Can you have a solo 401(k) and a job with a 401(k)?

Yes. Having a job with a 401(k) does not stop you opening a solo 401(k) for self-employed income on the side, and the combination is usually better than either alone.

The trap is that the two headline numbers behave differently. One is per person. The other is per employer. Almost every mistake here comes from applying the wrong one.

The employee deferral is per person

The elective deferral limit — $24,500 for 2026, plus catch-up if you are 50 or over — is yours, aggregated across every 401(k), 403(b) and SIMPLE plan you participate in.

It does not reset per job. Defer $24,500 at work and you have used all of it; your solo 401(k) can receive no employee deferral that year. Defer $10,000 at work and $14,500 is left for the solo plan.

The IRS holds you responsible for tracking this, not your employers. Neither payroll department can see the other plan, so nobody will stop you exceeding it.

The overall limit is per unrelated employer

The other number — $72,000 for 2026, covering employee deferral plus employer contributions — applies separately to each unrelated employer’s plan.

This is the part that makes the combination worth having. Your day job’s plan has its own $72,000 ceiling, and your business’s solo 401(k) has another one. You are not sharing a single overall cap between them.

So the realistic best case for someone with a job and a side business:

• At work: defer up to $24,500, plus whatever your employer matches.
• In your business: no employee deferral left, but the plan can still take an employer contribution of roughly 20% of your net earnings from self-employment — under its own $72,000 limit, and unaffected by anything the day job did.

At $50,000 of side profit that employer slice is worth something like $9,000 of additional sheltered income, entirely separate from the day job.

The exception that undoes it: related employers

“Unrelated” is doing real work in that sentence.

If you control both businesses — a controlled group, or an affiliated service group — the law treats their plans as one for the overall limit. In that case the two $72,000 ceilings collapse into one and the strategy above does not work.

For the ordinary case of an employee at a company they do not own, running an unrelated side business, this does not apply. It matters if you own a stake in your employer, if you own two businesses, or if your side work is providing services back to a company you have an ownership interest in. That last one is the least obvious and the most commonly missed.

Since the deferral is spoken for, consider a SEP instead

If your day job already absorbs the full $24,500, the employee deferral — the entire advantage a solo 401(k) has over a SEP-IRA — is worth nothing to you this year.

What is left is the employer percentage, and a SEP-IRA delivers exactly the same percentage with materially less administration: no plan document to maintain, no Form 5500 once assets pass $250,000, and a deadline that stretches to the extended filing date.

So the honest advice inverts the usual one:

• Day job deferral is maxed: a SEP-IRA is probably the better instrument.
• Day job deferral is partly used, or you have none: the solo 401(k) wins, because the leftover deferral is real money and the SEP cannot take it.
• Unsure, and your side income is growing: open the solo 401(k) before 31 December anyway. The deferral election has a year-end deadline the SEP does not, so keeping the option costs nothing and losing it costs a year.

The SEP vs solo 401(k) comparison runs that trade-off properly, and the deadlines guide explains why December matters more than April here.

If you do over-defer

It happens most often when changing jobs mid-year, because the new employer starts your deferral count from zero.

The correction is time-sensitive: notify one plan and withdraw the excess, plus its earnings, by 15 April following the year. Miss that window and the excess is taxed twice — once in the year it was deferred, and again when it eventually comes out.

Add the two plans up in December, not in April.

The summary

	Limit	Scope
Employee deferral	$24,500 (2026)	Per person, across all plans
Catch-up at 50+	$8,000	Per person
Overall plan limit	$72,000 (2026)	Per unrelated employer

Two plans, one deferral limit, two overall limits — provided the employers are genuinely unrelated. The calculator below sizes the employer contribution your business can make.