Yes, you can contribute to a Solo 401(k) for your self-employment income and to a 401(k) at a day job in the same year. But the IRS treats your employee elective deferral limit as one number shared by every 401(k) and 403(b) plan you participate in. Whatever you defer at your day job reduces how much you can defer as “employee” in your Solo 401(k).
Why the deferral limit follows you, not the plan
For one-participant plans, the IRS says: “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 applies to 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, however many employers or plans you split it across.
What isn’t shared: the employer side of your Solo 401(k)
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, isn’t shared. It applies separately to each plan maintained by an unrelated employer.
Your day job’s 401(k) is one employer’s plan, and your Solo 401(k) belongs to a different employer, your own business. So the employer contribution you can make to your Solo 401(k), based on your net self-employment earnings, isn’t reduced by anything in your day job’s plan. You can use the Self-Employed Retirement Contribution Calculator to estimate that piece.
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, which leaves $14,500 of deferral room for your Solo 401(k).
If your net self-employment profit is $50,000, your Solo 401(k)’s employer contribution is calculated from that $50,000 alone, following the IRS’s reduced-rate worksheet for self-employed plan contributions. The $10,000 you deferred elsewhere doesn’t affect it. The calculator has an “already contributed elsewhere” field for this case.
The full picture for that example
Putting both pieces together, this is what this person can save in 2026, using the reduced-rate method from Publication 560:
| Under 50 | Age 52 | |
|---|---|---|
| Day-job 401(k) deferral (already made) | $10,000 | $10,000 |
| Solo 401(k) deferral (rest of the $24,500) | $14,500 | $14,500 |
| Catch-up (age 50+, 2026) | — | $8,000 |
| Solo 401(k) employer contribution (20% × ($50,000 − $3,533 half of SE tax)) | $9,293 | $9,293 |
| Total across both plans | $33,793 | $41,793 |
The Solo 401(k) alone takes $23,793 under 50 and $31,793 at 52. Both fit within the $72,000 overall limit for that plan and the $46,467 of compensation it’s based on.
Catch-up contributions with two plans
If you’re 50 or older by the end of 2026, the catch-up adds $8,000 of deferrals on top of the $24,500. For ages 60 to 63 it’s $11,250 instead, per the IRS catch-up page. Like the regular deferral, it’s one amount for you, not one per plan. The IRS page on 401(k) contribution limits adds that:
- You can split it however you like. With plans of unrelated employers, the IRS says you can treat amounts as catch-up contributions “regardless of whether the individual plans permit those contributions.” Its example puts the whole catch-up in the solo plan, and says it can be split between the plans in any proportion.
- It sits outside the overall limit. Catch-up contributions don’t count toward the $72,000 cap on a plan’s annual additions.
- You have to keep track. In the IRS’s words, “it is up to you to monitor your deferrals.”
On the day-job side, starting in 2026, if a plan with Roth features offers catch-up contributions, your catch-up there must be Roth if your prior-year wages from that plan’s sponsor were over $150,000, per the same IRS catch-up page and Notice 2025-67.
Roth or pre-tax, it’s the same limit
Designated Roth deferrals aren’t a separate bucket. Publication 560’s deduction worksheet counts “allowable elective deferrals (including designated Roth contributions)” together, so deferring $10,000 as Roth at work uses up the same $10,000 of your $24,500 as pre-tax money would. Whether Roth or pre-tax is the better choice is covered in Roth IRA for Freelancers in 2026.
A simple tracking routine
- Before December, check your year-to-date 401(k) deferrals on your latest day-job pay stub. Include any employer you worked for earlier in the year: all your deferrals count.
- Subtract them from $24,500, plus your catch-up if you’re 50 or older. The result is your Solo 401(k) deferral room.
- Elect your Solo 401(k) deferral by December 31. The money can go in later. I explain the timing in Solo 401(k) vs. SEP IRA.
- Figure the employer contribution from your net profit once your books are closed. The calculator has a field for deferrals made elsewhere.
No single plan provider sees the whole picture, so run this check before you set a deferral percentage at either job.
Common mistakes
- Relying on the plans to catch it. Each plan only sees its own deferrals, and neither can stop you from going over the combined limit.
- Leaving the Solo 401(k) deferral room unused. If your day job doesn’t take the full $24,500, the rest can go into the Solo 401(k). That’s the main reason to pick it over a SEP, as Can You Have a SEP IRA and a 401(k) at the Same Time? shows.
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 be taxed twice, once when contributed and again when distributed. The “Treatment of excess deferrals” section of the IRS contribution limits page has the details.
