Solo 401(k) Contribution Optimizer

This is the most confusing aspect of Solo 401(k) contributions for self-employed individuals, and it trips up even experienced accountants. The circular dependency works like this:

The problem: Your employer contribution is based on your "net adjusted self-employment earnings." But to calculate net adjusted earnings, you need to subtract the deductible half of your SE tax. And the SE tax is calculated on your net SE income, which is your gross income minus business deductions. But here's where it gets circular: the employer contribution itself is considered a business deduction that reduces your net SE income.

The IRS solution (Keogh adjustment): Rather than making you solve a recursive equation, the IRS provides a simplified formula. Instead of using 25% as the employer contribution rate, self-employed individuals use an effective rate of 20%:

• Plan rate: 25% (what the plan document says)
• Effective rate: 25% ÷ (1 + 25%) = 25% ÷ 1.25 = 20%
• This 20% is applied to your net SE income minus the deductible half of SE tax

Step-by-step example with $150,000 net SE income:

• SE tax base = $150,000 × 92.35% = $138,525
• SE tax = ($138,525 × 12.4%) + ($138,525 × 2.9%) = $17,177 + $4,017 = $21,194
• Deductible half = $21,194 ÷ 2 = $10,597
• Adjusted net earnings = $150,000 - $10,597 = $139,403
• Employer contribution = $139,403 × 20% = $27,881

This Keogh adjustment is why self-employed individuals effectively contribute less on the employer side than an S-Corp paying the same total income as salary. It's a key factor when choosing between entity types.

The 2025 Solo 401(k) limits have been updated by the IRS and enhanced by SECURE Act 2.0 provisions. Here are the key numbers:

Employee deferral limits:

• Under age 50: $23,500 maximum employee deferral
• Age 50-59 or 64+: $23,500 + $7,500 catch-up = $31,000
• Age 60-63 (SECURE 2.0 super catch-up): $23,500 + $11,250 = $34,750. This enhanced catch-up is a new provision that started in 2025.

Total combined limits (employee + employer):

• Under age 50: $70,000
• Age 50-59 or 64+: $77,500
• Age 60-63: $81,250

Other SECURE 2.0 changes affecting Solo 401(k)s:

• Roth employer contributions: Since 2024, you can designate employer contributions as Roth (after-tax). This was previously not allowed.
• Roth catch-up mandate: Starting in 2026, catch-up contributions for those earning over $145,000 must be Roth. (Delayed from 2024.)
• Student loan matching: Employers can make matching contributions based on employee student loan payments, though this is more relevant for traditional 401(k)s.

Remember that the employer contribution portion is always limited to 25% of compensation (or 20% effective for self-employed). You need substantial income to reach the overall $70,000+ cap. For example, at the 20% Keogh rate, you would need adjusted net earnings of at least $232,500 to max out the employer side ($46,500 employer + $23,500 employee = $70,000).

Your business entity type significantly affects how Solo 401(k) contributions are calculated. The same gross income can yield very different contribution limits depending on your structure.

Sole Proprietor / Single-Member LLC:

• Compensation base = net SE income minus deductible half of SE tax
• Employer contribution uses the 20% Keogh rate (not 25%)
• You pay full 15.3% SE tax on net earnings
• Best for: Simplicity, lower income levels where the SE tax savings from S-Corp don't justify the complexity

S-Corporation:

• Compensation base = W-2 salary (must be "reasonable")
• Employer contribution = 25% of W-2 salary (no Keogh adjustment needed because you're an employee)
• Only pay FICA on the W-2 salary, not on distributions
• Trade-off: Higher employer contribution rate (25% vs 20%), but the contribution base is limited to salary, which is often lower than total income
• Best for: Higher-income individuals who benefit from the SE tax savings on distributions above reasonable salary

Partnership / Multi-Member LLC:

• Each partner's contribution is based on their guaranteed payments and distributive share
• Same Keogh 20% effective rate as sole proprietors
• Each partner's share of income is subject to SE tax
• Best for: Businesses with 2+ owners where each wants independent retirement contributions

The counterintuitive result: An S-Corp might allow a higher employer contribution rate (25% vs 20%), but the base is smaller because it's limited to salary. For example, with $200,000 net income and a $80,000 S-Corp salary, the employer contribution is $20,000 (25% of $80,000). As a sole prop, the employer contribution would be about $37,000 (20% of ~$189,000 adjusted earnings). The sole prop gets a higher retirement contribution but pays more in SE tax. You need to evaluate the total tax picture, not just the retirement contribution.

