tax subaccount is tracked.
What about money that leaves the plan?
If you later have a distributable event, such as leaving the plan, one allocation rule helps. Distributions sent to multiple destinations at the same time are treated as a single distribution for allocating pretax and after tax amounts (Notice 2014-54). That lets pretax dollars go to a traditional IRA and after tax dollars go to a Roth IRA in the same move.
Is there a five year rule?
Yes. If an in plan Roth rollover is the first contribution to your Roth account, the five year clock for a qualified, tax free distribution starts on the first day of the tax year of that rollover (Notice 2013-74). If you already had Roth money in the plan, the earlier start date controls (26 U.S.C. 402A(d)(2)(B)).
A second five year rule can apply the early withdrawal penalty to the taxable part of a conversion taken out within five years (2026 Instructions for Forms 1099-R), if you are under age 59½ and no exception applies (IRS Topic 558). Track both clocks from the first conversion year.
Step by step: running a mega backdoor Roth in a solo 401(k)
Confirm the plan document permits after tax contributions and in plan Roth rollovers.
Decide your employee deferral and employer contribution first, so you know the after tax room.
Make after tax contributions as a separate source, never labeled as deferrals.
Request the in plan Roth rollover and keep records of each contribution and conversion.
Track basis each year so the taxable earnings are reported correctly.
On timing, employer contributions can be made up to your business's tax return due date, including extensions, and still count for the prior year (26 U.S.C. 404(a)(6)).
Employee deferrals work differently depending on how you are taxed. For sole proprietors and other owner employees, the deferral election generally has to be made by the end of the tax year, and the deferrals can then be deposited by the filing deadline, including extensions (Publication 560).
If you work as an employee of your S corporation, the IRS says you make salary deferrals based on your Form W-2 compensation (IRS S corporation FAQ), so plan them around your payroll, not your tax return deadline. A deferral election can only apply to pay that is not yet available to you (Treas. Reg. 1.401(k)-1(a)(3)(iii)). After tax contributions follow your plan's rules, so pin down your dates before December.
If you are still setting the plan up, start with our guide to solo 401(k) setup for 1099 physicians.
Reporting: what your tax team files
The paperwork is where provider sales pages go quiet. Here is what shows up at tax time.
Form 1099-R. The plan reports in plan Roth rollovers on Form 1099-R. The total amount allocable to the rollover goes in box 1 (2026 Instructions for Forms 1099-R). On your return, only the taxable earnings should count.
Form 5500-EZ. Under the 2025 instructions, a one participant plan does not have to file Form 5500-EZ unless the assets of all the employer's one participant plans top $250,000 at year end, or it is the plan's final year (Instructions for Form 5500-EZ). The form is due by the last day of the seventh month after the plan year ends. If a filing was missed, the IRS has a Late Filer Penalty Relief Program for one participant plans.
A mega backdoor Roth can push a solo 401(k) past $250,000 faster than you expect. Put the 5500-EZ on your calendar the year it happens.
Is there a downside to a mega backdoor Roth?
Yes. Before you commit, weigh these:
You give up a current year deduction on every after tax dollar.
Your plan may need an amended document, which can carry a cost.
There is more record keeping for basis, conversions, and reporting.
Contributions and deadlines take coordination with your tax team through the year.
Pairing it with a cash balance plan
Some high earning physicians add a cash balance plan next to a solo 401(k). When an employer keeps both plan types, a combined deduction limit can apply. The code carves out defined contribution amounts that do not exceed 6% of compensation (26 U.S.C. 404(a)(7)).
The plan actuary sizes the cash balance contribution, and the 401(k) side has to be set with it in mind.
Can your spouse join?
A one participant 401(k) can cover a business owner with no employees, or that person and his or her spouse (IRS one participant 401(k) rules). Contributions can only be made from compensation (IRS S corporation FAQ), so a spouse can contribute only if they actually work in the practice and are paid for that work.
The deferral limit applies to each individual (26 U.S.C. 402(g)(1)), so a working spouse has their own employee deferral limit, up to what the practice actually pays them. The employer profit sharing option also applies to the spouse, subject to the annual 415(c) cap ($72,000 for 2026 or $80,000 with catch up) or 25% of their wages, whichever is lesser.
If you have ever put too much into a retirement account, our post on what happens if you contribute too much to a retirement account walks through the fix.
Talk to your tax team before year end
When your plan permits it, a mega backdoor Roth moves after tax dollars into Roth, and for federal purposes the conversion is taxed only on the earnings as long as basis is tracked correctly. That may reduce your tax liability over time. It goes wrong when someone contributes before checking the plan document or converts without the records to prove what was already taxed.
Figures and tax treatment are federal for the 2026 tax year unless stated, and state treatment of in plan Roth conversions can differ; contribution limits are indexed and change each year. Availability depends on your plan document; not every solo 401(k) permits after tax contributions or in plan Roth rollovers.
This is general information, not tax advice. Results depend on your facts; talk to your tax team about whether this fits your situation. Based on current law as of September 28, 2026; rules can change.
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Disclaimer: This material is intended for educational and informational purposes only and does not constitute tax, legal, accounting, or financial advice. The content is general in nature and may not apply to your specific circumstances. Tax laws and financial regulations are subject to change and interpretation, and the application of these laws can vary based on individual situations. Before making any decisions, you should consult with a qualified tax advisor, legal counsel, or financial professional.
