Solo 401k vs SEP IRA 2026: Contribution Limits, the Math at Every Income Level, and Which One Cuts More Tax

A Solo 401(k) and a SEP IRA both cap out at $72,000 for 2026, but at most real incomes the Solo 401(k) wins by exactly $24,500: the employee deferral a SEP IRA cannot take. On $100,000 of net self-employment profit the SEP IRA maximum is $18,587 and the Solo 401(k) maximum is $43,087; the two plans only converge around $378,000 of profit, and the Solo 401(k) then pulls ahead again with the $8,000 catch-up for anyone 50 or older ($11,250 at ages 60 through 63). The SEP IRA wins on paperwork and on deadline: you can open and fund one as late as October 15, 2027 for the 2026 tax year.

Key takeaways (2026 limits, IRS Notice 2025-67):

Save this cheat sheet — key numbers in one image.

Solo 401(k) vs SEP IRA: What Each Plan Actually Is

A Solo 401(k), which the IRS calls a one-participant 401(k) plan, is an ordinary 401(k) that covers only a business owner with no employees, or the owner and a spouse. The owner contributes twice: as the employee, through elective deferrals up to $24,500, and as the employer, through a profit-sharing contribution of up to 25% of compensation. What people search for as "solo 401k," "individual 401(k)," or "self-employed 401(k)" is the same plan.

A SEP IRA (Simplified Employee Pension) is a traditional IRA that the employer funds. Under IRC §408(k), contributions come only from the business, at up to 25% of compensation, and the IRS SEP contribution limits page is blunt about the rest: "Elective salary deferrals and catch-up contributions are not permitted in SEP plans." Our broader retirement plan deductions guide for the self-employed covers SIMPLE IRAs and personal IRAs as well; this article is the head-to-head between the two plans that let a one-person business shelter the most.

How Much Can You Contribute at Your Income? The Three-Row Ladder

The employer piece is identical in both plans, so the gap between them is the deferral plus any catch-up, until the $72,000 cap flattens both. The table runs the Publication 560 Deduction Worksheet for Self-Employed with the 2026 constants (Social Security wage base $184,500, deferral $24,500, cap $72,000):

Three things stand out. At $100,000 the Solo 401(k) more than doubles the SEP IRA, and the whole difference is the $24,500 deferral. At $250,000 the Solo 401(k) is a few hundred dollars under the $72,000 cap while the SEP IRA is still $25,000 short of it; above about $378,000 of profit both plans sit at $72,000 and only the catch-up separates them. And at $40,000 the worksheet's 100%-of-earnings rule bites: the 55-year-old's total is capped at her $37,174 of net earnings, and the employer piece shrinks to $6,337 so that deferral, employer contribution, and catch-up together never exceed what the business earned.

Run Your Own Net Profit Through Both Plans

Why 25% Becomes 20% for the Self-Employed

The 25% rate in both plans is written for W-2 compensation. A sole proprietor's compensation is "earned income" under IRC §415(c)(3)(B), which is net profit minus the deduction for half of self-employment tax minus the retirement contribution itself. Because the contribution reduces the base it is measured against, Publication 560 replaces 25% with a reduced rate from its Rate Table for Self-Employed: 25% ÷ 1.25 = 0.200000, applied to net earnings after the half-SE-tax deduction (Schedule 1, line 15) and before the contribution.

The worksheet then adds two guardrails that matter for a Solo 401(k). The employer piece cannot exceed one-half of net earnings remaining after your deferrals (step 12), and deferrals cannot exceed net earnings remaining after the employer piece (step 15), which together enforce the IRC §415(c) rule that annual additions cannot exceed 100% of compensation. A separate line multiplies the $360,000 compensation cap by 25%, giving $90,000, but the $72,000 dollar cap binds long before that. An S corporation owner skips all of this: the employer contribution is a flat 25% of W-2 wages, and the Solo 401(k) calculator handles both cases.

Which Plan Cuts More Tax?

The deduction goes on Schedule 1 (Form 1040), line 16, "Self-employed SEP, SIMPLE, and qualified plans" (2025 revision, confirmed against the form), and it reduces adjusted gross income for both plans. Neither plan reduces self-employment tax: Schedule SE runs on net profit before the contribution, so the $14,130 of SE tax on $100,000 is the same whether you contribute $0 or $43,087.

