For most one-person businesses, a Solo 401(k) is the stronger choice because it lets you contribute more at the same income level and adds features a SEP IRA simply does not offer, such as Roth contributions and the ability to borrow from your own account. A SEP IRA wins in the opposite situation: when you have (or expect to hire) employees beyond a spouse, or when you want the simplest possible plan with almost no paperwork. Both plans give you tax-advantaged savings well beyond a standard IRA, both are inexpensive to open at major brokerages, and both let you deduct contributions from business income. The right answer depends on three things: whether you have non-spouse employees, how much you want to shelter each year, and how much administrative complexity you are willing to accept in exchange for flexibility.
Key takeaways
- A Solo 401(k) usually allows a larger contribution than a SEP IRA at the same income because it stacks an employee salary deferral on top of an employer profit-sharing contribution.
- A SEP IRA is employer-funded only and requires the same contribution percentage for every eligible employee, which can get costly as you hire.
- A Solo 401(k) is limited to businesses with no full-time employees other than the owner and a spouse.
- Solo 401(k)s can offer Roth contributions, age-50 catch-ups, and plan loans; a traditional SEP IRA offers none of these.
- A SEP IRA can be opened and funded up to your tax deadline including extensions, making it the go-to for last-minute prior-year deductions.
- A Solo 401(k) requires an annual Form 5500-EZ once plan assets exceed the IRS threshold; a SEP requires no annual filing.
- 401(k)-type plans generally carry stronger federal creditor protection than IRA-based SEP plans, which depend on state law.
The Two Plans at a Glance
Both accounts are built for the self-employed, but they are structured very differently under the hood.
A SEP IRA (Simplified Employee Pension) is funded entirely by the business. There is no separate "employee" contribution and no salary deferral. The business sets aside a percentage of compensation, and that same percentage must be applied to every eligible employee. It is essentially a supercharged IRA with employer-only funding, and its defining trait is simplicity: no annual government filing, an easy setup form, and a contribution deadline that stretches to your tax-filing deadline including extensions.
A Solo 401(k) (also called an individual or one-participant 401(k)) is a full 401(k) plan for a business with no full-time employees other than the owner and a spouse. What makes it powerful is that you wear two hats. As the employee, you make salary-deferral contributions; as the employer, you make an additional profit-sharing contribution. Stacking the two lets you reach the annual maximum at a much lower income than a SEP requires.
Head-to-Head Comparison
The table below summarizes the practical differences that matter most when you are choosing. Figures shown are illustrative of how each plan behaves; exact dollar limits are set by the IRS and adjust for inflation each year, so confirm current numbers before you file.
| Feature | Solo 401(k) | SEP IRA |
|---|---|---|
| Who can have it | Owner-only business, spouse allowed; no other full-time employees | Any self-employed person or small business, including those with employees |
| Who contributes | You, as both employee and employer | The business only (employer) |
| Contribution structure | Salary deferral + profit-sharing (two buckets) | Single percentage of compensation |
| Roth option | Yes, if the plan document allows it | Traditionally no (pre-tax only) |
| Catch-up for age 50+ | Yes | No dedicated catch-up |
| Loans allowed | Yes, typically up to 50% of balance | No |
| Annual government filing | Form 5500-EZ once assets exceed a threshold | None required |
| Setup complexity | Moderate (adopt a plan document) | Very simple |
| Best for | Solo owners wanting maximum savings and flexibility | Owners wanting simplicity, or who have employees |
Contribution Limits: Why the Solo 401(k) Usually Wins
The single biggest reason owners choose a Solo 401(k) is that it reaches the maximum contribution at a lower income than a SEP IRA. Because a SEP is employer-only, you can generally contribute up to about 25% of compensation (roughly 20% of net self-employment earnings after the self-employment tax adjustment). A Solo 401(k) adds a flat salary-deferral bucket on top of that same profit-sharing percentage, so lower-earning years benefit dramatically.
