Solo 401(k): Contribution Limits (Employer + Employee) and Roth Option

The Solo 401(k) — also called the Individual 401(k) or One-Participant 401(k) — is the most powerful retirement vehicle available to self-employed people and owner-only businesses. It allows the owner to wear two hats: as the employee (making elective deferrals) and as the employer (making profit-sharing contributions). The result is annual contribution capacity that dwarfs what a W-2 employee can access through a regular 401(k).

If you're a sole proprietor, a single-member LLC owner, or the only employee of your own S corp or C corp (other than possibly your spouse), the Solo 401(k) should be on your radar. This guide walks through who qualifies, how the contribution math actually works, the Roth option, loan provisions, and the specific calculations that determine your real contribution capacity based on your entity type.

The Solo 401(k) is available to businesses where the only employees are the owner and the owner's spouse. The moment you have a non-spouse W-2 employee who works more than 1,000 hours per year (or meets the long-term part-time rules), you've exited Solo 401(k) territory and need a regular 401(k) plan with nondiscrimination testing.

Independent contractors are not employees for this purpose. If you have 1099 contractors doing work for you, you still qualify for a Solo 401(k). The line matters — misclassifying a worker as a contractor when they're actually an employee can retroactively disqualify your Solo 401(k) and create significant tax problems.

The entity types that work:

• Sole proprietorship / single-member LLC: Yes
• Partnership / multi-member LLC (spouses only): Yes, with each spouse having their own participant account
• S corporation with only owner-employees (and possibly spouse): Yes
• C corporation with only owner-employees (and possibly spouse): Yes

For partnerships where both spouses are partners, or for businesses where both spouses work, each spouse can make their own employee and employer contributions to the Solo 401(k), effectively doubling the household's contribution capacity.

The Solo 401(k) allows contributions through two mechanisms:

Employee elective deferral. The same type of contribution a W-2 employee makes to a regular 401(k). The annual limit is indexed (for 2024, it was $23,000; for 2025, $23,500; these limits increase over time). This limit is per person across all 401(k) plans — you can't contribute the max to your Solo 401(k) and another max to a W-2 employer's 401(k).

Age 50+ catch-up contributions add to this limit (an additional $7,500 for 2024, $7,500 for 2025 — the SECURE 2.0 Act created a higher "super catch-up" for ages 60-63 starting in 2025).

Employer profit-sharing contribution. A discretionary contribution from the employer side (which is you), separate from the elective deferral. The limit is 25% of compensation (for corporate owners) or roughly 20% of net self-employment earnings (for sole props and partnerships — the math is slightly more complicated because of the SE tax deduction interaction).

Combined, the overall cap is the 415(c) limit — $69,000 for 2024, $70,000 for 2025 — not including catch-up contributions. With catch-up, owners 50+ can contribute $76,500+ in 2024 and similar amounts going forward.

The math differs meaningfully based on your entity.

Sole Proprietorship / Single-Member LLC

Your "compensation" for retirement plan purposes is net self-employment earnings — defined roughly as Schedule C net profit minus the deductible half of self-employment tax.

The sequence:

1. Calculate Schedule C net profit.
2. Subtract the deductible half of SE tax.
3. Employer contribution is capped at approximately 20% of this adjusted number (the math actually works out to 20% because the 25% rate applies to the compensation figure after you've subtracted the employer contribution itself).
4. Employee elective deferral is in addition, up to the annual limit.

A worked example: Sole proprietor with $200,000 of Schedule C net profit. - Deductible half of SE tax: approximately $10,100 - Net earnings: approximately $189,900 - Employer contribution (20%): approximately $37,980 - Employee elective deferral (2024 limit): $23,000 - Total Solo 401(k) contribution: approximately $60,980

For owners age 50+, add the catch-up contribution on top: approximately $68,480.

S Corporation

For an S corp owner-employee, your "compensation" for retirement plan purposes is your W-2 wages — not your total business profit, and not your S corp distributions.

This is a critical distinction that trips up many owners. S corp distributions are not compensation. Only W-2 wages count toward retirement plan contribution calculations.