The interaction between Solo 401(k) contributions and the Qualified Business Income (QBI) deduction under Section 199A is often overlooked but can have a meaningful tax impact. Here's how they interact:

The key relationship: Your employer profit-sharing contribution to a Solo 401(k) is a deductible business expense. This means it reduces your net business income (Schedule C or K-1), which in turn reduces your QBI. Since the QBI deduction is 20% of qualified business income, a higher employer contribution means a lower QBI deduction.

Example impact:

• Net SE income before contribution: $150,000
• Employer contribution: $28,000
• QBI without contribution: $150,000 × 20% = $30,000 deduction
• QBI with contribution: ($150,000 - $28,000) × 20% = $24,400 deduction
• Lost QBI deduction: $5,600 × 24% bracket = ~$1,344 in additional tax

But this is still net positive: The $28,000 employer contribution saves you $28,000 × 24% = $6,720 in income tax (plus it grows tax-deferred). Even after losing $1,344 in QBI tax savings, you are still ahead by $5,376 in immediate tax savings, plus the compounding benefit.

Important nuances:

• Employee deferrals (the $23,500 part) do not reduce QBI because they are not a business deduction — they come from your income after it's been calculated
• For S-Corps, the employer contribution reduces QBI the same way (it's a corporate deduction)
• SSTB (specified service trade or business) rules may limit your QBI deduction regardless of contributions
• If you're above the QBI income threshold where it phases out, the contribution has no QBI impact at all

Both plans are popular with self-employed individuals, but they have fundamentally different structures that make the Solo 401(k) superior for most solo business owners.

Solo 401(k) advantages:

• Dual contributions: Both employee deferrals ($23,500) and employer profit-sharing (20-25% of compensation). This allows much higher contributions at lower income levels.
• Roth option: Employee deferrals can be Roth (after-tax). SEP-IRAs are always pre-tax.
• Loan provision: You can borrow up to $50,000 or 50% of the vested balance from a Solo 401(k). SEP-IRAs do not allow loans.
• Catch-up contributions: Age 50+ can contribute an extra $7,500-$11,250. SEP-IRAs have no catch-up provision.
• Backdoor Roth strategy: Solo 401(k) balances are not counted in the pro-rata rule for backdoor Roth IRA conversions, unlike SEP-IRA balances.

SEP-IRA advantages:

• Simpler to set up and maintain: No annual Form 5500-EZ filing (required for Solo 401(k) plans with assets over $250,000)
• Later establishment deadline: Can be set up and funded up to the tax filing deadline (including extensions). Solo 401(k) must be established by December 31.
• No plan document: Uses a simpler IRS model agreement (Form 5305-SEP)

When SEP-IRA wins: Only when you have very high income (over ~$350,000) where the 25% employer-only contribution hits the $70,000 cap anyway, or when simplicity is your top priority. For everyone else, the Solo 401(k) is the better choice because the employee deferral lets you contribute more at lower income levels.

Example at $100,000 net SE income:

• SEP-IRA: ~$18,587 (20% of adjusted net earnings)
• Solo 401(k): ~$23,500 employee + ~$18,587 employer = ~$42,087

That's more than double the contribution at the same income level.

Yes, you can participate in both a Solo 401(k) for your self-employment income and a traditional 401(k) through a W-2 employer. However, there are important aggregation rules that limit your total contributions across all plans.

The employee deferral limit is shared:

• The $23,500 employee deferral limit (2025) applies across all 401(k) plans combined, not per plan
• If you defer $20,000 at your day job's 401(k), you can only defer $3,500 more into your Solo 401(k)
• Catch-up limits ($7,500 or $11,250) are also shared across plans

Employer contributions are separate:

• Your W-2 employer's matching or profit-sharing contributions are independent of your Solo 401(k) employer contributions
• The $70,000 total annual limit applies per employer/plan, not in aggregate
• Your Solo 401(k) employer contribution can be up to 20% of adjusted net SE earnings, regardless of your W-2 plan contributions

Strategic tip: If your W-2 employer offers a 401(k) match, maximize that match first (it's free money). Then, if you have self-employment income, use the Solo 401(k) employer contribution to shelter additional income. You may not have much employee deferral room left in the Solo 401(k), but the employer side is still fully available.

