Solo 401(k) for Side Hustle: Setup, Roth Bucket, Mega-Backdoor

You already max out your employer 401(k). You also earn $20,000, $50,000, or $100,000 on the side — freelancing, consulting, selling products, or running a small business. That side income is fully exposed to federal income tax and 15.3% self-employment tax, with no retirement-plan shelter unless you create one. The Solo 401(k) is how you create it.

A Solo 401(k) — also called an individual 401(k) or one-participant 401(k) — is a qualified retirement plan under IRC Section 401(k) designed for self-employed individuals with no full-time employees other than a spouse. Because you act as both employer and employee, you can make contributions on both sides: employee elective deferrals (pre-tax or Roth) and employer profit-sharing contributions. Add after-tax contributions with the right plan document, and you unlock the mega-backdoor Roth. The combined ceiling under IRC Section 415(c) is $73,500 in 2026 — or $81,000 with the age-50 catch-up.

Eligibility: what counts as a side hustle for Solo 401(k) purposes

Any self-employment income reported on Schedule C (sole proprietorship), Schedule K-1 (partnership or S-corp), or Schedule F (farm income) qualifies. The IRS does not require a minimum income amount, a minimum number of hours, or a formal business entity. If you earn $2,000 driving for a rideshare service or $50,000 consulting on weekends, both qualify. The two requirements:

• Self-employment income. You must have net earnings from self-employment (or W-2 wages from your own S-corp). Passive income — rental income, dividends, interest — does not qualify.
• No full-time employees. The Solo 401(k) is limited to businesses with no common-law employees other than you and your spouse. If you hire a full-time W-2 employee (generally defined as someone working 1,000+ hours per year), you must convert to a standard 401(k) plan with nondiscrimination testing, or terminate the Solo 401(k). Part-time contractors paid via 1099 do not count as employees.

You can hold a Solo 401(k) simultaneously with an employer 401(k) at your day job. There is no prohibition on participating in multiple 401(k) plans, but the contribution limits interact in specific ways.

Contribution limits: the shared deferral, the separate profit-sharing

This is where most side hustlers get confused. Two limits govern Solo 401(k) contributions, and they work differently:

The key insight: if your day-job 401(k) already captures your full $24,000 employee deferral, you cannot defer additional employee contributions to the Solo 401(k). But you can still make employer profit-sharing contributions on your side-hustle income — and that contribution is not reduced by anything your day-job employer does.

Worked example: Priya, software engineer with freelance consulting

Priya is 38, earns $120,000 as a W-2 software engineer, and earns $45,000 in net Schedule C profit from freelance consulting on weekends. She defers $24,000 into her employer 401(k) (maxing the 2026 employee limit). Her employer matches 50% of the first 6% of salary, adding $3,600 in employer match. She opens a Solo 401(k) for her consulting business.

Step 1: Employee deferral to the Solo 401(k)

Priya already deferred $24,000 to her employer plan. The IRC 402(g) limit is shared. She has $0 remaining employee deferral space for the Solo 401(k). This is normal for most side hustlers who max their day-job 401(k).

Step 2: Employer profit-sharing contribution

As a sole proprietor, Priya’s employer contribution is up to 20% of net self-employment income after the SE tax deduction. The math:

• Net Schedule C profit: $45,000
• Self-employment tax: $45,000 × 92.35% × 15.3% = $6,358
• Deductible half of SE tax: $3,179
• Adjusted net SE income: $45,000 − $3,179 = $41,821
• 20% employer contribution: $41,821 × 20% = $8,364

Priya contributes $8,364 as an employer profit-sharing contribution to her Solo 401(k). This is a pre-tax deduction on her Schedule C, reducing both her income tax and her SE tax base for the following year.

Step 3: Mega-backdoor Roth (if plan supports it)

Priya’s Solo 401(k) plan (from MySolo401k) permits after-tax contributions and in-plan Roth conversions. Her IRC 415(c) limit for the Solo 401(k) is $73,500. She has used $0 in employee deferrals and $8,364 in employer contributions, leaving $65,136 in remaining 415(c) space. However, her total compensation from the side hustle (approximately $41,821 after the SE deduction) limits total contributions to 100% of compensation. Her after-tax contribution ceiling is $41,821 − $8,364 = $33,457.

Priya contributes $33,457 in after-tax (non-Roth) contributions, then immediately converts them to Roth inside the plan. Because she converts promptly, there are minimal earnings in the after-tax bucket, and the conversion is nearly tax-free (only the earnings portion, if any, is taxable).

Total sheltered in 2026

Without the Solo 401(k), Priya shelters $27,600 (employer 401(k) deferral plus match). With it, she shelters $69,421 — a $41,821 increase, entirely from her side-hustle income.

The Roth bucket: pre-tax vs. Roth deferral decision

If you have remaining employee deferral space (because your day job does not offer a 401(k) or you do not max it), you can direct your Solo 401(k) deferrals into a Roth bucket. The trade-off is the same as any Roth vs. traditional decision:

• Choose Roth if you expect your marginal tax rate in retirement to be equal to or higher than your current rate. Side-hustle income often pushes you into a higher bracket today, which weakens the Roth case — but if you plan to have substantial taxable retirement income (Social Security, pensions, RMDs from large pre-tax balances), future rates may be higher than you assume.
• Choose pre-tax if your side-hustle income puts you in the 32% or 35% bracket today and you expect to withdraw in the 22% or 24% bracket in retirement. The immediate deduction is worth more now than the tax-free withdrawal later.
• Split the difference by using the Roth bucket for employee deferrals and the pre-tax bucket for employer profit-sharing (which must be pre-tax regardless). This creates tax diversification: some money grows tax-free, some grows tax-deferred. In retirement, you draw from whichever bucket minimizes your effective rate in each year.

