Solo 401k vs SEP IRA: which plan wins for the self-employed in 2026?

The answer for most owners

This page compares two plan types for a self-employed person: the SEP IRA and the one-participant 401(k). If your question is instead which account should hold real estate or other alternative assets, that is a different comparison, covered in solo 401(k) vs self-directed IRA.

The SEP is an employer-only arrangement: the business contributes a percentage of pay into a traditional IRA for each eligible worker, owner included, and nothing else goes in under the plan. The solo 401(k) is an ordinary 401(k) that happens to cover only the owner and, if they work in the business, the owner's spouse. The IRS describes it as "a traditional 401(k) plan covering a business owner with no employees, or that person and his or her spouse", with "the same rules and requirements as any other 401(k) plan." The owner contributes in two capacities: an employee elective deferral and an employer nonelective contribution.

The employer contribution is the same in both plans: up to 25% of compensation, about 20% of net self-employment earnings for a sole proprietor or partner. So the comparison turns on what the 401(k) adds to that base and what it costs to run. The exceptions to a 401(k) win are owners with staff, owners deciding after the year has closed, and incomes high enough that both plans reach the cap.

Solo 401k vs SEP IRA: 2026 comparison chart

Dollar figures are from IRS Notice 2025-67, as summarized in the IRS COLA limits table. Features marked "plan permitting" depend on the plan document or custodian, not on the tax code.

2026 contributions at three income levels

For a sole proprietor, single-member LLC owner or partner, "compensation" is earned income: net earnings from self-employment after deducting half of self-employment tax and the employer contribution itself. Because the contribution reduces its own base, Publication 560 converts the 25% plan rate into a reduced rate of 0.25 ÷ 1.25 = 20%, applied to net earnings after the half-SE-tax deduction; the IRS solo 401(k) page points to the same computation for the employer piece of a 401(k). The employee deferral is a flat dollar amount, limited only by the $24,500 ceiling and by your earned income.

The table runs three Schedule C profit levels through that math. Self-employment tax is 15.3% of 92.35% of profit, with the 12.4% Social Security portion stopping at the 2026 wage base of $184,500. Figures are rounded down to the dollar and assume no W-2 wages from another job.

At $75,000 of profit the SEP shelters about 18.6% of profit; the solo 401(k) shelters more than half of it. At $300,000 the 401(k) is held back by the $72,000 cap rather than the formula: 20% of net earnings would allow $56,908 as the employer piece, but only $47,500 fits beside the deferral.

The 0.9% Additional Medicare Tax is not part of the half-SE-tax deduction, so it does not change these numbers. W-2 wages from another employer do: they use up part of the Social Security wage base, and deferrals to that employer's 401(k) reduce your $24,500, because the deferral limit is per person, not per plan. The detail on each limit is in solo 401(k) contribution limits and SEP IRA contribution limits.

Where the gap closes: the $252,000 and $376,000 crossovers

The solo 401(k) reaches the $72,000 ceiling when its employer piece hits $47,500, which requires $237,500 of net earnings after the half-SE-tax deduction. Working back through 2026 self-employment tax, that is about $252,300 of Schedule C profit. The SEP reaches $72,000 only when 20% of net earnings equals $72,000, or $360,000 of net earnings, which is about $376,500 of profit.

So the comparison has three zones for an owner under 50:

• Below about $252,300 of profit: the solo 401(k) shelters exactly $24,500 more than the SEP.
• Between about $252,300 and $376,500: the 401(k) is capped at $72,000 and the SEP is catching up; the advantage shrinks toward zero.
• Above about $376,500: both plans allow $72,000. The 401(k) then wins only through catch-up contributions ($8,000, or $11,250 at 60 through 63), after-tax contributions, loans and backdoor Roth treatment.

At 50 or older the 401(k) never fully converges: the catch-up sits on top of the $72,000 limit, and a SEP has no catch-up at all. Treat these crossover points as estimates; your own figures depend on other wages and on any other plans you participate in.

Roth options in each plan

Both plans now have a Roth route, but they are not equally usable. In a solo 401(k), Roth elective deferrals have long been available if the plan document provides for them, with no income limit, which makes them the simplest way for a high earner to put $24,500 a year into Roth money without the backdoor. SECURE 2.0 section 604 also allows employer contributions to be designated Roth at the participant's election, again only if the plan offers it.

The SEP gained a Roth option under SECURE 2.0 section 601, with mechanics set out in Notice 2024-2. It is optional for the employer, the election has to be made before the contribution, and the custodian has to be willing to accept employer money into a Roth IRA. Custodian support remains uneven, so check before you plan around it.

