S-Corp + Solo 401(k): Stacking Contributions

#Why the S-corp + Solo 401(k) combination is so powerful

The federal retirement system has a tier of plans designed for solo business owners: SEP-IRA, SIMPLE-IRA, Solo 401(k), defined benefit plans. Among them, the Solo 401(k) is the most flexible and the highest-contribution option for most solo S-corp owners under age 60.

The structural advantage isn’t subtle. A SEP-IRA tops out at 25% of compensation, capped at $72,000 for 2026. For sole proprietors it is ratably less, since the calculation runs against self-employment income minus deductible SE tax. A SIMPLE-IRA caps at $17,000 of employee deferral plus a small employer match. A traditional 401(k) at a regular employer has the same $24,500 deferral limit. But the employer match is whatever the company offers, usually 3% to 6% of pay.

The Solo 401(k) combines the best of both: full $24,500 employee deferral PLUS the full 25%-of-W-2-wages employer profit-sharing, with no other employees to dilute the contributions. For an S-corp owner clearing $180K W-2 wages plus distributions, that’s $24,500 + $45,000 = $69,500 going into a tax-deferred account every year. Compounded over 20 years at 7% returns, that’s $3.0M of tax-deferred wealth from contributions alone.

See how Solo 401(k) compares to a SEP-IRA for sole proprietors and other structures where the math works differently.

#The 2026 contribution limits, broken down

Total combined limit: $72,000 (under age 50) / $80,000 (age 50 with $8K catch-up) / $83,250 (ages 60–63 with $11,250 super catch-up)

Employee deferral:

• Standard: $24,500 (up from $23,500 in 2025)
• Age 50+ catch-up: additional $8,000 = $32,500 total deferral. Say your 2025 W-2 wages from the company were over $150,000. Then your 2026 catch-up must go in as Roth. Your plan must also allow Roth deferrals for you to make it.
• Ages 60–63 super catch-up: additional $11,250 = $35,750 total deferral (SECURE 2.0 enhancement)
• Roth option: any portion of the employee deferral can be designated Roth (no deduction now, tax-free withdrawals later)

Employer profit-sharing:

• Up to 25% of W-2 wages from the S-corp
• Subject to the $360,000 compensation cap (you can’t compute the 25% on wages above $360K)
• Tax-deductible at the corporation level
• The employer share is pre-tax unless your plan allows Roth employer contributions. Some plans now do. If it does, you pay tax on that amount in the year it goes in.

Combined:

• The employee deferral + employer profit-sharing cannot exceed $72,000 (under 50), even if 25% of your wages would mathematically support more
• Catch-up contributions sit on top of the $72,000 limit for those age 50+. See Roth catch-up contributions for S-corp owners for the related guide.

#The W-2 wages constraint (and why it’s the binding limit for most owners)

Here’s the catch that catches owners: the employer profit-sharing math runs against W-2 wages, not net business income.

If your S-corp generates $400K of net business income, and you pay yourself $80K in salary plus $320K in distributions:

• Maximum employer profit-sharing = 25% × $80K = $20,000
• Plus $24,500 employee deferral = $44,500 total contribution
• You’re leaving $27,500 of contribution capacity on the table

If you’d paid yourself $190K in salary plus $210K in distributions:

• Maximum employer profit-sharing = 25% × $190K = $47,500
• Plus $24,500 employee deferral = $72,000 total contribution
• You hit the cap

The contribution capacity is directly tied to W-2 wages. This creates a real tension with the SE-tax optimization that drove the S-corp election in the first place. Higher salary means more retirement contribution capacity, but more salary also means more payroll tax. The optimization is not “minimum reasonable comp”. It’s the salary level where retirement contribution capacity meets your actual contribution goals.

For an owner whose retirement contribution target is $40K/yr (employee $24.5K + employer $15.5K), the breakeven W-2 wages is about $62K. For an owner targeting the full $72K combined limit, the breakeven W-2 wages is $190K.

#The math at four common income levels

These are example scenarios for a solo S-corp owner under age 50. They assume reasonable comp is set at a defensible level for the income and goals:

The fourth scenario is where retirement planning becomes a primary tax-strategy lever. We’ll come back to the cash balance layer below.

#Setting up the Solo 401(k) correctly

The administrative setup matters because the plan document, the funding mechanism, and the W-2 reporting all need to align. Common mistakes:

1. Choosing the wrong plan provider. The big retail brokers (Fidelity, Vanguard, Schwab, E*TRADE) offer “free” prototype Solo 401(k) plans. They’re fine for basic setups. But they limit your investment options to public securities only, and may not support after-tax non-Roth contributions for mega backdoor Roth conversions. Some S-corp owners want maximum flexibility: real estate inside the plan, mega backdoor Roth strategies, brokerage windows. For them, a “self-directed” or “custom plan document” provider is worth it. Think My Solo 401k Financial, Solera, or Carry. The cost: $500 to $1,500 per year in admin fees.

