SEP IRA vs Solo 401(k) in 2026: IRS Limits, Math, and Examples

A SEP IRA is simple. A solo 401(k) is more powerful. That one sentence is directionally right, but it hides the part that costs self-employed people money: the answer changes with profit level, age, entity type, payroll status, employees, Roth goals, and how late in the year the plan is being set up. The IRS numbers for 2026 are now available in Notice 2025-67, and the contribution mechanics sit in IRS Publication 560.

The 2026 IRS Limits

Notice 2025-67 increased the 2026 defined contribution annual additions limit under section 415(c) from $70,000 to $72,000. The elective deferral limit under section 402(g) increased from $23,500 to $24,500. The regular age-50 catch-up limit for most 401(k)-type plans increased from $7,500 to $8,000. For participants who attain ages 60, 61, 62, or 63 in 2026, the higher catch-up limit remains $11,250. The compensation cap used for many plan calculations increased from $350,000 to $360,000.

Catch-up contributions are special. The $72,000 annual additions cap does not include catch-up contributions. That means a qualifying age-50 solo 401(k) owner may be able to contribute $72,000 plus an $8,000 catch-up, or $80,000 total, if compensation supports it. A person age 60 through 63 may be able to reach $83,250 because the higher catch-up is $11,250. The plan document has to allow the feature, and high earners should confirm the SECURE 2.0 Roth catch-up rules; Notice 2025-67 lists a $150,000 Roth catch-up wage threshold for 2025 wages used to determine 2026 treatment.

How a SEP IRA Works

A SEP IRA is an employer contribution arrangement. There is no current-year employee deferral for a new SEP IRA the way there is for a 401(k). Publication 560 says SEP contributions for a common-law employee cannot exceed the lesser of 25% of compensation or the annual dollar limit. For 2026 planning, Pub. 560's "What's New" section gives the 2026 annual additions limit of $72,000 and compensation limit of $360,000.

For a self-employed sole proprietor or single-member LLC taxed on Schedule C, the 25% headline becomes an effective 20% computation. Pub. 560 explains that excess SEP contributions are measured against 25% of an employee's compensation, or for the owner, 20% of net earnings from self-employment. The reason is circular: your plan contribution reduces the compensation base used to compute the contribution. Most tax software handles the worksheet, but the business owner still needs to understand the direction.

How a Solo 401(k) Works

A solo 401(k), also called an individual 401(k), has two layers. The owner wears the employee hat and can make an elective deferral up to the 2026 $24,500 limit, subject to earned income and coordination with other 401(k), 403(b), SARSEP, and 457(b) deferrals. The business then wears the employer hat and may make a profit-sharing contribution. For a corporation, that employer contribution is generally up to 25% of W-2 compensation. For a sole proprietor, it is the Pub. 560 self-employed computation, effectively 20% after the self-employment-tax adjustment.

That employee deferral is the lever. A SEP owner with $80,000 of Schedule C profit may be limited to a contribution around the high teens. A solo 401(k) owner with the same profit may contribute the same employer amount plus a $24,500 deferral. That is why solo 401(k)s dominate at moderate profit levels. The SEP catches up only when income is high enough that both plans hit the same $72,000 employer-plus-employee ceiling, ignoring catch-ups.

Worked Example: Elena, Schedule C Consultant

Elena is 37 and has no employees. Her consulting business shows $120,000 of Schedule C net profit before any retirement contribution. Because her net earnings are below the 2026 Social Security wage base, the rough self-employment-tax calculation is straightforward. Net earnings for self-employment tax are $120,000 × 92.35% = $110,820. At the 15.3% self-employment tax rate, her SE tax is about $16,955, and half is about $8,478. Her starting plan compensation after the half-SE-tax deduction is therefore about $111,522.

Under a SEP IRA using the standard self-employed computation, the maximum contribution is approximately 20% × $111,522 = $22,304. Under a solo 401(k), Elena can make a $24,500 employee deferral plus roughly the same $22,304 employer contribution, for a total around $46,804. The solo 401(k) allows about $24,500 more because the employee deferral sits on top of the employer contribution.