One 2026 change matters only to 401(k)s. Under SECURE 2.0 section 603, a participant whose prior-year FICA wages from the plan sponsor exceeded $150,000 must make any catch-up contributions as Roth. The final regulations key that test to wages under §3121(a), and the preamble gives "a partner who had only self-employment income" as an example of someone with no such wages, so a sole proprietor or partner is outside the rule. An S corporation owner paid more than $150,000 in 2025 W-2 wages is inside it, and needs a plan with a Roth feature to make any catch-up at all. Whether pre-tax or Roth is the better choice in the first place is the subject of pre-tax vs Roth.

Backdoor and mega-backdoor Roth compatibility

This is where many high-earning owners find the SEP's hidden cost. A SEP-IRA is a traditional IRA. When you convert a nondeductible IRA contribution in a backdoor Roth, the taxable share is figured on Form 8606 using your combined December 31 balance across all traditional, SEP and SIMPLE IRAs. Hold $300,000 in a SEP-IRA, contribute and convert $7,500, and roughly 97.6% of the conversion is taxable. A solo 401(k) balance is not an IRA and is left out of that calculation entirely.

That gives the 401(k) a second use: if its document accepts roll-ins, pre-tax IRA money, including an old SEP-IRA, can be moved into the plan before December 31 of the conversion year, clearing the way for a clean backdoor Roth. Roth IRA money cannot go the other way into a 401(k).

The solo 401(k) can also support a mega-backdoor Roth, if the document allows after-tax employee contributions and either in-plan Roth conversions or in-service distributions of those amounts. The room is whatever is left under the §415(c) limit after the deferral and employer contribution. At $150,000 of profit, that is $72,000 − $52,380 = $19,620 of after-tax capacity, which can then be converted to Roth. Many off-the-shelf brokerage solo 401(k) documents do not offer after-tax contributions, so this usually means a customized document with an annual fee. A SEP has no equivalent; it accepts employer contributions only.

The downsides of a solo 401(k)

The capacity advantage is real, but a solo 401(k) is a qualified plan with a trust, and it carries obligations a SEP does not.

• A filing with a penalty attached. Once plan assets reach $250,000 at year-end, Form 5500-EZ is due by July 31 for a calendar-year plan, and a final return is due when the plan terminates at any balance. The late-filing penalty under IRC §6652(e) is $250 a day, up to $150,000 per return, though the IRS runs a penalty-relief program for one-participant plans that come forward voluntarily.
• The deferral has a clock. It generally has to be elected by December 31 and cannot be added after the year closes, with one first-year exception for sole proprietors described below. A SEP contribution can be decided in October of the following year.
• One employee ends it. Once a common-law employee meets the plan's eligibility terms, the plan must cover them and becomes subject to coverage and nondiscrimination testing.
• Documents and restatements. The plan needs a written document, usually its own EIN for the trust, a signed deferral election each year, and a restatement when the IRS's pre-approved plan cycle requires it.
• Feature gaps in free versions. No-cost brokerage documents frequently omit Roth deferrals, after-tax contributions, loans or roll-ins; getting them means a third-party document and an annual fee.
• Loans can backfire. A participant loan that misses its repayment schedule becomes a taxable deemed distribution, with the 10% additional tax if you are under 59½.

None of these outweighs $24,500 of extra capacity for most owners, but they explain why a brand-new business is often started on a SEP and moved to a 401(k) once income is predictable.

Employees, and the SEP's 3-of-5 rule

The SEP's coverage rules are strict. Under the model Form 5305-SEP, contributions must be the same percentage of compensation for every eligible employee, and the most restrictive eligibility conditions the form allows are age 21, service in "at least 3 of the immediately preceding 5 years" and, for 2026, at least $800 of pay. Your own rate is the reduced 20%; an employee earning $70,000 of W-2 pay at a 25% plan rate receives $17,500. For a firm with several long-tenured staff, that is an expensive plan.

The 3-of-5 rule cuts both ways. The form defines service as "any work performed for you for any period of time, however short," so a seasonal worker who puts in a week in each of three years is in. The conditions also apply to the owner: a new sole proprietor who writes "3 years" into the agreement would exclude themself until the business has existed that long, which is why most one-person SEPs carry no service requirement. Used deliberately, the rule lets a growing firm keep new hires out of the SEP for up to three years, longer than a 401(k) can exclude them. A 401(k) may require at most one year of service for deferrals, and for plan years beginning in 2025 and later, SECURE 2.0 section 125 requires it to admit part-timers who log 500 hours in two consecutive years, as Notice 2024-73 confirms.

