Self-employed individuals and sole proprietors face a decision each year that directly affects their federal tax bill: SEP-IRA or Solo 401(k)? For 2026, both plans share the same $72,000 defined contribution ceiling under IRC § 415(c), but the Solo 401(k)'s unique employee deferral component means owner-only businesses earning under roughly $255,000 of net self-employment income can shelter significantly more income than a SEP-IRA permits. Choosing the wrong plan can cost tens of thousands of dollars in missed deductions—and the April 15, 2026 deadline to fund a 2025 SEP-IRA or Solo 401(k) is weeks away.
What Are the 2026 Contribution Limits for Each Plan?
IRS Notice 2025-67 increased the Solo 401(k) employee deferral limit to $24,500 for 2026 (up from $23,500 in 2025), while the IRC § 415(c) annual additions ceiling—which governs both plans—rose to $72,000. SECURE 2.0 also introduced an enhanced catch-up contribution of $11,250 for participants aged 60–63, the highest available under any defined contribution plan.
The SECURE 2.0 Act's age 60–63 super catch-up brings the Solo 401(k) maximum to $83,250 in 2026 for those in that age window—$11,250 more than what any age group can contribute to a SEP-IRA.
How Is the Self-Employed Contribution Actually Calculated?
The contribution formula for self-employed individuals is not simply 25% of Schedule C net profit. Because the retirement contribution reduces the net income used to calculate the contribution itself, the math is circular. IRS Publication 560 (2025 edition) provides a worksheet that resolves this. The effective rate for both plans' employer profit-sharing component works out to approximately 18.587% of gross Schedule C net profit.
For a Solo 401(k), the same $27,881 employer profit-sharing amount is supplemented by the $24,500 employee deferral, producing a total contribution of $52,381—nearly double the SEP-IRA deduction at this income level. Both contributions are deducted on Schedule 1, Line 16 of Form 1040 and reduce adjusted gross income directly.
How Do Deduction Amounts Compare Across Income Levels?
The Solo 401(k)'s advantage is largest at lower to moderate income levels, where the $24,500 employee deferral represents a large percentage of total net earnings. As income rises, the employer profit-sharing component grows until the Solo 401(k) hits the $72,000 ceiling at approximately $255,500 of net self-employment income. The SEP-IRA does not reach $72,000 until net SE income approaches $387,000.
| Net SE Income | SEP-IRA | Solo 401(k) |
|---|---|---|
| $60,000 | ~$11,152 deduction | ~$35,652 deduction (+$24,500 deferral) |
| $150,000 | ~$27,881 deduction | ~$52,381 deduction |
| $255,500 | ~$47,500 deduction | $72,000 (ceiling reached) |
| $387,000+ | $72,000 (ceiling reached) | $72,000 (ceiling reached) |
For a self-employed taxpayer in the 32% federal bracket with $150,000 of net SE income, choosing a Solo 401(k) over a SEP-IRA yields $7,840 in additional federal income tax savings ($24,500 × 32%) from the employee deferral alone. State income tax savings are additive.
What Are the Setup and Contribution Deadlines?
Deadline rules are a decisive factor when choosing between plans. Historically, the SEP-IRA held a major advantage because a Solo 401(k) plan document had to be signed by December 31 of the contribution year. SECURE 2.0 Act § 316 fundamentally changed that for sole proprietors.
2026 Contribution Deadlines by Plan Type
Deadline AlertFor 2025 tax year contributions (act now before April 15, 2026):
| Plan | Establish by | Fund by | Extended Deadline (w/ Form 4868) |
|---|---|---|---|
| SEP-IRA | April 15, 2026 | April 15, 2026 | October 15, 2026 |
| Solo 401(k) — Sole Proprietor | April 15, 2026 | April 15, 2026 | October 15, 2026 |
| Solo 401(k) — S-Corp / C-Corp | December 31, 2025 | March 15, 2026 | September 15, 2026 |
Important: SECURE 2.0 § 316's extended adoption deadline applies only to one-participant plans covering a sole proprietor (or self-employed individual and spouse). S-corporations and C-corporations with solo 401(k)s must have adopted the plan by December 31, 2025 for 2025 contributions. If your business is an S-corp that missed the December 31 deadline, a SEP-IRA remains available—it can be adopted on IRS Form 5305-SEP at any time through October 15, 2026.
Form 5500-EZ filing: Solo 401(k) plans with total assets exceeding $250,000 at December 31 must file Form 5500-EZ by July 31 of the following year. SEP-IRAs have no equivalent annual filing requirement.
When Does a SEP-IRA Make More Sense?
The SEP-IRA wins on administrative simplicity and is the only viable option once a business hires eligible employees. Under IRC § 408(k)(2), an employee is eligible for a SEP-IRA if they are at least age 21, have performed services for the employer in at least 3 of the last 5 years, and received at least $750 in compensation from the employer in 2026. Once an employee meets these criteria, the employer must contribute the same percentage of compensation to that employee's SEP-IRA as they contribute to their own account—at 20%, that is $10,000 for every $50,000 employee earning.
Choose the SEP-IRA When
The SEP-IRA is the right choice when: (1) net SE income exceeds $387,000, making the deferral advantage moot since both plans hit $72,000; (2) you have or anticipate hiring eligible employees, because a Solo 401(k) is legally restricted to businesses with no common-law employees other than the owner's spouse; (3) you want zero annual compliance paperwork—no Form 5500-EZ, no plan document maintenance, and no Roth catch-up wage threshold tracking; or (4) your business is an S-corp and you missed the December 31, 2025 plan adoption deadline for a Solo 401(k).
