The Solo 401(k) is one of the most powerful retirement plans available to self-employed individuals and small business owners. When structured and used correctly, it allows business owners to contribute far more than a traditional IRA, while gaining access to Roth strategies, participant loans, and alternative investments. However, the rules governing employer profit-sharing contributions, especially when combined with employee deferrals, are often misunderstood.

Many business owners know they can put away a significant amount in a Solo 401(k), but far fewer understand how those contributions are calculated, how deadlines vary by entity type, or how recent law changes now allow employer contributions to be made on a Roth basis. Getting these rules wrong can lead to missed opportunities, excess contributions, or compliance issues.

This guide explains how Solo 401(k) contributions work, the difference between employee deferrals and employer profit sharing contributions, the 2025 and 2026 limits, contribution deadlines by business structure, and why working with a true Solo 401(k) expert matters.

How Solo 401(k) Contributions Work

A Solo 401(k), also known as an individual 401(k), is designed for businesses with no full-time employees other than the owner and, in some cases, a spouse. What makes the Solo 401(k) unique is that the business owner wears two hats:

1. Employee
2. Employer

Because of this dual role, contributions come from two distinct sources:

• Employee salary deferrals
• Employer profit sharing contributions

Understanding how these two components interact is the foundation for maximizing a Solo 401(k).

Employee Deferral vs. Employer Contribution in a Solo 401(k)

Employee Salary Deferrals

The employee deferral portion of a Solo 401(k) functions much like a traditional employer-sponsored 401(k). As the employee, you can defer a portion of your compensation into the plan on either a pre-tax or Roth basis.

These deferrals are not tied to business profits. Even in a low-profit year, you may still be able to make an employee deferral as long as you have eligible compensation.

Employee Solo 401(k) Plan Deferral Limits for 2026

• Elective deferral limit: $24,500
• Catch-up contribution (age 50+): $8,000
• Total employee deferral (age 50+): $32,500
• Enhanced catch-up (ages 60–63): $11,250
• Total employee deferral (ages 60–63): $35,750

Employee Solo 401(k) Plan Deferral Limits for 2025

• Elective deferral limit: $23,500
• Catch-up contribution (age 50+): $7,500
• Total employee deferral (age 50+): $31,000
• Enhanced catch-up (ages 60–63): $11,250
• Total employee deferral (ages 60–63): $34,750

For sole proprietors and single-member LLCs, employee deferrals can be made up until the IRS Form 1040 filing deadline, including extensions. However, for C corporations, S corporations, and partnerships, employee deferrals must generally be elected by December 31.

Employer Profit Sharing Contributions

The employer contribution is where the Solo 401(k) truly differentiates itself from IRAs. Employer contributions are based on compensation or net self-employment income rather than a flat dollar limit.

Employer Contribution Limits for 2025 and 2026

• Up to 25 percent of W-2 compensation for corporations, including C corporations and S corporations
• Up to 20 percent of net self-employment income for sole proprietors and single-member LLCs filing Schedule C

These percentages are set by statute and cannot be exceeded.

Employer Profit Sharing Contribution Examples for 2026

Understanding how the employer contribution is calculated is critical to maximizing a Solo 401(k). The calculation depends on whether income is earned as self-employment income reported on Schedule C or as W-2 wages from a corporation. The examples below show how employee deferrals and employer profit sharing contributions work together in 2026.

Example 1: Schedule C Sole Proprietor (Under Age 50) for 2026

Facts:

• Net Schedule C income: $100,000
• Age: Under 50
• Business type: Sole proprietor or single-member LLC
• Solo 401(k) allows both pre-tax and Roth employee deferrals

Step 1: Employee Salary Deferral

As the employee, you may contribute up to the 2026 elective deferral limit:

• Employee deferral: $24,500

This deferral may be made on a pre-tax basis, a Roth basis, or a combination of both.

Step 2: Employer Profit Sharing Contribution Using the 20 Percent Rule

For Schedule C income, the employer contribution is capped at 20 percent of net self-employment income after the IRS adjustment for self-employment tax. For planning simplicity:

• $100,000 multiplied by 20 percent equals $20,000

Step 3: Total Solo 401(k) Contribution

• Employee deferral: $24,500
• Employer contribution: $20,000
• Total contribution: $44,500

This amount is well below the 2026 combined contribution limit of $72,000, so the full amount is permitted.

Example 2: S Corporation Owner Paid W-2 Wages (Under Age 50) for 2026

Facts:

• W-2 salary: $100,000
• Age: Under 50
• Business type: S corporation
• Solo 401(k) allows Roth and pre-tax contributions

Step 1: Employee Salary Deferral

• Employee deferral: $24,500

This deferral must be made through payroll and can be either pre-tax or Roth.

Step 2: Employer Profit Sharing Contribution Using the 25 Percent Rule

• $100,000 multiplied by 25 percent equals $25,000

Step 3: Total Solo 401(k) Contribution

• Employee deferral: $24,500
• Employer contribution: $25,000
• Total contribution: $49,500

This amount is also well below the 2026 combined contribution limit of $72,000, allowing the full contribution.

