What is a Solo 401(k)?
A Solo 401(k) (also called an individual 401(k) or one-participant 401(k)) is a regular 401(k) plan for a business with no employees other than the owner and, if applicable, the owner's spouse. It lets you contribute as both "employee" and "employer," which is why the 2026 combined limit reaches $72,000 ($83,250 if you are 60 to 63), far more than an IRA alone. The catch: it only works if the business truly has no other eligible employees. If you own more than one business, ownership rules can combine them into a single employer for this test and disqualify a "solo" plan, so check that before assuming you qualify.
What is a Solo 401(k)?
A Solo 401(k) is not a special product. It is an ordinary 401(k) plan, just adopted by a business that covers only its owner, or the owner and a spouse who also works in the business. The IRS calls it a "one-participant 401(k) plan" and defines it plainly: a traditional 401(k) plan covering a business owner with no employees, or that person and their spouse. You will also see it called a solo-k, individual-k, or uni-k. All four names describe the same structure.
Because there is no outside workforce to protect, a Solo 401(k) skips the nondiscrimination testing and employee-coverage paperwork that a normal small-business 401(k) needs. That is what makes it simple to run. It is not a looser set of tax rules, just a smaller set of people the plan has to serve.
Who can have a Solo 401(k)?
You qualify if your business has no common-law employees other than you (and your working spouse, if applicable). The IRS's own coverage rule comes down to a short checklist.
| Question | What it means for eligibility |
|---|---|
| Are you the 100% owner of the business (incorporated or unincorporated), or a partner in a partnership? | Required. You (and a co-owning spouse, or partners and their spouses) are the only people the plan is built to cover. |
| Does your spouse also work in the business and earn compensation from it? | Allowed, and does not disqualify you. A working spouse can also be a participant in the same plan. |
| Does the business have any other common-law employee who has met the plan's age and service eligibility (generally age 21 and one year of service, or the plan's own terms)? | Disqualifying. Once such a person exists, the plan must offer them coverage and it is no longer a solo plan. |
| Do you pay anyone as a 1099 independent contractor rather than a W-2 employee? | Does not disqualify you. Independent contractors are not employees for this test, only people on your own payroll count. |
| Do you (alone or with your spouse) own any other business, especially one with employees? | Stop and check the controlled-group rules before assuming you qualify. See the caveat below. |
Source: IRS, "One-Participant 401(k) Plans."
Can my spouse be in my Solo 401(k)?
Yes. The IRS definition explicitly includes "that person and his or her spouse." If your spouse works in the business and draws compensation from it, they can also be a participant in the same plan, each with their own employee deferral and employer contribution calculated on their own compensation. A spouse who does not work in the business, or who is not paid by it, cannot be added simply for tax purposes.
Can I have a Solo 401(k) if I have employees?
Not once someone other than you (or your working spouse) is a common-law employee who has met the plan's eligibility requirements. At that point the plan has to cover them like any other employer 401(k) would, and it is no longer a one-participant plan. Two things do not count against you: independent contractors you pay on a 1099, and employees who have not yet met the plan's age and service requirements. But a single eligible common-law employee, even part time, ends the "solo" structure at that business.
The caveat that comes before everything else: do you own more than one business?
This is the single most consequential question in the whole topic, and it is worth checking before you read anything else about contribution math. Owning "no employees" at the business that sponsors your Solo 401(k) is not enough on its own. If you (alone, with a spouse, or with family members) also own other businesses, IRS controlled-group and affiliated-service-group rules can treat all of those businesses as a single employer for this purpose. If any one of them has an eligible common-law employee, that can block a solo-only plan across the whole group, even at the business with zero employees. Spousal ownership is attributed to you by default in most cases, so "my spouse's separate business with staff" is a common trap, not a safe harbor.
This rule has real teeth and real exceptions, and it deserves its own careful read before you set up or rely on a Solo 401(k) across more than one business. See Solo 401(k) and multiple businesses: controlled groups explained for the full ownership thresholds, the spousal-attribution exception, and worked examples of what is and is not allowed.
What's the difference between a Solo 401(k) and a regular 401(k)?