The sequence:

1. Identify W-2 compensation paid to the owner.
2. Employer contribution is capped at 25% of W-2 compensation.
3. Employee elective deferral is in addition, up to the annual limit.

A worked example: S corp owner with $200,000 in combined W-2 plus distributions, split as $80,000 W-2 and $120,000 distribution. - Employer contribution (25% of W-2): $20,000 - Employee elective deferral (2024 limit): $23,000 - Total Solo 401(k) contribution: $43,000

This is meaningfully less than the sole proprietor at the same total income level — specifically because the S corp's "reasonable compensation" discipline that saves SE tax also limits the base on which retirement contributions are calculated.

This is one of the most important trade-offs in the S corp election. The SE tax savings from distributions come at the cost of reduced retirement contribution capacity. Owners who plan to max out retirement contributions need to balance their W-2 salary decision against this consideration — a higher W-2 salary increases both SE tax (payroll tax) and retirement capacity, while a lower W-2 salary saves SE tax but caps retirement contributions.

The S corp retirement planning math is specific enough that it's worth modeling with your CPA. The right W-2 salary for SE tax optimization may differ from the right W-2 salary for retirement maximization, and the real answer depends on which trade-off matters more to you.

C Corporation

Similar to S corp — compensation is W-2 wages paid to the owner-employee. Employer contribution is 25% of W-2 compensation. Employee elective deferral is separate. C corp owners sometimes have more flexibility in setting W-2 compensation because the dividend alternative has its own tax implications.

Partnership / Multi-Member LLC

For partnerships where spouses are partners, each partner's compensation is their net self-employment earnings from the partnership (K-1 box 14A for general partners). Each partner makes their own contributions. The math follows the sole proprietor framework.

Most Solo 401(k) plans (via quality plan documents) allow a Roth designated account option. This lets you make employee elective deferrals on a Roth (after-tax) basis rather than pre-tax.

Roth deferrals don't reduce your current taxable income. But they grow tax-free, and qualified distributions in retirement are tax-free. Over a long time horizon, this can be dramatically better than pre-tax deferrals if your current tax rate is lower than your expected retirement tax rate, or if you value tax diversification.

Key points on Roth Solo 401(k) contributions:

• Employee elective deferrals can be Roth, pre-tax, or a mix.
• Employer profit-sharing contributions were historically required to be pre-tax only, though SECURE 2.0 now permits Roth employer contributions — the mechanics of implementing this are evolving through plan document updates.
• There are no income limits on Roth 401(k) contributions (unlike Roth IRAs, which phase out at higher incomes). High earners who can't contribute to Roth IRAs can still make Roth 401(k) deferrals.
• Required minimum distributions historically applied to Roth 401(k) accounts (unlike Roth IRAs), but SECURE 2.0 eliminated RMDs for Roth 401(k)s starting in 2024.

For most business owners, having a Roth option available — even if you choose pre-tax contributions most years — is worth ensuring. It's plan-document dependent, so confirm your plan allows it.

Some Solo 401(k) plans allow after-tax employee contributions beyond the elective deferral limit — up to the full 415(c) combined limit. These after-tax contributions can then be converted to Roth (either via an in-plan Roth conversion or by rolling them to a Roth IRA upon separation).

This "mega backdoor Roth" strategy can allow tens of thousands of additional dollars into Roth treatment each year. It's not standard — your plan document has to allow after-tax contributions and in-service Roth conversions. Off-the-shelf Solo 401(k) plans from major brokerages often don't allow this. Custom plan documents (from specialized providers) typically do.

If maximizing Roth accumulation is important to you, this is a feature worth pursuing specifically when selecting a plan provider.

Solo 401(k) plans can allow participant loans — up to 50% of the account balance or $50,000, whichever is less. You can borrow, and you pay yourself back with interest.