2. Funding from the wrong account. Employee deferrals come out of your paycheck (Gusto can process them as a 401(k) deduction, sending the money directly to the plan custodian). Employer profit-sharing is funded by the corporation, from the corporate operating account, to the plan custodian. Mixing these up creates accounting and audit problems.

3. Funding employer contribution after the W-2 is finalized. You can fund the employer profit-sharing by the due date of the S corporation return with extensions. That is September 15 for most calendar-year S corporations. Employee deferrals must be made by year-end. Most owners make their employee deferral throughout the year via payroll deductions. They then fund the employer profit-sharing in January through March of the following year. That is well ahead of the September 15 deadline, after the books close.

4. Missing the W-2 reporting requirements. Employee deferrals reduce Box 1 of the W-2 (federal wages) but not Boxes 3 and 5 (FICA wages). Roth designated deferrals don’t reduce Box 1 (you pay the tax now). The 401(k) plan administrator should provide a year-end summary that ties to the W-2. Gusto handles this automatically when configured correctly.

5. Forgetting the $250,000 Form 5500-EZ trigger. File Form 5500-EZ once plan assets reach $250,000. Once you cross the threshold, Form 5500-EZ is required annually. Missing it triggers a penalty of $250 a day, up to $150,000 per return.

#When you have employees (or might soon)

The Solo 401(k) is specifically designed for owner-only businesses (owner + spouse counts as owner-only). The moment you hire a W-2 employee who meets the plan’s eligibility criteria, the Solo 401(k) is generally no longer viable. You’d need to convert to a regular 401(k) plan with employee participation, nondiscrimination testing, and the associated compliance overhead ($2K–$8K/yr in TPA fees).

The eligibility test typically requires 1,000+ hours per year. So short-term seasonal employees or part-timers often don’t trigger conversion. But a full-time W-2 hire usually does, typically with a 1-year wait before they become eligible to participate.

If you’re planning to hire in the next 2–3 years, factor this into the retirement plan choice. A SEP-IRA must cover eligible employees at the same percentage as the owner. It is simple to run, but it can get expensive when you hire. Some owners stick with it until the team stabilizes, then move to a Safe Harbor 401(k) with employee participation.

For owners who expect to stay solo (consultants, professional services, small e-commerce, real estate professionals), the Solo 401(k) is the durable choice.

#Adding a cash balance plan on top

Some high earners, typically $500K+ net business income, want to push tax-deferred contributions beyond the Solo 401(k) limits. For them, a defined benefit cash balance plan can be layered on top.

The mechanics:

• The Solo 401(k) handles the first $72K of annual contribution capacity
• When you add a cash balance plan, the profit-sharing part of the Solo 401(k) is often limited to about 6 percent of pay. Your actuary must check the total deduction limit before you plan the numbers.
• Cash balance contributions are calculated actuarially based on age, expected retirement date, and target benefit
• 2026 limits allow cash balance plan contributions of $80K to $300K+ depending on age. Older owners get higher limits because they have fewer years to fund the same benefit
• Combined Solo 401(k) + CBP contributions can reach $250K–$370K for age 55–65 owners

The cash balance plan adds real complexity. It needs an actuarial cert each year ($1,500 to $3,500) and Form 5500 filing. It also needs conservative investment requirements, typically a fixed-rate portfolio to match the plan’s interest crediting rate. It also needs a 3 to 5 year funding commitment minimum, plus ERISA compliance overhead.

It only makes sense for owners who:

1. Have $500K+ of net business income consistently
2. Are over 45 (better math for older owners)
3. Have already maxed the Solo 401(k)
4. Are willing to commit to multi-year funding
5. Have a long enough runway to amortize the setup costs

See our cash balance plan article for the deeper analysis.

#Roth vs traditional: the strategic choice

Solo 401(k) plans support both traditional (pre-tax) and Roth (after-tax) contributions on the employee deferral side. The employer profit-sharing side is always traditional.

The strategic call:

Traditional (pre-tax) makes sense when:

• You’re in your peak earnings years (top federal bracket 32%–37%)
• You expect lower tax rates in retirement
• You need the current-year tax deduction to manage cash flow
• You’re using the deduction to drop below QBI phase-out thresholds or other income-based phaseouts

Roth (after-tax) makes sense when:

• You’re in a lower bracket currently (younger years, transition years)
• You expect higher tax rates in retirement (large traditional IRA balances coming due for RMDs, estate planning considerations)
• You want tax-free growth + tax-free withdrawals after age 59½
• You’re building a Roth ladder for early retirement

For most S-corp owners in their 40s–50s with peak earnings, the traditional employee deferral is the better current-year math. Younger owners and owners with already-large traditional balances often lean Roth.