If Elena is trying to lower taxable income before making quarterly estimated tax payments, the solo 401(k) is the more flexible tool. If she only wants something fast and inexpensive to establish after year-end, the SEP may still be attractive. The best answer is not "SEP bad." The best answer is "SEP simple, solo 401(k) stronger."

Worked Example: Marcus, S-Corp Owner Age 52

Marcus owns an S corporation and pays himself $180,000 of W-2 wages in 2026. His reasonable-compensation memo supports that salary; he is not inflating wages only to boost retirement contributions. For a SEP IRA, the business can contribute up to 25% × $180,000 = $45,000, subject to the plan rules and the overall cap.

With a solo 401(k), Marcus can defer $24,500 as the employee, contribute $45,000 as the employer, and because he is over 50, add an $8,000 catch-up if the plan permits it. The employee-plus-employer amount before catch-up is $69,500, below the $72,000 annual additions limit. His total with catch-up is $77,500. For an S-corp owner, payroll design and retirement design are tied together, so run the wage side with the S-corp payroll calculator and check the owner-pay rules in our reasonable compensation guide.

When a SEP IRA Is the Better Choice

A SEP IRA is often the right answer when the owner wants speed, low administration, and no Roth or loan features. It can be established and funded relatively late compared with many qualified-plan deadlines, depending on the taxpayer's filing extension. It is also easier to explain to a small business that has variable profits and does not want annual 401(k) administration. For a high-income owner under age 50 who will hit the $72,000 cap either way, the SEP may produce the same deductible result with less paperwork.

The weak spot is employees. SEP contributions generally must cover eligible employees under the plan's allocation formula. A business owner cannot usually give herself 25% and give staff nothing if the employees meet SEP eligibility requirements. Solo 401(k)s have the same issue once the business has non-owner employees who cannot be excluded; at that point it may stop being a "solo" plan and become a regular 401(k) with testing and administration. The employee question should be answered before the plan is opened, not at tax filing.

When a Solo 401(k) Is the Better Choice

A solo 401(k) is usually better when profit is moderate, the owner is age 50 or older, Roth contributions matter, or the owner wants plan-loan flexibility. It also helps when the owner has W-2 income from another job but has not used the full elective deferral limit. The employee deferral can be traditional or Roth if the plan document permits Roth, while employer profit sharing is generally pre-tax unless the plan has newer Roth employer-contribution features and the provider supports them.

The tradeoff is administration. A solo 401(k) may require more careful setup, a written plan document, payroll coordination for S corporations, and Form 5500-EZ once plan assets exceed the filing threshold or when the plan terminates. Those tasks are manageable, but they are real. A person who will ignore plan paperwork may be better served by a SEP even if a spreadsheet says the 401(k) is better.

Backdoor Roth and IRA Basis Issue

One underappreciated SEP cost is the pro-rata rule for backdoor Roth IRA planning. A SEP IRA is an IRA for aggregation purposes. If you have pre-tax money in a SEP, SIMPLE, traditional, or rollover IRA at year-end, a backdoor Roth conversion can become partly taxable. A solo 401(k) can sometimes avoid that problem because qualified-plan balances are not included in the IRA pro-rata calculation. If backdoor Roth contributions are part of your annual routine, read our backdoor Roth IRA guide before opening a SEP.

Common Mistakes

• Comparing 25% to $24,500 as if they are alternatives. In a solo 401(k), the employee deferral and employer contribution can stack, subject to limits.
• Using 25% for Schedule C owners. Pub. 560's self-employed computation effectively turns the employer rate into 20% after the circular adjustment.
• Forgetting a day-job 401(k). The elective deferral limit is shared across plans.
• Ignoring employees. A plan that is perfect for a solo owner can become expensive once common-law employees become eligible.
• Letting tax savings override cash flow. Contributions are not free; they move cash out of the business. Coordinate with payroll, estimated taxes, and operating reserves.

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