Your Solo 401(k) Is Capped by Your Entity Choice. Here's the Math.
Your business clears $400,000 a year. You set up an S corp a few years ago to save on self-employment tax. You opened a solo 401(k) and maxed your employee deferral at $23,500. You figure that is the ceiling for retirement savings — the same ceiling every working American has.
It is not your ceiling. But your entity choice may be making it look that way.
The solo 401(k) has two contribution streams: the employee deferral — the $23,500 you know — and the employer profit-sharing contribution, up to 25% of compensation. That second stream is where the real money lives. And it is calculated differently depending on what entity you file under.
The ceiling you did not know existed
If you run your business as a sole proprietor or single-member LLC taxed as a sole prop, your solo 401(k) employer contribution is based on your net self-employment earnings — essentially the full profit of the business after deducting half the self-employment tax.
At $400,000 in net profit, that employer piece works out to roughly $46,500. Combined with the $23,500 employee deferral, your total into the solo 401(k) caps at around $70,000 for 2026. For someone 50 or older, the catch-up contribution pushes that to about $77,500.
That is a number that changes the conversation.
Now take the same $400,000 profit, but you elected S corp treatment. The employer contribution is based on your W-2 wages — what the IRS considers reasonable compensation for your role. Not your business profit. Your salary.
If you pay yourself a reasonable salary of $150,000, the employer contribution caps at 25% of that: $37,500. Plus your $23,500 employee deferral. Total into the solo 401(k): roughly $61,000 instead of $70,000.
The S corp saved you about $9,000 in self-employment tax. And it quietly cost you about $9,000 in retirement contribution room. The tradeoff is closer to even than most owners realize.
Three entities, three contribution ceilings
Let us walk the numbers side by side with $400,000 in business profit and an owner under 50:
| Entity type | Contribution base | Employer piece | Employee deferral | Total possible |
|---|---|---|---|---|
| Sole proprietor or LLC (sole prop tax) | Net earnings (~$372K after SE tax deduction) | ~$46,500 | $23,500 | ~$70,000 |
| S corp, $150K salary | W-2 wages of $150,000 | ~$37,500 | $23,500 | ~$61,000 |
| S corp, $250K salary | W-2 wages of $250,000 | ~$46,500 | $23,500 | ~$70,000 |
The S corp only matches the sole prop total if you pay yourself enough W-2 salary. But a higher salary costs you more payroll tax. At $150,000 you lose retirement room. At $250,000 you break even on contributions but lose more to payroll tax than the sole prop would have paid.
The ceiling is real, and your entity determines where it sits. For business owners in San Diego, Frisco, or Panama City Beach — or anywhere you run a profitable business — this is the kind of planning your preparer never runs.
When the S corp tradeoff flips
The classic advice — switch to an S corp to save self-employment tax — is correct for most owners past a certain income. But it is incomplete. The S corp changes your retirement contribution math, and no one mentions that when the election is filed.
For an owner making $300,000 to $500,000, the self-employment tax savings run roughly $8,000 to $15,000 per year. The retirement contribution gap between an S corp with a modest salary and a sole prop runs roughly $5,000 to $9,000 per year. At higher profits, the gap grows.
The inflection point depends on your salary. The lower your reasonable compensation, the more self-employment tax you save — and the more retirement room you lose. The higher your salary, the narrower the gap narrows but the payroll tax savings shrink.
There is no single right answer. The right answer depends on your age, how much you want to save for retirement, and whether you plan to add a second plan alongside the solo 401(k).
A stronger structure: pairing entity and plan
For the owner who wants both the S corp's self-employment tax savings and a retirement contribution above the solo 401(k) ceiling, the answer is not choosing one — it is adding a defined benefit or cash balance plan alongside the 401(k).
A cash balance plan layered on top of an S corp's solo 401(k) can push total retirement contributions well past $150,000 per year. The contribution is based on the plan's actuarial target, not your salary — so the W-2 cap on the 401(k) side is no longer your ceiling. You keep the self-employment tax savings from the S corp, and you add a retirement contribution that the sole prop structure alone cannot match.
For the owner who is still a sole proprietor and values maximum retirement savings above self-employment tax savings, the solo 401(k) with full profit sharing already delivers roughly $70,000. And you have the option to add a defined benefit plan on top of that, too.
The question is not which one is better. The question is which combination fits where you are.
What to check before year-end
If you run a profitable business and have a solo 401(k), here is what to verify before Q4 closes:
Your salary is reasonable but not unnecessarily high. If your S corp pays you $80,000 on $500,000 of profit, your retirement contribution is capped at around $43,500. Raising your salary to $200,000 raises the ceiling to about $73,500 — and the extra payroll tax is modest.
Your plan type matches your contribution goal. If you want to put away more than the solo 401(k) ceiling allows, a cash balance plan is the next step. They are not as complicated as they sound when a strategist sets them up.
Your contribution percentage is set correctly. Many solo 401(k) providers default to the employee deferral only. You have to elect the profit-sharing contribution separately, and it must be calculated correctly for your entity type.
None of these are hard to fix before year-end. After year-end, the contribution limits are locked.
FAQ
What is a solo 401(k)?
A solo 401(k) is a retirement plan for self-employed business owners with no employees. It allows you to contribute as both employee (the standard deferral) and employer (a profit-sharing contribution), with total limits far higher than a traditional IRA or a standard employee 401(k).
How does the entity type affect my solo 401(k) contribution?
Your entity type determines the base used to calculate the employer profit-sharing contribution. A sole proprietor uses net self-employment earnings. An S corp owner uses W-2 wages only. A C corp owner can use total compensation including salary and bonuses. This can create a difference of $5,000 to $10,000 or more in total contribution room.
What is reasonable compensation for an S corp owner?
Reasonable compensation is what a similar role would pay in an arm's-length transaction. For a working owner of a profitable business, that generally means a salary aligned with industry benchmarks for your role. Setting it too low saves payroll tax but limits retirement contributions. Setting it too high triggers unnecessary payroll tax.
Can I have a solo 401(k) and a defined benefit plan at the same time?
Yes. A cash balance or defined benefit plan can run alongside a solo 401(k). The combined contribution can exceed $150,000 per year for a high-earning owner, depending on age and plan design. This is one of the most powerful structures for owners over 50 who entered the retirement savings game late.
When does an S corp stop making sense for retirement?
An S corp stops making sense for retirement when the self-employment tax savings are smaller than the retirement contribution room you are giving up. This usually happens when you want to maximize retirement savings above all else, you are over 50 with catch-up eligibility, or your reasonable compensation is much lower than your total business profit.
Ready to see whether your entity and retirement plan are working together or against each other? A 15-minute discovery call is enough to spot the gap. Call (619) 280-2700 or email info@RoadmapTax.com to book yours. We serve clients in San Diego, Frisco Texas, Panama City Beach Florida, and nationwide.