#SECURE 2.0 super catch-up for ages 60–63

The SECURE 2.0 Act of 2022 added a “super catch-up” provision that took effect in 2025. For Solo 401(k) participants ages 60, 61, 62, and 63, the catch-up contribution increases from $8,000 to $11,250 in 2026.

So a 61-year-old S-corp owner can make:

• Standard employee deferral: $24,500
• Super catch-up: $11,250 (this replaces, rather than adds to, the regular $8,000 catch-up)
• Employer profit-sharing: up to 25% of W-2 wages
• Combined limit: $72,000 + $11,250 super catch-up = $83,250 maximum (subject to W-2 wage support)

The provision only applies to ages 60–63. At age 64, the catch-up reverts to the standard $8,000 amount. This is intentionally a “compress 4 years of higher contributions before normal retirement age” provision.

#Common questions

Can I contribute to a Solo 401(k) if I also have a day-job 401(k)? Yes, but the employee deferral limit ($24,500) is aggregate across all 401(k) plans. If you defer $20K through your day-job plan, you can only defer $4,500 to the Solo 401(k). The employer profit-sharing side is separate per-employer, so you can still get the full 25% of W-2 wages from your S-corp.

What if I’m a sole proprietor instead of an S-corp. Do the limits change? Yes. For sole proprietors and single-member LLCs taxed as disregarded entities, the employer profit-sharing math runs against net self-employment earnings. That is after subtracting the deductible portion of SE tax. For a sole proprietor, the employer share is about 20 percent of net profit after the SE tax deduction. That can be higher than an S corp owner’s share when the S corp pays a modest salary.

Can I roll my old employer 401(k) into the Solo 401(k)? Yes. Solo 401(k) plans generally accept rollovers from prior 401(k) plans, IRAs, 403(b) plans, and 457 plans. This is useful for consolidating accounts and accessing self-directed investment options. Roth funds and traditional funds need to be tracked separately.

Can my spouse also participate? Yes, if your spouse is a legitimate employee of the S-corp (real work, reasonable comp, W-2 wages). Each spouse can make their own employee deferral ($24,500) and each spouse can receive employer profit-sharing on their own W-2 wages. Say each spouse earns $100,000 of W-2 wages. Each can defer $24,500 and receive 25% of $100,000, which is $25,000, from the employer. That is $49,500 each, or $99,000 for the household. It is not the full $72,000-per-spouse limit. Each spouse needs about $190,000 of W-2 wages to reach their own $72,000 employer-plus-employee limit.

Can I take a loan from my Solo 401(k)? Yes. Solo 401(k) plans can be designed to permit participant loans up to 50% of vested balance, max $50K. Repayment terms run 5 years, longer for primary residence purchases. Useful for short-term cash needs without triggering distribution tax. Not all plan providers support loans. Verify before opening the account.

Does the Solo 401(k) protect assets from creditors? A one-owner 401(k) is usually not covered by ERISA, since ERISA generally exempts plans that cover only the owner (and spouse). Its assets may still be protected under federal bankruptcy law and under state law. Those rules differ by state, so check yours before you rely on this. Section 404(d) is a deduction rule for payments to independent contractors; it has nothing to do with creditor protection.

What happens to the plan if I close the business? You can leave the plan open and continue to manage the assets, with no new contributions without earned income from the business. Or you can roll the assets into an IRA. Most owners eventually roll to an IRA after closing the business: simpler administration, no Form 5500 obligation, equivalent tax treatment.

Can I do a backdoor Roth IRA if I have Solo 401(k) traditional balances? Yes, but be careful of the pro-rata rule. The pro-rata rule applies to traditional IRA balances, NOT to 401(k) balances. So your Solo 401(k) traditional balances don’t pollute backdoor Roth conversions of traditional IRA contributions. This is actually an advantage of the Solo 401(k) over the SEP-IRA (which DOES count for pro-rata).

Can I do a mega backdoor Roth through my Solo 401(k)? Yes, if the plan document supports after-tax non-Roth contributions and in-plan Roth conversions. Most retail prototype plans don’t support this. A custom plan document from a specialty provider does. The strategy can push Roth savings well above the $7,500 Roth IRA limit.

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Say you’re running an S-corp and you don’t yet have a Solo 401(k). Or you have one but you’re not maxing the employer profit-sharing side. Either way, the Discovery call is the right next step. We model the optimal salary level and set up the right plan for your contribution goals and investment preferences. We also integrate the plan with the Gusto payroll setup, so the deferrals run automatically.