Solo 401(k) Plans

Solo 401(k) (aka Individual 401(k), Solo-k, Uni-k, One-participant k) and Solo Roth 401(k) plans are like 401(k) retirement plans, but they apply only to 1-person businesses, regardless of the type of business entity chosen, such as a sole proprietorship or an S or C corporation. Although these plans only cover a business owner with no employees, spouses can be covered. If the business later acquires employees, then the Solo 401(k) plan must be terminated and replaced with another retirement plan, such as a regular 401(k).

Contributions to the Solo 401(k) plan can be made with either pre-tax or after-tax dollars. The Solo 401(k) plans are much like the Simplified Employer Pension (SEP) plan for the self-employed, including contribution limits, which are adjusted annually for inflation and allowing both employee and employer contributions. However, a major benefit to the Solo 401(k) is that much higher contributions can be made at lower income levels than for any other type of defined contribution retirement plan. Like the SEP plan, the employer's nonelective contribution is limited to 20% of self-employment compensation, unless the business owner is an employee of his own S or C corporation, in which case, the maximum percentage rate = the 25% limit for employees. The other major benefit to the Solo 401(k) plan is that the employee portion of the contribution can be made to a designated Roth account, which does not reduce current taxes, but both principal and earnings are tax-free when withdrawn, just as with a Roth IRA. Note, however, that the taxpayer must wait at least 5 years, including the year when the first designated Roth account was set up, to withdraw the earnings tax-free. Thereafter, earnings can be withdrawn tax-free from any other designated Roth account, if the withdrawals would otherwise be qualified. This rule is like the 5-year rule that applies to Roth IRAs.

The employee contribution is limited to the lesser of 100% of compensation or the statutory contribution limits listed in the table below. Total contributions cannot exceed the statutory limits + any allowed catch-up contribution. These limits apply to each person, so if a business owner is also employed by another business with a 401(k) plan, then the elective deferral limit applies to the total contributions to both plans.

Contribution Limits for Solo-401(k) Plans
Tax Year Maximum Employee Contribution Catch-Up Contribution Employee Contribution + Catch-Up Contribution Maximum Account Contribution,
Including Employer Contribution
Maximum Account Contribution +
Catch-Up Contribution
2024 $23,000 $7500 $30,500 $69,000 $76,500
2023 $22,500 $7500 $30,000 $66,000 $73,500
2022 $20,500 $6500 $27,000 $61,000 $67,500
2021 $19,500 $6500 $26,000 $58,000 $64,500
2020 $19,500 $6500 $26,000 $57,000 $63,500

Compensation is defined, in calculating the limits, as self-employment net earnings minus ½ of the self-employment tax. The taxpayer's contribution to the plan is also subtracted, but in an indirect way, by using the following formula that depends on the contribution percentage for employees as defined in the plan. (Although the business has no employees now, it may acquire them later on, so the plan must specify the employee contribution percentage, used to calculate the contribution rate for the owner):

Employer's Contribution Rate = Employees' Rate
1 + Employees' Rate

So to claim the full 20% of net business income, the contribution rate for employees in the retirement plan must also be set to the maximum employee rate of 25%:

Employer's Contribution Rate = .25/1.25 = .20 = 20%

Example: Calculating the Maximum Solo 401(k) Contribution

For instance, if you are at least 50 and earned $100,000 in self-employment income, then to calculate the employer contribution, you must deduct ½ of your self-employment tax, which would be about $7065. So:

Note that if a taxpayer earns more than the defined-benefit wage base, then the maximum contribution under either plan would equal the statutory limit, the defined contribution dollar limit.

Retirement Plan Limits
Year Defined
Contribution (DC)
Benefit (DB)
2024 $69,000 $345,000
2023 $66,000 $330,000
2022 $61,000 $305,000
2021 $58,000 $290,000
2020 $57,000 $285,000
2019 $56,000 $280,000
2018 $55,000 $275,000
  • DC dollar limit is the lesser of 100% of the
    participant's compensation or the above limits.
  • DB wage base to calculate benefits is the lesser of
    100% of the participant's average compensation for the
    3 highest consecutive calendar years or the above limits.

Source: COLA Increases for Dollar Limitations on Benefits and Contributions | IRS

Loans can also be taken out from Solo 401(k) plans. The maximum loan amount is limited to the lesser of $50,000 or 50% of the account balance, which must be repaid within 5 years. The frequency of repayments must be at least quarterly and a reasonable interest rate must be charged.

Solo 401(k) plans must be set up prior to the tax yearend. Contributions for unincorporated businesses must be made by the tax filing deadline, including extensions. Profit contributions made by corporations must be made by the tax filing date for the business, including extensions. Corporations must also fund the accounts within 15 days after the fiscal year end for salary deferral contributions.

A business owner with no common-law employees does not need to perform the nondiscrimination tests that 401(k) plans generally require. However, if the business acquires employees, then any eligible employees must be included in the plan and, thenceforth, elected deferrals will be subject to nondiscrimination testing. Form 5500, Annual Return/Report of Employee Benefit Plan also must be filed annually if the Solo 401(k) plan has at least $250,000 by year-end.

Single taxpayers can choose anyone as a beneficiary of their retirement account, but married taxpayers require the consent of their spouse to choose a nonspouse beneficiary.

Secure Act 2.0

The Secure Act 2.0, passed at the end of 2022, makes it easier for employers to help workers open emergency savings accounts, assist employees who are repaying student loan debt and allow part-time workers access to retirement plans in 2 years instead of 3. There are also new benefits to both Roth IRAs and Roth 401(k)s, including moving leftover money from a 529 college savings account to a Roth I.R.A. Certain rules must be followed to avoid taxes and penalties, and there are some income restrictions. However, some provisions start in different tax years.

Secure Act 2.0 Changes for Employees with 401(k) Plans:

Secure Act 2.0 Changes for Employers with 401(k) Plans:

Setting Every Community Up for Retirement Enhancement (SECURE) Act of 2019 (SECURE Act)

Setting Every Community Up for Retirement Enhancement (SECURE) Act of 2019 (SECURE Act) has changed the law regarding qualified tax-deferred retirement accounts:

Choosing a Solo 401(k) Provider

Remember that what you can do with the solo 401(k) depends on both tax law and the retirement plan. Although many financial institutions offer solo 401(k) plans, most of them do not allow loans and some of them do not provide for a designated Roth account. Another major consideration is the investment options offered by the plan provider. Many plans offer few options. For instance, as of this writing, Vanguard only offers some of its mutual funds as an investment option. Investing in stocks, bonds, Treasuries, or even exchange traded funds is not permitted.