Solo 401(k) vs SEP IRA: Which Is Better?

1. Our verdict on solo 401(k) vs SEP IRA

Quick Answer: Pick the solo 401(k) if you have no employees other than a spouse and you want the largest possible deduction. Pick the SEP IRA only if the tax year has already ended and you missed the solo 401(k) deadline, or if you want zero ongoing paperwork and your profit is under about $60,000.

Most comparisons of these two plans start with the same sentence: both let you put away up to $72,000 in 2026. True, and almost useless. What decides the question is not the ceiling. It is how much profit you need to reach it.

A SEP IRA needs roughly $376,500 of Schedule C profit to hit $72,000. A solo 401(k) gets there at about $252,300. That is a $124,200 difference in required income for the identical outcome, and it is the whole argument in one line.

We built this the way we build every comparison at DollarVisor: the real numbers first, then the state-level value of the deduction, then the two situations where the SEP still wins. Every 2026 figure traces to the IRS, the SSA or a named state source: the same standard behind our investing guides. No company pays for placement in anything we publish.

2. Solo 401(k) vs SEP IRA: what actually differs

Quick Answer: Twelve rules separate solo 401(k) vs SEP IRA, and only four of them matter to a typical one-person business: the employee deferral, the catch-up, the Roth option and the setup deadline. The rest are paperwork differences that never change the answer.

Most feature grids weight every row equally, which hides the point. Ours marks the rows that actually move money. Figures come from IRS Notice 2025-67 and the IRS pages on one-participant 401(k) plans.

The last row is the one high earners overlook. A SEP IRA balance is a pre-tax IRA balance, so it lands inside the pro-rata calculation that governs a backdoor Roth IRA conversion. A solo 401(k) balance does not. If you run that strategy each year, the SEP quietly taxes it.

3. How much more will a solo 401(k) hold at your profit level?

Quick Answer: Exactly $24,500 more, at every profit level from $40,000 up to about $252,300. The gap is flat, not proportional, which is why the solo 401(k) helps modest earners far more than it helps high earners: in percentage terms.

The table below runs the math at eight profit levels. It uses the IRS method for calculating your own retirement plan contribution and the 2026 Social Security wage base of $184,500 published by the SSA. Bars show each plan’s total as a share of profit.

Two things stand out. First, the extra room is a flat $24,500 until the overall cap starts biting near $252,300 of profit. Second, a $40,000 freelancer can shelter almost 80% of profit in a solo 401(k) against 18.6% in a SEP: the smaller your business, the larger the advantage. That inverts how solo 401(k) vs SEP IRA is usually framed, as a question for high earners.

4. Why the employer half is identical in both plans

Quick Answer: Both plans use the same rule: 25% of compensation, which for a sole proprietor works out to about 20% of profit after the self-employment tax adjustment. That means the employer piece can never be the reason you pick one plan over the other in solo 401(k) vs SEP IRA.

Both plans cap the employer contribution at 25% of compensation. For someone with no W-2 wages, “compensation” is not your Schedule C profit: it is profit reduced by half your self-employment tax and by the contribution itself. Run that circular math out and 25% collapses to 20%.

Here is the calculation at $120,000 of profit, following IRS Publication 560:

1. Net earnings subject to self-employment tax. $120,000 × 92.35% = $110,820.
2. Self-employment tax. 15.3% of $110,820 = $16,955.
3. Deduct half of it. $120,000 − $8,477 = $111,523.
4. Apply 20%. $111,523 × 20% = $22,304: your employer contribution in either plan.
5. Add the deferral, solo 401(k) only. $22,304 + $24,500 = $46,804.

The five steps are the same in both plans until step five. That is the entire mechanical difference, and it explains why the extra room stays flat at $24,500 rather than scaling with income.

One caveat if you pay yourself a W-2 salary. Owners of an S corporation use 25% of W-2 wages instead, so the profit needed to reach $72,000 is different again: closer to $288,000 of salary. The $376,500 figure in this article is the sole proprietor number. If you see a much lower threshold quoted elsewhere, check which of the two it describes.

