Solo 401(k) and the Mega Backdoor Roth: How Self-Employed Owners Can Save $70,000+ Per Year

By Yarik Yarosh, CPA ·September 5, 2026 ·Updated September 23, 2026 ·8 min read

Written by Yarik Yarosh, CPA (US & Canada) September 5, 2026 · FL CPA license AC61704 · CPA Ontario

The solo 401(k) (also called an individual 401(k) or one-participant 401(k)) is the most powerful retirement savings vehicle available to self-employed individuals with no employees (other than a spouse). It allows contributions far exceeding IRA limits, with three contribution types: (1) employee elective deferrals ($23,500 for 2025, or $31,000 with standard catch-up for age 50+, or $34,750 with the SECURE 2.0 super catch-up for ages 60-63), (2) employer profit-sharing contributions (up to 25% of W-2 compensation or 20% of net self-employment income), and (3) after-tax employee contributions (which enable the mega backdoor Roth strategy). The total of all three contribution types can’t exceed $70,000 for 2025 ($77,500 with standard catch-up, or $81,250 with super catch-up). The mega backdoor Roth strategy involves making after-tax contributions to the solo 401(k) and immediately converting them to the Roth sub-account, allowing Roth savings far beyond the $7,000 Roth IRA limit.

✓Key takeaway

2025 solo 401(k) contribution limits:

Contribution Type	Under 50	Age 50-59/64+	Age 60-63 (SECURE 2.0 Super Catch-Up)
Employee deferral (pre-tax or Roth)	$23,500	$31,000 (+$7,500)	$34,750 (+$11,250)
Employer profit-sharing (pre-tax only)	Up to 25% of compensation	Same	Same
After-tax employee (mega backdoor Roth)	Remainder up to total limit	Remainder up to total limit	Remainder up to total limit
Total annual limit (all sources)	$70,000	$77,500	$81,250

Solo 401(k) vs. SEP-IRA vs. SIMPLE IRA:

Factor	Solo 401(k)	SEP-IRA	SIMPLE IRA
Max contribution (2025)	$70,000	$70,000	$16,500 (+$3,500 catch-up)
Employee deferral	Yes ($23,500)	No	Yes ($16,500)
Employer contribution	Yes (25% of comp)	Yes (25% of comp)	Yes (match up to 3%)
Roth option	Yes (Roth 401(k))	No	No (until 2026 per SECURE 2.0)
After-tax contributions	Yes (mega backdoor Roth)	No	No
Loan provision	Yes (up to $50,000)	No	No
Eligible employees	None (or spouse only)	Any employee with $750+ comp	Any employee with $5,000+ comp
Administration	Moderate (annual Form 5500-EZ if >$250K)	Simple	Simple

Why solo 401(k) beats SEP-IRA for most self-employed:

Advantage	Details
Employee deferral	Solo 401(k) allows $23,500 as employee deferral regardless of income. SEP-IRA only allows employer contributions (25% of comp), so low-income years produce lower contributions.
Roth option	Solo 401(k) offers Roth 401(k) sub-account. SEP-IRA has no Roth option.
Mega backdoor Roth	Solo 401(k) allows after-tax contributions + in-plan Roth conversion. Not available in SEP-IRA.
Loans	Solo 401(k) allows loans up to $50,000. SEP-IRA doesn’t.

How does the mega backdoor Roth work in a solo 401(k)?

Hypothetical example

Round numbers. For current guidance, see IRS 401(k) plan overview. A sole proprietor consultant, age 45. Net Schedule C income: $200,000.

Step 1: Calculate maximum contributions

Contribution Type	Calculation	Amount
Employee deferral (pre-tax or Roth)	$23,500 (statutory limit)	$23,500
Employer profit-sharing	20% of net SE income (after 50% SE tax deduction)	$200,000 x 92.35% x 20% / (1 + 20%) = approx. $30,783
Subtotal (traditional sources)		$54,283
After-tax employee contribution	$70,000 - $54,283	$15,717
Total		$70,000

Step 2: The mega backdoor Roth conversion The $15,717 in after-tax employee contributions can be immediately converted to the Roth sub-account within the solo 401(k). This is called an “in-plan Roth conversion.”

Item	Tax Treatment
$23,500 employee deferral (if Roth elected)	Contributed after-tax, grows tax-free forever
$30,783 employer profit-sharing	Pre-tax (deductible), taxed on withdrawal
$15,717 after-tax + Roth conversion	Converted to Roth (no tax on conversion since already after-tax), grows tax-free
Total Roth savings	$39,217 ($23,500 Roth deferral + $15,717 mega backdoor Roth)

Compare this to the Roth IRA limit of $7,000. The solo 401(k) allows 5.6x more Roth savings ($39,217 vs. $7,000).

