A Solo 401(k) lets a self-employed person contribute in two separate capacities — as employee and as employer — which is exactly why it allows far higher limits than a traditional IRA. Knowing both halves of the formula, and the deadlines attached to each, is what determines how much you can actually shelter from tax.
The Two-Part Contribution Structure
As the 'employee,' you can contribute up to the standard elective deferral limit (the same limit that applies to a 401(k) at a traditional job), either pre-tax or as a Roth contribution if your plan allows it. As the 'employer,' your business can additionally contribute up to 25% of your net self-employment earnings (after adjusting for the deductible portion of self-employment tax). The two pools stack, which is why a profitable freelancer can often shelter far more here than through a SEP IRA at the same income level.
Catch-Up Contributions After Age 50
If you're 50 or older, you can add an additional catch-up contribution on top of the standard employee deferral limit. This applies only to the employee side of the equation — the employer profit-sharing contribution formula doesn't change with age.
The Deadline That Trips People Up
The plan itself must be established by December 31 of the tax year you want it to apply to — you cannot open a Solo 401(k) in February and have it count for the prior year, unlike a SEP IRA. However, you generally have until your tax filing deadline (including extensions) to actually fund the employer contribution for that year. Missing the December 31 setup deadline is the most common reason freelancers lose an entire year of Solo 401(k) eligibility.
Who Actually Benefits Most
The Solo 401(k) tends to outperform a SEP IRA for freelancers earning a moderate income who want to maximize the employee deferral portion, since a SEP IRA only offers the employer-style contribution (capped at roughly 20% of net self-employment income after adjustments) with no separate employee deferral. At higher income levels, the two plans converge, and the decision shifts toward features like Roth availability or a loan provision, which Solo 401(k) plans can offer and SEP IRAs cannot.
Filing Requirements Once the Balance Grows
Once your Solo 401(k) balance passes a certain reporting threshold, you're required to file an annual Form 5500-EZ with the IRS. This is a compliance step many self-employed account holders forget, since traditional 401(k) filings are usually handled by an employer's plan administrator.
Frequently asked questions
Can my spouse also participate in my Solo 401(k)?
Yes, if your spouse earns income from the same business, they can make their own employee and employer contributions under the same plan, effectively doubling the household's sheltered savings.
Is a Solo 401(k) only for sole proprietors?
No. It's available to any self-employed individual or owner-only business with no full-time employees other than a spouse, including single-member LLCs and S-corps.
Can I still contribute to a Solo 401(k) and an IRA in the same year?
Yes, but your ability to deduct traditional IRA contributions may be limited since you're covered by an employer-type retirement plan.