5. What the extra $24,500 saves you in your state

Quick Answer: Between $0 and $2,279, depending entirely on where you file. In Texas and Florida the deferral saves no state tax at all. In California it saves $2,279 a year on top of the federal saving: roughly a month of median rent.

National guides stop at the federal number. That skips most of the story, because the deferral is deducted from state taxable income too, wherever the state has an income tax and follows the federal treatment. The table applies each state’s 2026 marginal rate, from the Tax Foundation’s 2026 state income tax rates, to the $24,500 of extra room the solo 401(k) gives you.

The Pennsylvania note matters. Pennsylvania has a 3.07% income tax, but its rules for net income from a business, profession or farm do not let a self-employed owner deduct retirement plan contributions made for themselves. The state saving is zero even though the rate is not.

Add the federal layer and the picture changes scale. A filer in the 24% federal bracket saves $5,880 on that same $24,500, per the 2026 federal brackets. A Californian therefore banks roughly $8,159 a year in combined tax that a SEP IRA would have left on the table.

6. The one thing a SEP IRA still does better

Quick Answer: It buys you an extra six months. A SEP IRA can be created and funded up to your filing deadline including extensions, so an October filer can still open one for the prior year. A first-year solo 401(k) has to be in place by the April deadline, extensions excluded.

This is where the old advice was right and the new advice usually is not. Plenty of guides still say a solo 401(k) must exist by December 31. That stopped being true for sole proprietors after the SECURE 2.0 Act. A one-person unincorporated business can now adopt a plan after year end and make first-year deferrals up to the return due date, but, per the IRS, without regard to extensions.

So the deadline ladder for a 2026 tax year looks like this:

• SEP IRA. Open and fund by April 15, 2027, or by October 15, 2027 if you extend.
• Solo 401(k), first year. Open and make deferrals by April 15, 2027. An extension does not move this.
• Solo 401(k), employer piece. Deposit by the return due date including extensions, per the IRS issue snapshot on late employer contributions.

If it is already May and you are looking at a big prior-year profit with no plan open, the solo 401(k) vs SEP IRA question answers itself. The SEP is the only plan you can still open.

7. What a solo 401(k) costs that a SEP does not

Quick Answer: One annual form and one disqualifying event. Once the plan holds more than $250,000, you must file Form 5500-EZ each year. And the day you hire a non-spouse employee who qualifies, the solo 401(k) stops being a solo plan.

The extra room is not free of admin. Three costs are worth knowing before you open one:

• Form 5500-EZ. Required once total assets across your one-participant plans exceed $250,000 at year end, per the IRS Form 5500-EZ page. It is a short form, but missing it carries a penalty of $250 a day up to $150,000 a year.
• The headcount trigger. Hire one eligible non-spouse employee and the plan becomes a regular 401(k), with testing and disclosure obligations attached.
• Provider quality varies. Free brokerage solo 401(k) plans often exclude loans or Roth deferrals. A SEP IRA at the same broker has no such variation. Read the plan document as well as the fee schedule when you choose where to open the account.

None of these outweigh $24,500 a year of extra room. But they explain the one case where solo 401(k) vs SEP IRA is genuinely close. A very small side business earning $15,000, with no intention of growing, can pick the SEP and never think about it again.

8. What choosing the SEP costs you over 20 years

Quick Answer: About $1,004,000. A $120,000-profit business that uses the full $24,500 of extra deferral every year for 20 years, earning 7% a year, ends with just over a million dollars that a SEP IRA would never have held.

The annual gap looks modest. Compounded, it is the largest number in this article. This is a modeled projection, not a guarantee: it assumes a steady 7% return and a full $24,500 contributed each year end, with no fees or withdrawals.