Mega-backdoor Roth: the advanced play

The mega-backdoor Roth through a Solo 401(k) requires a plan document with two features: (1) the plan accepts after-tax (non-Roth) employee contributions, and (2) the plan permits either in-plan Roth conversions or in-service withdrawals to a Roth IRA. Standard free plans from Fidelity, Schwab, and Vanguard do not include these provisions.

The mechanics: after you make your employee deferral and employer profit-sharing contribution, any remaining space under the IRC 415(c) limit ($73,500 in 2026) can be filled with after-tax contributions. You then convert those after-tax dollars to Roth immediately — either inside the plan (in-plan Roth conversion) or by rolling them out to a Roth IRA. Because the after-tax contributions have already been taxed, only the earnings (which are minimal if you convert promptly) are taxable at conversion.

The practical value depends on your side-hustle income. The IRC 415(c) limit and the 100%-of-compensation limit both apply. If your net SE income after the deduction is $30,000, your total Solo 401(k) contributions (all types) cannot exceed $30,000 — even though the 415(c) ceiling is $73,500. This means the mega-backdoor Roth is most powerful for side hustlers earning $50,000+ in net self-employment income.

Provider comparison: free vs. paid plan documents

If you only need pre-tax and Roth deferrals plus employer profit-sharing, a free Fidelity or Schwab plan is sufficient. If you want the mega-backdoor Roth, you need a paid plan document from a specialized provider. The $500–$1,500 setup fee pays for itself in one year if you convert $20,000+ of after-tax contributions to Roth — the tax-free growth on that amount over 20+ years far exceeds the one-time cost.

Setup checklist: opening a Solo 401(k) for your side hustle

1. Confirm self-employment income. You need net earnings from self-employment. Even $1 of Schedule C profit qualifies, though the contribution math only becomes meaningful at $5,000+.
2. Get an EIN. Apply at IRS.gov for free. Takes 5 minutes online. You need this for the plan application.
3. Choose a provider. Fidelity or Schwab for basic pre-tax and Roth. MySolo401k or Nabers Group if you want after-tax contributions and mega-backdoor Roth.
4. Complete the plan document. The provider generates the plan adoption agreement. Sign it before December 31 of the tax year.
5. Fund the account. Make employee deferrals by December 31. Make employer profit-sharing contributions by your tax filing deadline (April 15 or October 15 with extension).
6. Track your 402(g) limit. If you also defer into a day-job 401(k), add both deferral amounts together. Do not exceed $24,000 combined ($31,500 if 50+). Excess deferrals are taxable and must be corrected.
7. File Form 5500-EZ. Required when plan assets exceed $250,000 at year-end. Due by the last day of the 7th month after the plan year ends (July 31 for calendar-year plans). Penalty for late filing: $250/day up to $150,000.

The plan-establishment deadline: December 31 is not negotiable

The most expensive mistake side hustlers make is waiting too long. The plan must be established — not just applied for, but formally adopted with a signed plan document and an open account — by December 31 of the tax year. If you miss this deadline, you cannot make employee elective deferrals for that year. At $24,000 in deferrals and a 32% marginal rate, that is $7,680 in federal tax savings lost. With the mega-backdoor Roth factored in, the lost opportunity can exceed $15,000 in tax-advantaged savings.

The employer profit-sharing contribution has a longer deadline (your tax filing due date), but the plan itself must still exist by December 31 for even the profit-sharing contribution to count.

Rolling a prior employer 401(k) into your Solo 401(k)

If you have an old 401(k) from a previous employer, you can roll it into your Solo 401(k) — provided your plan document accepts rollovers (most do). This is strategically important if you use the backdoor Roth IRA. Rolling old 401(k) money into a traditional IRA creates a pre-tax IRA balance that triggers the pro-rata rule under IRC Section 408(d)(2), making backdoor Roth conversions partially taxable. Rolling into the Solo 401(k) instead keeps the money in a qualified plan, which is excluded from the pro-rata calculation. Your backdoor Roth stays clean.

Key takeaways

• Any self-employment income — freelancing, consulting, gig work, e-commerce — qualifies you for a Solo 401(k). You can hold it simultaneously with your employer 401(k). The employee deferral limit ($24,000 in 2026) is shared across all plans, but the employer profit-sharing contribution is calculated independently on your side-hustle income.
• The employer profit-sharing contribution for a sole proprietor is up to 20% of net self-employment income after the SE tax deduction. At $45,000 net side-hustle profit, that is approximately $8,364 in additional tax-deferred savings — on top of whatever you defer at your day job.
• A Roth bucket lets you designate employee deferrals as Roth (after-tax in, tax-free out). The mega-backdoor Roth adds $20,000 to $40,000+ of additional Roth space through after-tax contributions converted to Roth — but requires a plan document from a specialized provider like MySolo401k or Nabers Group.
• The plan must be established by December 31 of the tax year for employee deferrals. Missing this deadline forfeits the entire year. Start the setup process by early December at the latest.
• If you use the backdoor Roth IRA, roll old employer 401(k) balances into the Solo 401(k) — not into a traditional IRA — to avoid the pro-rata rule that makes backdoor Roth conversions taxable.