A solo 401(k) avoids testing only because there is no one to test. The IRS states that "the no-testing advantage vanishes if the employer hires employees" who meet the plan's eligibility rules. At that point you have an ordinary 401(k) with coverage and nondiscrimination obligations, unless it is redesigned as a safe harbor plan. Because of the long-term part-time rule, a part-timer at 500 hours a year is not automatically outside the plan after two years; track hours before assuming someone can be excluded.

Loans, paperwork and cost

Loans. A 401(k) may lend to a participant if the plan document provides for it: generally up to the lesser of $50,000 or half the vested balance, repaid in level payments over no more than five years, per the IRS loan FAQs. An IRA, including a SEP-IRA, can never lend to its owner; any loan is a prohibited transaction under IRC §4975.

Paperwork and cost. A SEP is a one-page agreement and an IRA at a custodian; it files nothing annually, and the custodian reports contributions on Form 5498. A solo 401(k) carries the document, restatement, election and Form 5500-EZ obligations listed above. Brokerage versions cost little but offer limited features; documents enabling Roth, after-tax contributions, loans and roll-ins cost more.

Deadlines, and moving a SEP into a solo 401(k)

The SEP is the most forgiving plan on timing. You can establish it and fund it as late as your return due date, including extensions: October 15, 2027 for a 2026 Schedule C filer who extends. That makes it the default when a profitable year ends without any plan in place.

The solo 401(k) is more demanding. Employer contributions can generally be made up to the extended due date, and since the SECURE Act a plan can be adopted after year-end for employer contributions. Employee deferrals are tighter. IRC §401(b)(2), as amended by SECURE 2.0 section 317, lets "an individual who owns the entire interest in an unincorporated trade or business, and who is the only employee" treat deferrals made "before the time for filing the return ... (determined without regard to any extensions)" as made in the plan's first year. So a sole proprietor or single-member LLC owner who adopts a plan after year-end can still defer for that first year, but only by April 15, and only once. S corporation owners do not get that relief, and in later years the election generally has to be in place by December 31. Confirm the dates for your entity with your plan provider.

Moving from a SEP to a solo 401(k) is common and usually simple. There is no conversion as such: you adopt the 401(k), stop contributing to the SEP, and decide what to do with the SEP-IRA balance. Leaving it in place is fine. Rolling it into the new plan, where the document accepts roll-ins, is a direct rollover of pre-tax money to a pre-tax account, not taxable, and it removes the balance from the backdoor Roth calculation. Because the model SEP may not be used by an employer that maintains another qualified plan, the cleanest switch is at the start of a year, with no SEP contribution made for any year the 401(k) also covers.

Can you have both?

You can own both accounts, but for the same business the limits do not stack. Form 5305-SEP says plainly: "Do not use this form if you currently maintain any other qualified retirement plan." A non-model or prototype SEP may coexist with a 401(k), but under IRC §415(k)(1)(D) a SEP is a defined contribution plan for §415 purposes, so your combined annual additions from both plans are still tested against one $72,000 limit for 2026, before catch-up.

Separate limits can exist where you own two unrelated businesses, but the controlled-group and affiliated-service-group rules decide whether they are unrelated, and those rules are easy to fail. For one business, a solo 401(k) already reaches the cap on less income than a SEP, so adding a SEP buys paperwork and no capacity.

Decision rules: when each plan wins

The solo 401(k) wins when:

• You have no employees and profit under about $376,500. It shelters up to $24,500 more for an owner under 50.
• You are 50 or older, at any income. Only the 401(k) allows catch-up contributions.
• You use or plan to use the backdoor Roth. Keep pre-tax money out of IRAs, or at least stop adding SEP balances.
• You want a mega-backdoor Roth or a loan facility. Both require a 401(k) document that allows them.
• You are an S corporation owner keeping salary modest. The deferral does not depend on the size of the W-2.

The SEP wins when:

• The year is over and no plan exists. A SEP can still be opened and funded by the extended due date; a solo 401(k) deferral is only possible for a sole proprietor's first plan year, by the unextended due date.
• You have, or will soon have, eligible employees. The solo plan is off the table. Price the SEP's uniform contribution and 3-of-5 exclusion against a safe harbor 401(k).
• Profit is reliably above about $376,500 and you are under 50. Both plans reach $72,000; the SEP gets there with no filing, no document and no deferral election, provided the backdoor Roth is not part of your plan.
• You value minimal administration above capacity, with eyes open about the backdoor Roth interaction.

Once you know how much you can shelter, the next question is whether to shelter it pre-tax or as Roth. The retirement tax calculators model what each choice is worth in retirement, including the effect on future RMDs and conversions.

This article is educational and is not tax, legal or investment advice. Plan features depend on your plan document and provider, and the 2027 limits had not been announced at the time of writing. Confirm the figures for your situation with a qualified professional before contributing.