Solo 401(k): The Self-Employed Retirement Plan That Stacks Two Contributions

A solo 401(k) — also called an individual 401(k) or one-participant 401(k) — is designed specifically for self-employed individuals and business owners with no full-time W-2 employees other than a spouse. What makes it unusually powerful is its dual-contribution structure: you contribute once as the employee and again as the employer, which is why it can shelter more income than most alternatives at moderate income levels. For a broader look at how a solo 401(k) compares to other self-employed retirement options, see Retirement Plans For Self-Employed.

Solo 401(k) Contribution Limits for 2025

For 2025, the combined solo 401(k) contribution cap is $70,000 (2025 IRS rules). That total breaks into two separate buckets.

• Employee elective deferral: up to $23,500, or $31,000 if you're age 50 or older (the $7,500 catch-up applies)
• Employer profit-sharing contribution: up to 25% of your net self-employment compensation, after the 50% self-employment tax deduction
• Combined ceiling: $70,000, or $77,500 with catch-up (2025 IRS rules)

The employee deferral is what sets the solo 401(k) apart — you can shelter $23,500 before the 25% employer calculation even begins. For a full contribution comparison with the SEP IRA, see Solo 401(k) vs. SEP IRA.

Who Qualifies for a Solo 401(k)

You qualify if you have self-employment income and no full-time W-2 employees other than a spouse — covering sole proprietors, single-member LLCs, S-corp owners, and spousal partnerships. If you hire a qualifying full-time employee (generally age 21+, 1,000+ hours/year), the plan must be converted or terminated.

One hard requirement: you need an EIN (Employer Identification Number) to open a solo 401(k). Your Social Security Number alone won't work. If you don't have one, IRS Form SS-4 gets you there quickly — most online applications process in minutes.

Key Rules and Deadlines

The plan establishment deadline is December 31 of the tax year — you must open the account before year-end to make contributions for that tax year. For how this deadline compares to other self-employed retirement plans, see Solo 401(k) vs. SEP IRA.

Once the plan is established, the employee deferral election must be made by December 31. The employer profit-sharing contribution can be funded up to your tax filing deadline (April 15, or October 15 with an extension). Once plan assets cross $250,000, you're required to file Form 5500-EZ each year — a straightforward annual filing, but one worth putting on your calendar.

Roth Solo 401(k) and Loan Provisions

Many solo 401(k) providers offer a Roth designation for the employee deferral portion. Roth contributions go in after tax and grow tax-free — a meaningful option if you expect to be in a higher bracket in retirement or if your income is lower this year. The employer profit-sharing side stays traditional (pre-tax) regardless.

Solo 401(k) plans can also allow participant loans — up to 50% of your vested balance, capped at $50,000 (2025 IRS rules). Neither this feature nor a Roth option exists in a SEP IRA; see Solo 401(k) vs. SEP IRA for a full feature comparison.

Where to Open a Solo 401(k): Fidelity, Schwab, and Vanguard

Three major self-directed options cover most solo 401(k) needs:

• Fidelity solo 401(k): no account fees, broad investment selection including mutual funds and ETFs, Roth option available, and an online setup process. A frequently chosen starting point for first-year freelancers.
• Schwab solo 401(k): no annual fees, access to commission-free ETFs and Schwab's full fund lineup, Roth option included. Setup is online with a plan adoption agreement.
• Vanguard solo 401(k): strong for low-cost index investing; historically required paper-based setup, though the process has been updated. Best suited for those committed to a Vanguard-fund strategy long term.

All three require your EIN and a completed plan adoption agreement, and none charge annual maintenance fees at the solo level. For checkbook control, alternative investments, or more flexible loan provisions, third-party administrators like Ascensus handle custom plan documents — though that adds cost and paperwork.

How a Solo 401(k) Reduces Your Tax Bill

Traditional solo 401(k) contributions are deductible on Schedule 1 of Form 1040 (your annual individual tax return), reducing your adjusted gross income dollar-for-dollar — though they don't reduce your self-employment tax base. At $80,000 net self-employment income, maxing the employee deferral alone moves $23,500 out of your ordinary income calculation. This is an estimate based on 2025 IRS rules, not a substitute for professional tax advice.