Notice where growth overtakes contributions: somewhere between year 15 and year 20. That crossover is why solo 401(k) vs SEP IRA matters most in your first decade of self-employment, when there is still time for the extra money to compound. Steady annual contributions of this kind are simply dollar-cost averaging applied to a retirement plan.

9. How to choose between them in 2026

Quick Answer: Work through four questions in order: do you have employees, has the tax year already closed, do you run a backdoor Roth, and is your profit above $376,500. The first question that returns a “yes” gives you your answer.

Rather than weighing a dozen features, run solo 401(k) vs SEP IRA as a sequence. Stop at the first line that describes you.

1. Do you have an eligible employee who is not your spouse? Then a solo 401(k) is off the table. Your realistic choices are a SEP IRA or a full 401(k) plan.
2. Has the tax year already ended and you filed an extension? Take the SEP IRA. It is the only plan you can still open for that year.
3. Do you contribute to a backdoor Roth IRA every year? Take the solo 401(k). A SEP balance triggers the pro-rata rule and makes the conversion partly taxable.
4. Is your profit above $376,500 and steady? Both plans reach $72,000, so pick on admin preference, unless you are 50 or older, where only the solo 401(k) adds the $8,000 catch-up.
5. None of the above? Take the solo 401(k). It matches the SEP on every shared rule and adds $24,500 of room.

One nuance for the fourth line. If you also draw a salary elsewhere, your $24,500 deferral is shared across every plan you take part in. The solo 401(k) advantage then shrinks to whatever deferral room your day job leaves unused. The employer contribution is unaffected.

Where a plan sits inside your wider mix is covered in our guide to how much you need to retire. If your day job offers a governmental plan instead, compare it against how a 457(b) stacks up against a 401(k).

10. So which one wins?

On solo 401(k) vs SEP IRA, the solo 401(k) is the better plan for the large majority of one-person businesses, and the margin is not close. Same employer formula, same $72,000 ceiling, same $360,000 compensation cap, plus $24,500 of deferral, a catch-up the SEP does not offer, and no interference with a backdoor Roth.

The SEP IRA earns its place in two situations only: when you already employ people you would have to fund equally, and when the calendar has run out on you. Both are real, and both are narrow.

If you are choosing today for the 2026 tax year, open the solo 401(k) before April 2027. Keep the SEP in your back pocket as the plan that can still be created after the year closes.

11. Frequently Asked Questions

1. Can I have both a solo 401(k) and a SEP IRA?

Yes, but it rarely helps. The $72,000 annual additions cap applies across both plans for the same business, so running two accounts adds paperwork without adding room. The only common reason is a transition year, where you keep an old SEP balance while contributing to a new solo 401(k).

2. Which is better for someone earning $100,000 self-employed?

The solo 401(k), by a wide margin. At $100,000 of Schedule C profit a SEP IRA allows $18,587 while a solo 401(k) allows $43,087. That is 43% of profit sheltered instead of 18.6%, for the same underlying employer formula.

3. Does the solo 401(k) vs SEP IRA answer change if my spouse works in the business?

No, and it usually strengthens the case for the solo 401(k). A spouse who earns income from the business counts as a participant, not an outside employee. The plan stays a one-participant plan, and your spouse gets their own $24,500 deferral plus an employer contribution.

4. Can I switch from a SEP IRA to a solo 401(k) later?

Yes. You can stop funding the SEP and open a solo 401(k) for future years. Many providers also let you roll the old SEP balance into the new solo 401(k), which removes it from the pro-rata calculation if you use a backdoor Roth.

5. Do I need an LLC or corporation to open either plan?

No. A sole proprietor filing Schedule C can open either one. What matters to the IRS is that you have self-employment income and, for the solo 401(k), no eligible employees besides a spouse. The IRS page on retirement plans for self-employed people sets out the options.

This article is information, not financial or tax advice. Contribution limits, state rules and plan deadlines change; confirm current figures with the IRS and your state revenue department, or a qualified tax professional, before you act. Our sourcing and ranking standards are set out in our methodology, and the full terms are in our disclaimer.