The Basic Differences
Solo 401(k): a 401(k) plan for businesses with no employees other than the owner (and spouse). Allows both employee elective deferrals (up to $23,500 in 2026, $31,000 with age 50+ catch-up) and employer profit-sharing contributions (up to 20% of net SE income or 25% of W-2 wages). Combined limit: $69,000 ($76,500 catch-up).
SEP-IRA: a Simplified Employee Pension. Allows only employer contributions, up to 20% of net SE income or 25% of W-2 wages. Same overall limit of $69,000. No employee elective deferrals, no Roth option (mostly), no loans.
Both must be set up by tax year end (December 31 typically) to make contributions for that year. Both can be funded after year-end up to your tax filing deadline (including extensions, so up to October 15 of the following year).
Eligibility: Solo 401(k) requires no employees other than spouse. Hire one part-time employee and you may have to start covering them or convert to a regular 401(k) plan. SEP-IRA requires you to cover all employees aged 21+ who worked 3 of the last 5 years and earned $750+ in 2026 (under SECURE 2.0). For pure freelancers with no employees, both work.
Solo 401(k) Advantages
Higher contributions at lower income levels. The employee elective deferral ($23,500) is available regardless of business income — even at low income, you can contribute the full deferral. SEP-IRA contributions are capped at 20% of net SE income, so at $50K income, max SEP is $10K but Solo 401(k) is $33,500 ($23,500 + $10K profit share).
Roth option. Solo 401(k) plans can offer Roth elective deferrals — your $23,500 employee contribution can go to a Roth sub-account, growing tax-free. SEP-IRAs technically allow Roth contributions starting in 2024 under SECURE 2.0, but most providers haven't implemented the option. If you want Roth tax-free growth on your contributions, Solo 401(k) is the practical choice.
Loan provision. Solo 401(k)s allow loans up to $50,000 or 50% of vested balance (whichever is less). SEP-IRAs don't allow loans. The Solo 401(k) loan can be useful for short-term cash needs (e.g., bridging between contracts) without triggering early withdrawal penalty.
Mega Backdoor Roth potential. Some Solo 401(k) plans allow after-tax (non-Roth) contributions that can be converted to Roth. This unlocks the Mega Backdoor Roth strategy at the solo level — potentially adding $46K+ of additional Roth contributions per year. Requires a custom Solo 401(k) plan; Vanguard, Fidelity, and Schwab don't offer this in their basic plans.
Catch-up contributions. Age 50+ Solo 401(k) participants can contribute an additional $7,500 elective deferral (raising employee deferral to $31,000), totaling $76,500 with profit sharing. SEP-IRA has no separate catch-up provision.
SEP-IRA Advantages
Simpler administration. SEP-IRA is essentially a Traditional IRA with higher contribution limits. Set it up at any major brokerage in 15 minutes; contributions and distributions follow IRA rules. No annual Form 5500 filing required (Solo 401(k)s require Form 5500-EZ once balance exceeds $250,000).
Easier setup deadline. SEP-IRA can be established AND funded for a tax year by your tax filing deadline (April 15, plus extensions to October 15). Solo 401(k) must be ESTABLISHED by December 31 of the tax year, even though contributions can be made until tax filing.
No annual paperwork. Once a SEP-IRA is set up, no annual filings or reports beyond your normal tax return. Solo 401(k) requires Form 5500-EZ filings once balance exceeds $250,000 — not difficult, but additional paperwork.
Better for inconsistent income. If you have years where you barely earn anything from self-employment, SEP-IRA's profit-share-only structure is straightforward — contribute 20% of whatever you made. Solo 401(k) lets you also do the elective deferral, but you have to track whether contributions came in as deferrals or profit shares.
No covered-employee complications. If your business grows and hires part-time helpers, SEP-IRA handles them straightforwardly (you must cover them). Solo 401(k) is technically not allowed if you have non-spouse employees — you'd need to convert to a full 401(k) plan.