Loan provisions can be useful for:

• Bridge financing for business needs
• Short-term personal liquidity
• Avoiding early withdrawal penalties on other accounts

The loans have rules. They must be repaid within 5 years (longer for primary residence purchase). Repayments must be made at least quarterly. If you default on a Solo 401(k) loan, the outstanding balance becomes a taxable distribution, subject to income tax and possibly the 10% early withdrawal penalty.

Not all plan documents include loan provisions. If you want this feature, confirm the plan you're opening allows it. Most major brokerage Solo 401(k) plans do not; specialized plan providers typically do.

Using Solo 401(k) loans to fund business operations on a sustained basis is generally a bad idea — you're essentially borrowing from your retirement to fund ongoing operating capital, and if the business fails, you lose both the business and the retirement funds. But for specific short-term uses, the loan provision is a tool.

Setting up a Solo 401(k) is straightforward but has deadlines:

Plan must be established by the end of the tax year for which you want to contribute. SECURE Act changes created some flexibility for sole proprietors to establish a plan up until their tax filing deadline, but the rules are nuanced and don't apply to every situation. The safe answer: establish the plan by December 31 of the tax year you want contributions for.

Contributions can be made up to the tax filing deadline, including extensions. Employee elective deferrals have more restrictive deadlines for established plans, but the overall contribution window extends into the following year.

Form 5500-EZ is required once plan assets exceed $250,000. Below that threshold, no annual filing is required. Above it, an annual return is due by July 31 of the following year (or October 15 with extension).

Keep plan documents current. Plan documents need to be updated periodically to reflect legislative changes. Most providers handle this automatically for their plan participants, but custom plan documents may require active maintenance.

Solo 401(k)s can generally invest in anything a regular 401(k) or IRA can invest in — mutual funds, ETFs, individual stocks, bonds, and for self-directed plans, real estate, private equity, precious metals, and other alternative investments.

Off-the-shelf Solo 401(k) plans from major brokerages (Fidelity, Schwab, Vanguard, etc.) typically limit investments to the brokerage's platform — mutual funds, ETFs, stocks, bonds. Self-directed Solo 401(k) plans through specialized providers allow alternative investments but carry higher administrative fees and require careful attention to prohibited transaction rules.

For most owners, a brokerage-platform Solo 401(k) invested in low-cost index funds is sufficient. Self-directed plans make sense only when there's a specific alternative investment strategy you actually want to pursue.

The Solo 401(k) vs. SEP IRA decision (SEP IRA is covered in 3.2) comes down to a few factors:

Maximum contribution at lower income levels: Solo 401(k) wins because of the $23,000+ employee elective deferral that's available before any employer contribution. A SEP IRA with 20% of $100,000 earnings can accept $20,000. A Solo 401(k) at the same income can accept $23,000 + $20,000 = $43,000.

At higher income levels: The two converge, because both max out at the 415(c) limit.

Administrative simplicity: SEP IRA wins. No annual filing, simpler paperwork.

Loan provisions: Solo 401(k) only.

Roth option: Solo 401(k) only.

Catch-up contributions: Solo 401(k) only.

Spouse participation: Solo 401(k) handles this cleanly.

For most owner-only businesses, Solo 401(k) wins on flexibility and contribution capacity. SEP IRA wins on simplicity. For owners who want maximum contributions, a Roth option, and loan flexibility, Solo 401(k) is the answer.

If you're an owner without a retirement plan and meet the eligibility criteria, setting up a Solo 401(k) is one of the highest-leverage financial moves you can make. The rough action sequence:

1. Calculate your current and projected contribution capacity based on your entity type and income.
2. Decide whether you need loan provisions, Roth option, or after-tax contributions — this determines whether you go with a standard brokerage plan or a specialized provider.
3. Open the plan before year-end for the current tax year.
4. Make contributions by your tax filing deadline.
5. File Form 5500-EZ if plan assets exceed $250,000.

The tax savings from a maxed-out Solo 401(k) for a high-earning owner can easily exceed $20,000 per year — more at higher income levels. Over a 20-30 year working career, this compounds into a retirement account that measures in the millions. Few decisions a business owner can make have a higher lifetime financial impact.