Mechanically, none. Both run on the same 401(k) rules in the tax code, with the same 2026 contribution limits and the same tax treatment. The difference is who they cover. A regular employer 401(k) has to pass nondiscrimination testing and cover eligible employees broadly. A Solo 401(k) covers only the owner (and working spouse), so that testing is unnecessary, the paperwork is lighter, and the owner can wear two roles at once, employee and employer, on the same compensation. That two-role structure is also why the numbers work out so differently, covered next.
Why is the Solo 401(k) contribution limit so much higher than a regular IRA?
Because the owner is contributing to the plan in two separate capacities, and the two add together. As employee, the owner can defer part of their own compensation, exactly like any W-2 worker in a 401(k). As employer, the business can separately make a profit-sharing (nonelective) contribution on the owner's behalf, calculated as a percentage of compensation. A personal IRA only gives you one contribution bucket. A Solo 401(k) gives you two, stacked up to a combined ceiling, which is why the dollar amounts are several times larger.
What are the 2026 Solo 401(k) contribution limits?
The IRS sets these limits annually. For 2026, the employee elective deferral limit is $24,500, and the combined employee-plus-employer ceiling is $72,000 (higher with catch-up contributions for older savers).
| Item | 2026 amount | IRC section |
|---|---|---|
| Employee elective deferral | $24,500 | §402(g)(1) |
| Catch-up, age 50+ | +$8,000 ($32,500 total) | §414(v)(2)(B)(i) |
| Super catch-up, ages 60 to 63 | +$11,250 ($35,750 total) | §414(v)(2)(E)(i) |
| Combined employee + employer (total additions) | $72,000 (under 50); $80,000 (50+); $83,250 (60 to 63) | §415(c)(1)(A) |
| Annual compensation cap | $360,000 | §401(a)(17) |
Source: IRS Notice 2025-67, "2026 Amounts Relating to Retirement Plans and IRAs."
The employer piece is capped at roughly 20% to 25% of compensation depending on business structure (see below), so the full $72,000 generally requires a fairly high income. For the complete limits breakdown, including how the employer percentage is actually calculated, see Solo 401(k) contribution limits for 2026.
Sole proprietor vs. S-corp: how the Solo 401(k) numbers differ
The two contribution types work the same way in both structures, but the employer piece is calculated on a different base.
| Sole proprietor / single-member LLC | S-corp owner-employee | |
|---|---|---|
| Employer contribution base | Net earnings from self-employment (net Schedule C profit, adjusted) | W-2 wages actually paid, not net profit or K-1 distributions |
| Employer contribution rate | Roughly 20% of adjusted net earnings (a reduced rate that accounts for the circular math) | Up to 25% of W-2 wages |
| Employee deferral | Up to $24,500 (2026), same for both | Up to $24,500 (2026), withheld through payroll |
| When it can be finalized | Only after year-end net profit is known | Deferral election must be set before the wages are earned |
Source: IRS, "Self-employed individuals: Calculating your own retirement plan contribution and deduction"; IRS, "One-Participant 401(k) Plans."
Both routes land in the same combined 415(c) ceiling. The difference is entirely in how the employer contribution's base and percentage are figured, and the exact step-by-step math for each is its own topic.
The bottom line
A Solo 401(k) is a normal 401(k) simplified for a business with no outside workforce. It qualifies you (and a working spouse) because there is no one else to cover, and it pays off with a much larger contribution ceiling because you fund it twice, once as employee and once as employer. The number that matters for 2026 is $72,000 combined, $83,250 if you are 60 to 63. But eligibility is not just about the business in front of you. If you own any other business, especially one with employees, get the controlled-group question answered before you rely on a solo plan at all. Skipping that check is the single most common way people assume they qualify when they do not.
Related reading: how does a 401(k) work? for the underlying mechanics this all builds on, Solo 401(k) vs. SEP-IRA if you are weighing the simpler alternative, and which retirement account should you use? if you are comparing account types more broadly.
Sources
- Definition of a one-participant 401(k), owner-and-spouse coverage, and the common-law-employee eligibility trigger: IRS, "One-Participant 401(k) Plans".
- 2026 elective deferral limit ($24,500), catch-up and super catch-up amounts, the combined §415(c) limit, and the §401(a)(17) compensation cap: IRS Notice 2025-67, "2026 Amounts Relating to Retirement Plans and IRAs".
- How the sole-proprietor employer contribution is calculated (net earnings from self-employment, the reduced-rate mechanic): IRS, "Self-employed individuals: Calculating your own retirement plan contribution and deduction".