A solo 401(k) has two separate slots you can fill. One is the employee deferral — the money you choose to set aside from your own pay. The other is the employer profit-sharing contribution — money "the business" puts in on your behalf. You're both the employee and the employer here, so you get to use both slots.
Here's the part that trips people up when they run more than one business. The employee deferral limit is one number per person. It doesn't matter if you have one business or five. You get a single deferral limit, total, across everything. The employer profit-sharing slot is different. It's calculated per employer, based on what that business pays you. So the real question is whether the IRS sees your two businesses as one employer or two.
That answer comes down to how each business is taxed — not what it's called. A sole proprietorship and a single-member LLC (a Single-Member LLC, or SMLLC) can end up being treated as the exact same thing for tax purposes. Or they can be treated as two separate employers. Let's walk through how to tell which situation you're in.
The two buckets inside a solo 401(k)
Think of your solo 401(k) as having two compartments.
The first is your employee elective deferral. For 2026, you can defer up to $24,500 of your own earnings. If you're 50 or older, you add an $8,000 catch-up, for a total of $32,500. And if you're between ages 60 and 63, a special "super catch-up" lets you add $11,250 instead, for a total of $35,750 for 2026. This number is yours as a person. You cannot multiply it by stacking businesses.
The second compartment is the employer profit-sharing contribution. The business contributes a percentage of your compensation. For a corporation paying W-2 wages, that's up to 25% of your wages. For a sole proprietor or a disregarded LLC, the math works out to roughly 20% of your net self-employment (SE) income after a couple of adjustments. This contribution is calculated per employer.
Both compartments together can't exceed the 415(c) limit, which is the overall cap on everything that goes into a single plan in a year. For 2026, that's $72,000 per plan (or more if catch-up contributions apply on top). And the compensation that counts toward the profit-sharing math is capped at $360,000 for 2026.
Here are the figures that drive everything below, all for tax year 2026:
| 2026 limit | Amount |
|---|---|
| Employee deferral (under 50) | $24,500 |
| Employee deferral with catch-up (50+) | $32,500 |
| Employee deferral with super catch-up (ages 60–63) | $35,750 |
| 415(c) total annual additions (per plan / per controlled group) | $72,000 |
| Compensation cap for profit-sharing math | $360,000 |
The disregarded entity rule
Now the key concept. A single-member LLC, by default, is what the IRS calls a disregarded entity. That phrase means exactly what it sounds like. For federal income tax, the IRS ignores the LLC as a separate thing and looks straight through it to you, the owner.
So if you have a sole proprietorship and a default SMLLC, both report their income on Schedule C of your personal tax return. The LLC doesn't file its own business tax return. It doesn't have its own employer identity for this purpose. As far as the IRS is concerned, both businesses are one self-employment activity carried on by one person — you.
That has a direct consequence for retirement plans. You don't get two separate employer contribution calculations. You don't get two separate 415(c) limits. The two businesses are combined and treated as a single employer. One pool, one set of limits.
This is different from how it might feel day to day. You might keep separate bank accounts, separate clients, separate invoices for the two businesses. None of that changes the tax treatment. What matters is that a default SMLLC and a sole proprietorship both land on Schedule C as one taxpayer's self-employment income.
The combined Schedule C math
Let's put real numbers on it. Say your sole proprietorship nets $80,000 for 2026, and your disregarded SMLLC nets $40,000. Because both are treated as one self-employment activity, you combine them. Your total net SE income is $120,000.
Your employer profit-sharing contribution is based on that combined $120,000 — not on each business separately. The rough rule of thumb for a sole proprietor is about 20% of net SE income. So your employer contribution is approximately 20% of $120,000, or about $24,000 for 2026.
I'm using the 20% shortcut here on purpose to keep it readable. The precise calculation is a little lower. You first subtract the deductible portion of your self-employment tax from your net income, then apply the rate. The actual figure runs slightly under that $24,000, and a contribution calculator or your plan administrator will give you the exact number. But 20% gets you close enough to plan around.
Now stack the two buckets. You add your employee deferral of $24,500 for 2026 (assuming you're under 50) on top of the roughly $24,000 employer piece. That's about $48,500 going into your single solo 401(k) for the year. That sits comfortably under the $72,000 per-plan 415(c) limit for 2026, so you've got room.