Priyanka, a freelance structural engineer in Denver with $100,000 of net profit and no other income, sits in the 22% federal bracket for 2026. A maximum SEP IRA contribution of $18,587 saves her about $4,089 of federal income tax; a maximum Solo 401(k) contribution of $43,087 saves about $9,479, before the knock-on reduction in her QBI deduction (which is figured on income net of the retirement deduction) and before state tax. At 24% or 32% the gap widens in proportion. State treatment varies: Massachusetts, for one, does not allow the deduction at all for its own income tax (2025 Form 1 instructions, line 11), so the state saving there is zero under either plan.

Deadlines: When Each Plan Must Be Open and Funded

A SEP IRA can be set up and funded as late as the due date of your return including extensions: for tax year 2026, April 15, 2027, or October 15, 2027 with an extension. That is the SEP IRA's genuine advantage; you can decide in September 2027 that 2026 was a good year and fund it then.

A Solo 401(k) has two clocks. The employer contribution follows the same return-due-date rule. The employee deferral does not. Under Treasury Regulation §1.401(k)-1(a)(6)(iii), a sole proprietor's compensation is deemed currently available on the last day of the taxable year, so the election to defer must be made by December 31, 2026, even though the cash can move by the filing deadline. SECURE 2.0 softened this for a brand-new plan: Publication 560 confirms that a sole proprietor with no employees can adopt a 401(k) after the end of the tax year, provided it is adopted by the tax filing deadline without regard to extensions, and the same provision (SECURE 2.0 §317) lets first-year deferrals made by that date count for the year just ended. The safe practice is unchanged: adopt the plan and sign the deferral election by December 31, then fund by April 15.

Form 5500-EZ, when it applies, is due by the last day of the seventh month after the plan year ends, July 31, 2027 for a calendar-year 2026 plan, with an automatic extension to your extended return due date if the plan year and tax year match.

Roth Options and the Backdoor Roth Problem

Most Solo 401(k) providers offer designated Roth deferrals, so the $24,500 can go in after-tax with tax-free growth while the employer piece stays pre-tax. SEP IRAs can be Roth too, in principle: Publication 560 confirms that for tax years beginning after 2022, SECURE 2.0 §601 lets an employer's SEP plan allow contributions to a Roth SEP IRA. In practice the custodian has to offer it, and many still do not, so ask before assuming.

The quieter Roth issue is the pro-rata rule. A SEP IRA is a traditional IRA, and Form 8606 line 6 asks for the value of all your traditional IRAs at December 31 when you convert; a large SEP balance makes a backdoor Roth IRA contribution mostly taxable. A Solo 401(k) balance is not an IRA and never enters that line. For a high earner who relies on the backdoor Roth every year, that alone can decide the question.

What Happens When You Hire? Employees and the Eligibility Cliff

A SEP IRA survives hiring but gets expensive. Under Publication 560, an eligible employee is anyone who has reached age 21, worked for you in at least 3 of the last 5 years, and received at least $800 of compensation in 2026 (up from $750 in 2025), and the IRS model Form 5305-SEP requires the same percentage of compensation for every eligible employee as for you. A 20% contribution for yourself means 20% for the part-time assistant in year four.

A Solo 401(k) does not survive hiring at all. The IRS one-participant page says the plan covers a business owner with no employees, or the owner and spouse, and that employees who meet the plan's eligibility requirements must be included and tested for nondiscrimination. Traditional eligibility is 1,000 hours in a year, and the long-term part-time rule now reaches further: for plan years beginning after 2024, an employee who works at least 500 hours in 2 consecutive 12-month periods must be allowed to defer (Publication 560, restriction on conditions of participation). A spouse who works in the business is the one exception; a spouse counts as an owner, not a disqualifying employee.

Paperwork, Loans, and What the SEP IRA Does NOT Do

The SEP IRA has no annual filing, no plan document beyond the one-page Form 5305-SEP, and no trustee duties. The Solo 401(k) needs a plan document from the provider and, per the 2025 Form 5500-EZ instructions, an annual return once the total assets of all your one-participant plans exceed $250,000 at year-end, and always for the final plan year when the assets are distributed. The penalty for a missing 5500-EZ is $250 a day, up to $150,000 per plan year.