Example: You earn $120,000 from a W-2 job (deferring $23,500 into their 401(k)) and $80,000 from freelancing. Your Solo 401(k) strategy:

• Employee deferral: $0 (already used at W-2 job)
• Employer contribution: ~$13,900 (20% of adjusted net SE earnings)
• Total additional retirement savings: $13,900 from the side business alone

Exceeding Solo 401(k) limits can trigger significant tax penalties. Understanding the different limits and their consequences is important for proper planning.

Excess employee deferrals (over $23,500):

• Must be corrected by April 15 of the following year by withdrawing the excess plus any earnings
• If not corrected, the excess is taxed twice — once in the year of contribution and again in the year of distribution
• Earnings on excess contributions are also taxable in the year of withdrawal
• This is especially common when you have multiple 401(k) plans and lose track of the aggregate limit

Excess employer contributions (over 25% or annual limit):

• Excess employer contributions are not tax-deductible for the business
• A 10% excise tax applies to non-deductible contributions that are not corrected
• Can be corrected by removing the excess before the tax filing deadline (with extensions)

Exceeding the total annual limit ($70,000 / $77,500 / $81,250):

• The 6% excess contribution penalty applies to amounts in the plan above the limit
• The penalty applies each year the excess remains in the plan
• Must remove excess contributions plus earnings to stop the penalty

How to avoid excess contributions: Track your contributions carefully, especially if you have multiple employer plans. Use a calculator like this one to determine your exact limits before making contributions. Set up your plan custodian to reject contributions that would exceed the limit. Review your total contributions in December before making year-end contributions.

The traditional vs. Roth decision for Solo 401(k) contributions depends on your current and expected future tax rates. Unlike Roth IRAs, there are no income limits for Roth Solo 401(k) contributions, making this a powerful option for high earners.

Choose traditional (pre-tax) when:

• You're in a high tax bracket now and expect to be in a lower bracket in retirement
• You need the tax deduction today to reduce your quarterly estimated payments
• You're maximizing other Roth sources (backdoor Roth IRA, Roth employer contributions) and want tax diversification
• Your state has high income taxes now but you plan to retire in a no-income-tax state

Choose Roth (after-tax) when:

• You expect higher tax rates in the future — either personally or due to tax law changes
• You have a long time horizon — decades of tax-free growth can be worth more than the upfront deduction
• You have lower income this year (early in your business, transition year, etc.)
• You want tax-free income in retirement — Roth distributions are not included in AGI, which affects Social Security taxation, Medicare IRMAA surcharges, and ACA subsidy eligibility
• No RMDs (since 2024): SECURE 2.0 eliminated RMDs for Roth 401(k)s starting in 2024, so your money can grow tax-free indefinitely without forced distributions

The split strategy: Many Solo 401(k) plans allow you to split contributions between traditional and Roth. For example, $12,000 traditional + $11,500 Roth = $23,500 total employee deferral. This provides tax diversification and hedges against uncertain future tax rates.

Important: Employer profit-sharing contributions have historically been pre-tax only. Since 2024, SECURE 2.0 allows designated Roth employer contributions, but not all plan custodians support this yet. Check with your provider.

Solo 401(k) plans have relatively light administrative requirements, but there are some filings you cannot skip.

Annual requirements:

• Form 5500-EZ: Required when plan assets exceed $250,000 at the end of the plan year. This is a simple one-page form filed with the IRS. Deadline: July 31 (with extension to October 15). Penalty for late filing: up to $250/day, capped at $150,000.
• Under $250,000: No annual filing required. This is a major advantage over traditional 401(k) plans.
• Final Form 5500-EZ: Required when you terminate the plan, regardless of asset level.

Plan document requirements:

• You must have a written plan document (your custodian typically provides this)
• The plan document must be adopted by December 31 of the year you want to start contributing
• Plan amendments may be needed for SECURE 2.0 provisions (Roth employer contributions, super catch-up, etc.)

Contribution deadlines:

• Employee deferrals: Must be made by December 31 of the plan year
• Employer contributions: Can be made up to the business tax filing deadline, including extensions (March 15 or April 15, extendable to September 15 or October 15)
• Plan establishment: Must be done by December 31 (unlike SEP-IRAs, which can be established up to the filing deadline)

Record-keeping: Keep records of all contributions, plan documents, loan paperwork (if applicable), and beneficiary designations. While there's no formal audit requirement for Solo 401(k)s, maintaining clean records protects you in case of an IRS examination.