Notice what didn't happen. You did not get a second $24,000 employer contribution just because you have a second business. Combining the income gave you one calculation, not two.
The S-corp exception
Here's where the answer flips. Everything above assumes your LLC is a default disregarded entity. But an LLC can elect to be taxed as an S corporation (S-corp). That's a choice you make by filing an election with the IRS. Many self-employed people do it to manage self-employment tax.
If your SMLLC has elected S-corp taxation, it's no longer disregarded. It becomes a separate entity that pays you W-2 wages and files its own corporate return. A separate employer like this can sponsor its own solo 401(k) plan, separate from any plan your sole proprietorship sponsors.
Here's where the dangerous myth lives. It's tempting to conclude that two separate plans means two separate $72,000 (2026) limits — and that you could therefore push something like $144,000 in total. For a single owner, that is not how it works. When you own 100% of both businesses, the IRS treats them as a controlled group under common control (IRC 414(b)/(c)). IRC 415(f) then requires every defined-contribution plan of a controlled group to be aggregated and tested against a single $72,000 annual-additions limit for 2026. Two plans, one combined cap. You cannot put $72,000 into the S-corp plan and another $72,000 into the sole-prop plan — combined annual additions across both are capped at one $72,000.
This is not a rare "sometimes" exception, which is how it often gets described. For one person who wholly owns both businesses, controlled-group aggregation is the default, not the edge case. (Affiliated service group and controlled group rules get genuinely complicated when ownership is shared with other people — that's where professional advice matters — but a solo owner of two wholly-owned businesses sits squarely inside a single controlled group.)
So what does the S-corp actually buy you for retirement? Not a doubled cap. The real difference is the employer contribution basis. A sole proprietor's profit-sharing contribution is roughly 20% of net SE income; an S-corp's is up to 25% of the W-2 wages it pays you. Depending on how you set your salary, the 25%-of-wages basis can let you reach the (single) $72,000 ceiling more efficiently — but it does not raise that ceiling above $72,000.
And as always, your single employee deferral limit applies across both plans combined. You don't get to defer $24,500 into the S-corp plan and another $24,500 into the sole prop plan. The $24,500 (or your catch-up total) for 2026 is one number for you as a person, split however you like between the two plans.
So do you actually need two 401(k)s?
For most people in this spot, the practical answer is simpler than they fear. Here's the side-by-side:
| Disregarded SMLLC + sole prop | S-corp LLC + sole prop | |
|---|---|---|
| Number of plans | One solo 401(k) | Can be two (one per employer) |
| Employee deferral limit | One shared $24,500 (2026) | One shared $24,500 (2026) |
| Employer contribution basis | ~20% of combined Schedule C net SE income | 25% of S-corp W-2 wages + ~20% of separate sole-prop Schedule C |
| Overall 415(c) limit | One $72,000 (2026) | One $72,000 (2026) — controlled group aggregated under 415(f) |
If your LLC is a default disregarded entity alongside your sole proprietorship, you almost certainly don't need two plans. Both businesses are one self-employment activity. One solo 401(k) covers everything. You combine the Schedule C income, run one employer contribution calculation, add your one employee deferral, and you're done. A second plan would add paperwork without adding any room to contribute.
If your LLC has elected S-corp taxation, two separate plans are possible — one sponsored by the S-corp on your W-2 wages, one by the sole proprietorship on its Schedule C income. But don't expect a bigger overall cap: because you control both businesses, controlled-group aggregation under 415(f) holds the combined total to a single $72,000 for 2026. The S-corp's advantage is a different employer-contribution basis (25% of W-2 wages), not a doubled ceiling — and it comes with more administration and separate filings.
The single most important thing to confirm is your LLC's tax election. People often don't remember whether they filed an S-corp election years ago, or they assume their LLC is "an LLC" as if that alone settles the tax treatment. It doesn't. Check your records or ask your accountant. Find out whether your LLC is disregarded or taxed as an S-corp before you set up a second plan. That one fact decides whether you're looking at one shared limit or two separate ones.
Once you know how each business is taxed, run your combined or separate income through the [Roth vs. Traditional analyzer](/calculators/roth-vs-traditional) to see exactly how much fits in each bucket for 2026 — and whether those contributions belong in pre-tax or Roth dollars.