Key Takeaways from These Examples

• Employee deferrals are the same regardless of entity type
• Employer contribution percentages differ
  • 20 percent for Schedule C filers and single-member LLCs
  • 25 percent for corporations paying W-2 wages
• Employer contributions are made in addition to employee deferrals
• Contributions can be structured as pre-tax, Roth, or a combination, depending on plan design
• Proper planning ensures you maximize contributions without exceeding IRS limits

Contribution Deadlines by Business Entity Type

Sole Proprietors and Single-Member LLCs Filing Schedule C

• Employee deferrals must be made by the tax filing deadline, including extensions
• Employer profit sharing contributions must also be made by the tax filing deadline, including extensions

This allows Schedule C filers to fund both employee and employer contributions well into the following year if an extension is filed.

S Corporations and C Corporations

• Employee deferrals must be withheld from W-2 wages during the calendar year
• Employer contributions are due by the corporate tax filing deadline, including extensions

Late payroll adjustments are not permitted, which makes year-end planning essential.

Partnerships

• Employee deferral elections must be made by December 31
• Contributions must be made by the partnership tax filing deadline, including extensions

Because partnership income calculations are more complex, careful coordination with a tax advisor is critical.

How Solo 401(k) Employer Profit Sharing Contributions Are Calculated

Schedule C Example Using the 20 Percent Rule

• Net Schedule C income: $150,000

After adjusting for self-employment tax, the employer contribution is approximately:

• $150,000 multiplied by 20 percent equals $30,000

This contribution is deductible to the business and does not reduce the employee deferral limit.

S Corporation Example Using the 25 Percent Rule

• W-2 salary: $100,000
• $100,000 multiplied by 25 percent equals $25,000

This contribution is in addition to any employee deferral made from wages.

Roth Employer Contributions and a Major Planning Upgrade

Under SECURE Act 2.0, employer profit sharing contributions can now be made as Roth contributions, provided the plan allows it.

• The employee pays income tax on the Roth employer contribution
• The business still receives a tax deduction
• Future qualified distributions are tax-free

This creates a powerful planning opportunity, particularly for high-growth businesses or those expecting higher tax rates in the future.

Not all Solo 401(k) plans permit Roth employer contributions, making plan design a critical distinction.

Putting Employee and Employer Contributions Together

Example 1: 2026 Schedule C Business Owner (Age 52)

• Net income: $200,000
• Employee deferral: $32,500
• Employer contribution at 20 percent: $40,000
• Total contribution: $72,500
• Capped at: $80,000 for age 50 and older

Result: $72,500 is fully allowable.

Example 2: 2026 S Corporation Owner (Age 61)

• W-2 salary: $150,000
• Employee deferral with enhanced catch-up: $35,750
• Employer contribution at 25 percent: $37,500
• Total contribution: $73,250

This amount is within the $83,250 enhanced catch-up limit for ages 60 through 63.

Example 3: 2025 Schedule C Business Owner (Age 45)

• Net income: $120,000
• Employee deferral: $23,500
• Employer contribution: $24,000
• Total contribution: $47,500

This amount is well under the $70,000 total limit for 2025.

Why IRA Financial for Solo 401(k) Profit Sharing Strategies

Few firms understand Solo 401(k) plans at the depth required to maximize both compliance and opportunity. IRA Financial has spent nearly two decades focused exclusively on self-directed retirement strategies and quite literally wrote the book on Solo 401(k) plans.

• Proper plan design, including pre-tax, Roth, or blended strategies
• Employer contribution calculations
• Annual tax compliance and reporting
• Recordkeeping and plan maintenance
• Ongoing consulting as income levels and laws evolve

Their Annual Compliance Shield™ service ensures that contributions, deadlines, and reporting obligations are handled correctly year after year, reducing audit risk and costly errors.

Why Not All Solo 401(k) Plans Are the Same

• Prohibit alternative investments
• Do not offer checkbook control
• Do not allow participant loans
• Do not support Mega Backdoor Roth strategies
• Do not permit Roth employer contributions
• Provide little or no compliance guidance

By contrast, IRA Financial’s Solo 401(k) is designed as a true retirement plan, not just a brokerage account. Clients can invest in real estate, private equity, cryptocurrency, private lending, and more, while maintaining full IRS compliance.

Final Thoughts

The Solo 401(k) employer profit sharing contribution is one of the most powerful, and most misunderstood, retirement planning tools available to business owners. When combined properly with employee deferrals, Roth strategies, and thoughtful tax planning, it can dramatically accelerate retirement savings.

However, these benefits only materialize when the plan is designed correctly, contributions are calculated accurately, and deadlines are respected. That level of precision requires expertise rather than guesswork.

With the right guidance, the Solo 401(k) is not just a retirement account. It is a strategic financial engine.