Math: Real Income Scenarios
Scenario 1: $40,000 of net SE income, age 35. SEP-IRA max: $40K × 20% = $8,000. Solo 401(k) max: $23,500 employee deferral + $8,000 profit share = $31,500. Solo 401(k) wins by $23,500.
Scenario 2: $100,000 net SE income, age 40. SEP-IRA: $20,000. Solo 401(k): $23,500 + $20,000 = $43,500. Solo 401(k) wins by $23,500.
Scenario 3: $300,000 net SE income, age 45. SEP-IRA: $60,000. Solo 401(k): $23,500 + $60,000 = $83,500, BUT capped at $69,000 total. So Solo 401(k) is $69,000 and SEP is $60,000. Solo 401(k) wins by $9,000.
Scenario 4: $500,000 net SE income, age 55. SEP-IRA: $69,000 (capped). Solo 401(k): $76,500 (with catch-up). Solo 401(k) wins by $7,500.
Conclusion: Solo 401(k) wins on contribution capacity in essentially every income scenario, especially at lower income levels. SEP-IRA only matches at very high income where both hit the $69K cap (and Solo 401(k) still wins by the catch-up amount for those 50+).
But: simplicity has value. If you're in your first year of self-employment with $30K of income, the Solo 401(k) advantages may not be worth the marginally more complex setup. As income grows, the advantages become more obvious.
Setup and Administration
Solo 401(k) setup: choose a provider (Vanguard, Fidelity, Schwab, ETrade all offer free Solo 401(k)s with limited investment menus; Solo 401k Brokers offers expanded features for $200-$400/year). Set up by December 31 of the tax year. Establishing as 'self-administered' lets you handle paperwork yourself; or use a third-party administrator.
Investment options: Vanguard Solo 401(k) restricts you to Vanguard mutual funds. Fidelity Solo 401(k) allows any Fidelity fund or ETF. Schwab is similar. For maximum flexibility (Roth options, after-tax contributions, custom investments), use a self-directed Solo 401(k) provider — but expect $200-$1,000/year in fees.
SEP-IRA setup: any brokerage that offers IRAs offers SEP-IRAs. Open the account, fund it. Done. The investment options are whatever the brokerage offers in IRAs (typically thousands of mutual funds, ETFs, individual stocks).
Both plans allow rollovers in and out. You can roll old 401(k) balances into either plan (consolidation) or roll them out when you eventually wind down the self-employment business. Rollovers are non-taxable events.
Backdoor Roth interaction: existing SEP-IRA balances trigger pro-rata rules for Backdoor Roth conversions. Solo 401(k) balances do NOT trigger pro-rata rules. If you also do Backdoor Roth annually, Solo 401(k) is significantly cleaner — your existing pre-tax balances stay isolated from the IRA pro-rata calculation.
Advanced: Defined Benefit Plans for Higher Earners
Beyond Solo 401(k) and SEP-IRA, very high-income self-employed people can establish Defined Benefit pension plans with contribution limits well above $69K. These plans target a specific retirement benefit (e.g., $100K/year at age 65) and require actuarial calculations to determine annual contributions.
Typical contribution capacity: at age 55 with $400K of consistent SE income, a defined benefit plan could allow $200,000+ of annual contributions. The exact amount depends on age (older = higher limits because shorter funding horizon), target benefit, and IRS limits.
Cost: defined benefit plans require an actuary, which costs $2,000-$5,000/year for setup and annual administration. Form 5500 filing is more complex than Solo 401(k). Total annual cost: $3,000-$8,000.
Best for: high-income consultants and professionals over age 50 with stable income who want to maximize tax-deductible retirement savings. The contribution limits dwarf any other retirement account, but the complexity and minimum funding requirements (you must contribute every year) make it inappropriate for variable income.
Cash Balance Plans are a hybrid (defined benefit with individual account features) that have grown in popularity. They're easier to administer than traditional DB plans and allow more flexibility in contribution amounts. Combined with a Solo 401(k), some high-earning professionals contribute $200K+ to retirement accounts annually.