Loans are the other one-sided feature. The IRS loan FAQs state that loans are not permitted from IRAs or IRA-based plans such as SEPs, SARSEPs, and SIMPLE IRA plans. A Solo 401(k) that provides for loans can lend you the lesser of $50,000 or the greater of $10,000 or 50% of your vested balance.

When the Solo 401(k) does NOT win

• Net profit above roughly $378,000 for someone under 50. Both plans hit $72,000; the SEP IRA gets there with no plan document and no 5500-EZ
• You already have, or will soon hire, a non-spouse employee working 1,000 hours a year (or 500 hours in two consecutive years). The Solo 401(k) is unavailable; a SEP IRA or a regular 401(k) is the choice
• The year is over and no plan exists. After December 31 the deferral election is gone for the general rule, and after the unextended return due date the SECURE 2.0 first-year window closes too; a SEP IRA still accepts a full contribution until October 15, 2027
• You want to skip contributions in lean years without paperwork. Both plans allow it, but a SEP IRA with no employees has nothing to file either way
• The contribution rate itself is the issue. With no employees and profit below the cap, a SEP IRA never beats a Solo 401(k) on the number; it only beats it on effort and deadline

Common Mistakes With Solo 401(k) and SEP IRA Contributions

1. Applying 25% to net profit

Twenty-five percent of $100,000 is $25,000; the correct SEP maximum is $18,587, because the rate is 20% and the base is net earnings after the $7,065 half-SE-tax deduction. The excess is a nondeductible contribution subject to a 10% excise tax until corrected.

2. Expecting the contribution to lower self-employment tax

Schedule SE runs on line 31 profit. A $43,087 Solo 401(k) contribution changes income tax only; the $14,130 of SE tax in the $100,000 example is untouched.

3. Deciding on the Solo 401(k) deferral in February

The deferral election for 2026 belongs on paper by December 31, 2026 under the regulation; only a first-year plan adopted under SECURE 2.0 §317 gets until April 15, 2027. A February decision on an existing plan means the employer piece only, which is the SEP IRA number.

4. Forgetting the final Form 5500-EZ

The $250,000 threshold is irrelevant in the year you close a Solo 401(k) and roll the assets out; the instructions require a final return for every one-participant plan, and the $250-a-day penalty starts the day after the July 31 due date.

The Right Contribution Number by December: How Jupid Helps

Both plans key off one figure, net profit from self-employment, and most freelancers do not know it with any precision until March. Jupid connects to your business bank account, categorizes income and expenses at 95.9% accuracy as they land, and keeps a running Schedule C profit you can query any day of the year. Ask the AI accountant in WhatsApp or iMessage "what is my profit so far and what could I put in a Solo 401(k)?" and you get the current number and the 20% math in chat, in time to sign the deferral election before December 31. Try Jupid.

Sources

• IRS Notice 2025-67: 2026 limits of $24,500, $8,000, $11,250, $72,000, $360,000, and the $800 SEP compensation threshold
• IRS: One-participant 401(k) plans: contribution structure, Form 5500-EZ, employee coverage
• IRS: SEP contribution limits (including grandfathered SARSEPs): 25% or $72,000, no deferrals or catch-ups
• IRS Publication 560 (2025), Retirement Plans for Small Business: Rate Table and Deduction Worksheet for Self-Employed, SEP eligibility, deadlines, Roth SEP, long-term part-time rule, Schedule 1 line 16
• IRS: Instructions for Form 5500-EZ (2025): $250,000 threshold, final return, due date, penalty
• IRS: Retirement plans FAQs regarding loans: no loans from SEP IRAs; loan limits
• Schedule 1 (Form 1040), 2025: line 15 and line 16 labels
• Instructions for Form 8606 (2025): line 6, value of all traditional IRAs
• IRC §402(g), IRC §415(c), and IRC §408(k): deferral limit, annual additions and earned income, SEP participation rules
• Treasury Regulation §1.401(k)-1(a)(6)(iii): timing of a self-employed individual's deferral election
• Massachusetts 2025 Form 1 instructions: SEP, Keogh, and IRA payments not deductible for Massachusetts purposes

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This guide is for general educational purposes and does not constitute tax, legal, or investment advice. Contribution limits are indexed annually, plan documents differ by provider, and the earned-income calculation depends on your entity type and other income. For advice specific to your situation, consult a qualified tax professional or plan administrator.