Tax savings analysis:

Contribution	Immediate Tax Deduction?	Tax-Free Growth?	Tax-Free Withdrawal?
Employee deferral (pre-tax, $23,500)	Yes ($23,500 x 32% = $7,520)	Yes	No (taxed at withdrawal)
Employee deferral (Roth, $23,500)	No	Yes	Yes
Employer profit-sharing ($30,783)	Yes ($30,783 x 32% = $9,851)	Yes	No (taxed at withdrawal)
After-tax + Roth conversion ($15,717)	No	Yes	Yes

If the consultant maximizes pre-tax contributions:

Item	Amount
Pre-tax deferral + employer contribution	$54,283
Tax deduction (32% bracket)	$54,283 x 32% = $17,371
SE tax reduction	None (SE tax is figured before the retirement deduction)
After-tax + Roth conversion	$15,717 (no deduction, but tax-free growth)

Comparison: Solo 401(k) vs. no retirement plan

Factor	No Plan	Solo 401(k)
Taxable income	$200,000	$200,000 - $54,283 = $145,717
Federal tax savings	$0	$17,371
Retirement savings	$0	$70,000
Roth savings (tax-free forever)	$0	$39,217

Adding a spouse: If the consultant’s spouse works in the business (even part-time), the spouse can also contribute to the solo 401(k):

Person	Employee Deferral	Employer Contribution	After-Tax (Mega Backdoor)	Total
Consultant	$23,500	$30,783	$15,717	$70,000
Spouse (salary $50,000)	$23,500	$12,500 (25% of $50,000)	$34,000	$70,000
Combined	$47,000	$43,283	$49,717	$140,000

A married couple can save $140,000/year in tax-advantaged retirement accounts through the solo 401(k).

Setup and administration:

Item	Details
Providers that support mega backdoor Roth	Fidelity, Schwab, Vanguard (check if their plan document allows after-tax + in-plan conversion)
Plan document requirement	Must specifically allow after-tax contributions and in-plan Roth conversions
Form 5500-EZ	Required annually if plan assets exceed $250,000
Deadline to establish plan	December 31 of the year you want to start contributions
Deadline to fund	Tax filing deadline (including extensions) for employer contributions
Employee deferrals deadline	Must be elected and withheld during the year

Illustrative numbers only, not a real client and not advice for your facts.

Which retirement plan allows the highest contribution for a self-employed business owner?

A solo 401(k) or a defined benefit plan typically allows the largest contributions. A solo 401(k) allows up to $23,500 in employee deferrals ($31,000 if age 50+) plus an employer profit-sharing contribution of up to 20% of net self-employment income, for a combined maximum of $70,000 ($77,500 if 50+) in 2025. A SEP-IRA allows employer contributions of up to 20% of net self-employment income (after the self-employment tax deduction), maxing at $70,000. A cash balance or other defined benefit plan can allow contributions exceeding $200,000 per year depending on your age and plan design, making it the best option for high-income business owners over 50. All contributions reduce ordinary income dollar for dollar, though they don’t reduce self-employment tax.

How does a SEP-IRA affect my ability to do a backdoor Roth IRA?

A SEP-IRA balance triggers the pro-rata rule under IRC 408(d)(2), which makes backdoor Roth conversions partially taxable. When you convert a non-deductible traditional IRA contribution to a Roth, the IRS treats all your traditional IRA balances (including SEP-IRAs and SIMPLE IRAs) as one pool. The taxable portion of the conversion is proportional to the pre-tax balance across all IRAs. If your SEP-IRA holds $100,000 in pre-tax funds and you convert a $7,000 non-deductible contribution, roughly 93% of the conversion is taxable. The solution is to roll the SEP-IRA into a solo 401(k) or employer 401(k) before doing the conversion, because 401(k) balances are not included in the pro-rata calculation.

Related guides:

What are the catch-up contribution rules for business owners over 50?

Business owners aged 50 and older can make additional catch-up contributions beyond the standard limits for most qualified retirement plans. For 401(k) plans including solo 401(k)s, the catch-up contribution is an additional $7,500 for 2025 on top of the standard $23,500 employee deferral limit. SEP IRAs don’t have catch-up provisions because contributions are employer-only and already allow up to 25% of compensation or $70,000. SIMPLE IRAs permit a $3,500 catch-up for those 50 and older, and starting in 2025, participants aged 60-63 get an enhanced catch-up of $5,000 under the SECURE 2.0 Act.

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Cite this page

Yarik Yarosh, CPA. "Solo 401(k) and the Mega Backdoor Roth: How Self-Employed Owners Can Save $70,000+ Per Year." Blue Cloud CPA, September 5, 2026, updated September 23, 2026. https://bluecloudcpa.com/guides/small-business-solo-401k-mega-backdoor-roth-strategy

This guide is general information, not tax advice for your specific situation. Which points apply, and how, depends on your facts.