The example below shows the mechanism, not a promise. It uses round numbers to illustrate how the two plans compare for an owner earning a modest net profit. The employee-deferral figure and overall cap are IRS-set and change yearly.
| Scenario (for example) | Solo 401(k) | SEP IRA |
|---|---|---|
| Net business profit | $60,000 | $60,000 |
| Employee salary deferral | ~$23,000 (for example) | Not available |
| Employer/profit-sharing (~20% of net) | ~$11,000 (for example) | ~$11,000 (for example) |
| Approximate total contribution | ~$34,000 | ~$11,000 |
At higher incomes the gap narrows, because the profit-sharing side eventually reaches the same overall cap in both plans. But for the many owners netting under six figures, the Solo 401(k) can allow roughly two to three times the annual contribution. Both plans share the same combined ceiling in a given year, and both cap the compensation that can be counted.
Roth, Catch-Ups, and Loans: The Features Owners Overlook
Comparisons often stop at contribution limits, but three Solo 401(k) features quietly tip the decision for many owners.
Roth contributions. A Solo 401(k) can accept Roth salary deferrals if the plan document supports them, letting you pay tax now and take qualified withdrawals tax-free in retirement. A traditional SEP IRA does not offer a Roth bucket. For younger owners in a lower bracket today, or anyone who expects higher rates later, this flexibility is meaningful.
Catch-up contributions. Once you reach age 50, a Solo 401(k) lets you add an extra deferral on top of the normal limit. A SEP IRA has no equivalent catch-up, so older owners running out of runway to save can shelter noticeably more in a Solo 401(k).
Plan loans. A Solo 401(k) can permit a loan against your balance, commonly up to half of the vested amount subject to a dollar cap, repaid with interest to yourself. A SEP IRA prohibits loans entirely. This is not a reason to raid retirement savings, but it can be a useful backstop that a SEP cannot match.
The Employee Question That Changes Everything
Eligibility is where the two plans diverge most sharply, and it is the deciding factor for many businesses.
A Solo 401(k) is strictly for a business with no full-time common-law employees other than you and your spouse. The moment you hire a non-spouse employee who meets the plan's service requirements, the Solo 401(k) generally no longer fits, and you would need to convert to a traditional 401(k) with its added testing and cost.
A SEP IRA, by contrast, is built to include employees. But that inclusion comes with a rule owners sometimes miss: whatever percentage of compensation you contribute for yourself, you must contribute the same percentage for every eligible employee. If you put away 20% for yourself, you generally owe 20% of each eligible worker's pay too. For a growing team that can become expensive fast, which is why some employers with staff eventually move to a SIMPLE IRA or a full 401(k) instead. If you are truly a solo operator today but plan to hire soon, factor that trajectory in before you commit.
Deadlines, Setup, and Ongoing Paperwork
Timing and administration are real, practical differences.
Setup and funding deadlines. A SEP IRA is famously forgiving: you can open and fund it up to your business tax-filing deadline, including extensions, which makes it a popular last-minute tax move. A Solo 401(k) is stricter about the employee-deferral side, which generally must be elected during the tax year, though the employer profit-sharing portion has more flexibility. If you are reading this after year-end and want a deduction for last year, a SEP is often the only option still open.
Ongoing filing. A SEP requires no annual IRS return. A Solo 401(k) requires an annual Form 5500-EZ once total plan assets cross a set threshold. It is a short form, but it is a real obligation with penalties for missing it, so put it on your calendar.
| Task | Solo 401(k) | SEP IRA |
|---|---|---|
| Open the account | Adopt a written plan document by year-end | Complete a short form (or IRS Form 5305-SEP) |
| Fund employee deferrals | Elect during the tax year | Not applicable |
| Fund employer portion | By tax deadline plus extensions | By tax deadline plus extensions |
| Annual filing | Form 5500-EZ above asset threshold | None |
Rollovers, RMDs, and Creditor Protection
Beyond the setup mechanics, three later-stage issues deserve attention.
Rollovers. Both plans can generally receive and send rollovers with other pre-tax retirement accounts, so you are not locked in. Many owners consolidate old employer 401(k)s or IRAs into a Solo 401(k) precisely because it can also serve as a rollover hub, and because assets inside a 401(k) may allow the plan-loan feature described above.
