Yes — you can contribute to a Solo 401(k) for your self-employment income and a 401(k) at a day job in the same year. The catch is that the IRS treats your employee elective deferral limit as one shared number across every 401(k) and 403(b) plan you participate in, not a separate limit per plan. Whatever you defer at your day job reduces how much you can defer as “employee” in your Solo 401(k) — the two don’t stack.
Why the deferral limit follows you, not the plan
The IRS is explicit about this for one-participant plans: “A business owner who is also employed by a second company and participating in its 401(k) plan should bear in mind that his limits on elective deferrals are by person, not by plan. He must consider the limit for all elective deferrals he makes during a year,” according to the IRS page on one-participant 401(k) plans.
The same rule is stated more generally for anyone with more than one plan: “Generally, you aggregate all elective deferrals you made to all plans in which you participate to determine if you have exceeded these limits,” per IRS retirement topics on 401(k) contribution limits.
For 2026, the employee elective deferral limit is $24,500, with an additional $8,000 catch-up if you’re 50 or older (or $11,250 instead, for the calendar year you turn 60, 61, 62, or 63), per IR-2025-111. That total is yours alone — it doesn’t matter how many employers or plans you split it across.
What isn’t shared: the employer side of your Solo 401(k)
Here’s the part that trips people up in the other direction: the elective deferral limit is shared across plans, but the overall limit on your Solo 401(k) — the one that governs the “employer” (profit-sharing) contribution you make to yourself, on top of your deferral — is not. That overall limit applies separately to each plan maintained by an unrelated employer.
Your day job’s 401(k) is one employer’s plan. Your Solo 401(k) is a plan of a different employer — your own business. So the employer/profit-sharing contribution you can make to your Solo 401(k), based on your net self-employment earnings, isn’t reduced by anything happening at your day job’s plan. You can use our Self-Employed Retirement Contribution Calculator to estimate that piece on its own.
A quick example
Say you defer $10,000 into your day job’s 401(k) in 2026 and you’re under 50. Your total employee deferral limit for the year is $24,500. That leaves $14,500 of deferral room for your Solo 401(k) — not the full $24,500.
Separately, if your net self-employment profit is $50,000, your Solo 401(k)’s employer/profit-sharing contribution is calculated from that $50,000 alone, following the IRS’s reduced-rate worksheet for self-employed plan contributions — completely unaffected by the $10,000 you already deferred elsewhere. Plug your own numbers into the calculator to see the exact split for your situation, including the “already contributed elsewhere” field built for exactly this scenario.
If you go over the limit
If you defer more than the combined limit across your plans, the IRS calls it an excess deferral. You’re expected to notify your plan administrator before April 15 of the following year and have the excess — plus any earnings on it — distributed back to you. Excess deferrals withdrawn by that date are taxable in the year you contributed them; amounts left in the plan past the deadline can effectively be taxed twice, once when contributed and again when eventually distributed. See the “Treatment of excess deferrals” section of the IRS contribution limits page for the full mechanics.
Bottom line
Having both plans is completely normal and legal — plenty of people with a day job and a side business do exactly this. The thing to actually manage is your own tracking: no single plan provider sees the whole picture, so before you set a deferral percentage at either job, add up what you’ve already put away for the year and check it against the combined limit.