A SEP-IRA can be established as simply as completing IRS Model Form 5305-SEP, which requires no IRS approval and no plan document filing. Contributions are flexible—you are not required to contribute every year, and the percentage can vary (as long as it is uniform across all eligible employees).
When Does a Solo 401(k) Generate a Larger Deduction?
The Solo 401(k) is structurally superior for most owner-only businesses earning under $255,500 in net self-employment income—which covers the vast majority of self-employed professionals, consultants, freelancers, and small business owners. The incremental tax savings from the employee deferral is real money, not a technicality.
Choose the Solo 401(k) When
Prioritize the Solo 401(k) when: (1) net SE income is under $255,500, where the $24,500 employee deferral creates a material additional deduction unavailable through a SEP-IRA; (2) you are between ages 60–63 and want the SECURE 2.0 super catch-up of $11,250 for a total contribution of $83,250; (3) you want the flexibility of Roth deferrals—the employee deferral portion of a Solo 401(k) can be designated as Roth, while SEP-IRA contributions are always pre-tax; (4) you may need access to funds via a participant loan under IRC § 72(p), which SEP-IRAs do not permit; or (5) you want to contribute the employer profit-sharing portion after year-end using the SECURE 2.0 extended adoption window.
Roth catch-up rule: If your FICA wages in 2025 exceeded $150,000, any catch-up contributions to a Solo 401(k) in 2026 must be designated as Roth contributions under IRC § 402(g) and the mandatory Roth catch-up requirement enacted by SECURE 2.0.
How Does a Solo 401(k) Interact With a W-2 Job?
If you have self-employment income alongside a W-2 position, the $24,500 employee deferral limit under IRC § 402(g) is an aggregate limit across all plans. If your employer's 401(k) plan has already absorbed the full $24,500 deferral, you cannot make additional employee deferrals to your Solo 401(k) for 2026. Exceeding the limit results in a 10% excise tax on the excess amount if not corrected by April 15 of the following year.
However, the employer profit-sharing contribution to your Solo 401(k)—which is separate from and not subject to the § 402(g) deferral limit—remains fully available. A self-employed consultant earning $150,000 from their practice while also receiving a W-2 salary can still contribute approximately $27,881 as an employer profit-sharing contribution to their Solo 401(k), regardless of W-2 401(k) deferrals. This is a high-value planning opportunity that is frequently overlooked. For individual tax planning that coordinates deferrals across multiple compensation sources, a mid-year review is essential.
Combining Retirement Plans With Other Business Tax Strategies
Both the SEP-IRA and Solo 401(k) reduce self-employment income, which in turn lowers the base for the Section 199A QBI deduction for pass-through businesses. Because the QBI deduction is 20% of qualified business income (reduced by retirement contributions under Treas. Reg. § 1.199A-1), maximizing retirement contributions can reduce the QBI deduction dollar-for-dollar. At certain income levels below the W-2 wage limitation thresholds, contributing less to a retirement plan may preserve more in QBI deduction savings than the retirement contribution itself generates.
For a comprehensive tax planning analysis that models the retirement-QBI interaction before you fund a plan, schedule a review now. The April 15 deadline does not allow for the extended luxury of post-filing reconsideration.
2026 Key Reference: SEP-IRA vs Solo 401(k) at a Glance
Quick Reference: 2026 Plan Limits and Rules
IRS Notice 2025-67SEP-IRA (IRC § 408(k))
- 2026 contribution limit: Lesser of $72,000 or 25% of compensation (20% effective rate for self-employed)
- Who can contribute: Employer only; no employee deferrals permitted
- Roth option: No
- Eligible employees: Must contribute same % for all workers with 3-of-5 years service, age 21+, $750+ wages
- Establish by: Tax filing deadline including extensions (Form 5305-SEP)
- Annual filing: None required below $250,000 in assets
Solo 401(k) / One-Participant 401(k) (IRC § 401(k))
- 2026 employee deferral limit: $24,500 (IRC § 402(g))
- 2026 employer profit-sharing: 25% of compensation / ~18.59% of net SE income
- 2026 total ceiling: $72,000 / $80,000 (age 50+) / $83,250 (age 60–63)
- Roth deferrals: Permitted
- Eligible employees: Owner and spouse only; terminates if common-law employees are hired
- Adopt by: December 31, 2025 (S-corp/C-corp) or extended filing deadline (sole proprietors, SECURE 2.0 § 316)
- Annual filing: Form 5500-EZ by July 31 when plan assets exceed $250,000
Governing authority: IRS Notice 2025-67; IRC §§ 401(k), 402(g), 404(a), 408(k), 415(c), 164(f), 72(p); SECURE 2.0 Act § 316; IRS Publication 560 (2025)
For self-employed individuals and small business owners deciding between plans before the April 15, 2026 deadline, the decision turns on three factors: your net SE income level, whether you have or plan to hire employees, and whether Roth or loan access has value to you. At income levels under $255,500, the Solo 401(k) almost always produces the larger deduction.
Have questions about SEP-IRA vs Solo 401(k) planning for 2026? Contact TS CPA for a free consultation. We respond within the same day.