Required minimum distributions. Both traditional SEP and Solo 401(k) balances are subject to required minimum distributions once you reach the age set by law. Roth money inside a Solo 401(k) has different treatment than pre-tax money, another reason the Roth bucket matters for long-term planning.
Creditor protection. This is a genuine but often unstated advantage of the Solo 401(k). Assets in a 401(k)-type plan generally receive strong federal protection from creditors, while IRA-based plans like the SEP typically rely on state law, which varies. If asset protection is a priority for you, discuss the difference with a qualified advisor.
Funding the Business So You Can Fund Your Retirement
A retirement plan only helps if you can actually contribute to it, and for many owners the real constraint is cash flow, not tax strategy. Money that could go into a Solo 401(k) or SEP often gets pulled back into payroll, inventory, or a slow-paying season. That is where working-capital financing can play a supporting role: by smoothing revenue gaps, it frees up the profit you need to make a meaningful year-end contribution rather than skipping one.
If your business has steady deposits but an uneven month-to-month picture, a revenue-based financing marketplace can be worth exploring. These lenders lean on your bank-deposit history and monthly revenue more than your credit score, which helps owners who are strong operators but not perfect on paper. Typical parameters look like the example below; they are illustrative, not an offer, and approval is never guaranteed.
| Factor (for example) | Typical range |
|---|---|
| Minimum funding amount | Around $10,000 and up |
| Credit score considered | FICO 500+ |
| Primary qualification | Monthly revenue and bank deposits |
| Funding speed | Often 24 to 48 hours after approval |
Used deliberately, this kind of financing is a bridge, not a crutch. The goal is to keep operations funded so the profit you have worked for can go toward the long-term account, Solo 401(k) or SEP, that fits your situation best.
Frequently asked questions
Can I have both a Solo 401(k) and a SEP IRA at the same time?
It is possible but rarely useful, because the two plans share the same overall annual contribution ceiling per person. Contributing to both does not raise your total limit, and running two plans adds paperwork. Most owners are better served choosing the single plan that fits, and for solo operators that is usually the Solo 401(k).
Which plan lets me contribute more money?
For most incomes, the Solo 401(k). Because it combines a flat employee salary deferral with an employer profit-sharing contribution, it reaches the annual maximum at a much lower income than a SEP IRA, which relies on the profit-sharing percentage alone. At very high incomes the two converge on the same overall cap.
What happens to my Solo 401(k) if I hire an employee?
A Solo 401(k) is only valid while your business has no full-time common-law employees besides you and your spouse. Once you hire a non-spouse employee who meets the plan's eligibility rules, you generally must convert to a traditional 401(k), which involves nondiscrimination testing and higher administration, or move to a different plan such as a SEP or SIMPLE IRA.
Does a SEP IRA allow Roth contributions?
A traditional SEP IRA is pre-tax only, so it does not include a Roth bucket the way a Solo 401(k) can. If tax-free withdrawals in retirement are a priority for you, that difference often points toward a Solo 401(k) with Roth deferrals enabled in the plan document.
Which plan is easier to set up and maintain?
The SEP IRA. It uses a short setup form, requires no annual government filing, and can be funded up to your tax deadline including extensions. A Solo 401(k) needs a written plan document and an annual Form 5500-EZ once assets pass a threshold, in exchange for higher contribution potential and more features.
Can I borrow money from either account?
Only from a Solo 401(k), and only if the plan document permits it. Loans are commonly limited to about half of your vested balance up to a dollar cap and must be repaid with interest to yourself. A SEP IRA does not allow loans of any kind.
I am behind on saving and short on cash. What should I do first?
Prioritize stabilizing cash flow so you can contribute consistently, since a plan only helps if it gets funded. If uneven revenue is the obstacle, a revenue-based financing marketplace that evaluates bank deposits and monthly revenue rather than credit score can bridge gaps, often funding within 24 to 48 hours. Approval is never guaranteed, so treat it as a short-term bridge, not a substitute for profit.
Do both plans have required minimum distributions?
Yes. Pre-tax balances in both a SEP IRA and a Solo 401(k) are subject to required minimum distributions once you reach the age set by law. Roth funds held inside a Solo 401(k) are treated differently, which is another reason the Roth option can matter for long-term planning.
