Here is a number that surprises most freelancers: a sole proprietor earning $60,000 can shelter $12,000 in a SEP-IRA but $36,000 in a Solo 401(k) — for the exact same income, in the exact same year, with the exact same paperwork burden. The plan you pick is not a footnote. It can triple how much of your income escapes tax.
Self-employment hands you a freedom that W-2 employees never get: you are both the employee and the employer, so you get to fund both sides of a retirement plan. But that freedom comes with a fork in the road. The two most popular options for one-person businesses — the Solo 401(k) and the SEP-IRA — look similar on a brochure and behave very differently in practice. This guide walks through how each one works, who should pick which, and the mistakes that quietly cost people thousands.
The Core Difference: One Lever vs. Two
Both plans let you contribute as the "employer." That contribution is capped at 25% of your net self-employment earnings. (For a sole proprietor, the math works out closer to 20% of net profit after you subtract the deductible half of self-employment tax — more on that trap later.)
The SEP-IRA stops there. One lever: the employer profit-sharing contribution. That is the whole plan.
The Solo 401(k) gives you a second lever: an employee elective deferral. In 2026, you can defer up to $24,500 of your earnings as the "employee," and that amount is not limited to a percentage of profit — it is a flat dollar cap. Then, on top of that, you still get the 25% employer contribution.
That second lever is why the Solo 401(k) wins so decisively for lower and moderate earners. The employee deferral lets you contribute a large share of a modest income. The SEP-IRA, working only off a percentage, simply cannot keep up until your profit is high enough that 25% exceeds the deferral-plus-profit-sharing total.
2026 Contribution Limits, Side by Side
Here are the numbers that matter for the 2026 tax year.
Solo 401(k):
- Employee elective deferral: up to $24,500
- Catch-up contribution (age 50–59 or 64+): additional $8,000, bringing the deferral to $32,500
- Enhanced catch-up (ages 60–63, under SECURE 2.0): additional $11,250 instead of $8,000
- Employer profit-sharing: up to 25% of net self-employment earnings
- Combined maximum: $72,000 (under 50), $80,000 (50–59 and 64+), or $83,250 (ages 60–63)
SEP-IRA:
- Employer contribution only: up to 25% of net self-employment earnings
- Combined maximum: $72,000
- No employee deferral, and traditionally no catch-up contribution
Notice that the ceiling is identical — $72,000 of total contributions for someone under 50. A high earner who maxes out either plan ends up in the same place. The difference is entirely about how fast you reach that ceiling at lower income levels, and that is where the Solo 401(k) pulls ahead.
A Worked Example
Consider a freelance designer with $90,000 in net self-employment earnings (after the self-employment tax adjustment).
- SEP-IRA: 25% of $90,000 = $22,500 maximum contribution.
- Solo 401(k): $24,500 employee deferral + 25% employer contribution ($22,500) = $47,000.
Same income, same person, more than double the tax-advantaged savings. The gap only closes once earnings climb high enough that the 25% employer piece alone approaches the overall cap — generally north of $200,000 in net earnings. Below that, the Solo 401(k) is the more powerful tool for almost everyone.
Where the SEP-IRA Still Wins
If the Solo 401(k) saves more, why does the SEP-IRA exist? Because simplicity has real value, and a few situations genuinely favor it.
Setup speed and deadline flexibility. A SEP-IRA is famously easy: a one-page form (IRS Form 5305-SEP) or a prototype document from your brokerage, and you are done. More importantly, you can open and fund a SEP-IRA as late as your tax filing deadline, including extensions. Miss the new year and realize in March that you had a great prior year? You can still open a SEP-IRA for that prior year and contribute. That retroactive flexibility is the SEP-IRA's signature advantage.
You have, or plan to have, employees. A SEP-IRA can cover employees. But here is the catch that surprises owners: a SEP-IRA requires you to contribute the same percentage of compensation for every eligible employee that you contribute for yourself. Want to put 20% away for yourself? You owe 20% for each qualifying employee too. For a true solo operator that is irrelevant. For a small team it can get expensive fast — but it is still administratively simpler than the alternative.
You want zero ongoing paperwork. A Solo 401(k) has a small administrative tail: once plan assets exceed $250,000, you must file IRS Form 5500-EZ each year. It is not hard, but it is a deadline you have to remember. A SEP-IRA has no such filing requirement, ever.
Where the Solo 401(k) Pulls Ahead
Beyond the raw contribution math, the Solo 401(k) carries several features the SEP-IRA simply does not offer.
The Roth option. A Solo 401(k) can include a Roth bucket, letting you make after-tax contributions that grow and come out tax-free in retirement. For younger self-employed people who expect to be in a higher bracket later, this is a major advantage. Under SECURE 2.0, employer contributions can even be designated as Roth. Traditional SEP-IRAs are pre-tax only; while the law now permits Roth SEP contributions, few providers actually support them, so in practice the Solo 401(k) is your realistic Roth path.
Plan loans. A Solo 401(k) can let you borrow from your own balance — up to 50% of the vested amount, capped at $50,000. It is not something to do casually, but it is an emergency lever a SEP-IRA does not have.
No drag on the backdoor Roth IRA. This one is subtle and costs high earners real money. If your income is too high to contribute directly to a Roth IRA, the standard workaround is the "backdoor Roth" — a non-deductible traditional IRA contribution converted to Roth. But the pro-rata rule taxes that conversion proportionally based on all your pre-tax IRA balances. A SEP-IRA is an IRA, so its balance gets dragged into that calculation and makes your backdoor Roth largely taxable. A Solo 401(k) is not an IRA — its balance is invisible to the pro-rata rule. High earners who want a clean backdoor Roth strongly prefer the Solo 401(k). In fact, a common fix is to roll an existing SEP-IRA into a Solo 401(k) precisely to clear the pre-tax IRA balance to zero.
The Mistakes That Cost Real Money
Picking the right plan is half the job. The other half is not tripping over the rules.
Missing the December 31 deferral election
The single most expensive Solo 401(k) mistake. The employee deferral portion — that valuable $24,500 lever — must be elected by December 31 of the tax year. You do not have to move the cash by then; the employer profit-sharing piece can wait until your tax deadline. But the deferral election has to be documented in writing to your plan administrator before year-end. Forget, and you lose access to the deferral entirely. This is the difference between a $47,000 contribution and a $22,500 one.
The 25% that is really 20%
For a sole proprietor or single-member LLC, "25% of net self-employment earnings" does not mean 25% of your Schedule C profit. You first subtract the deductible portion of self-employment tax, and the formula is circular because the contribution itself reduces the base. The effective rate lands around 20% of net profit, not 25%. People who plug 25% of gross profit into a calculator routinely over-contribute, which triggers a 10% excise tax and a correction headache. Use a proper retirement-plan contribution calculator, and run the numbers against your actual business structure (sole proprietor, S-corp, and partnership all calculate differently).
Hiring a full-time employee
A Solo 401(k) is, by definition, for a business with no full-time W-2 employees other than you and a spouse. Hire someone who works enough hours to qualify, and your Solo 401(k) is no longer "solo." You must convert it to a standard 401(k), which brings nondiscrimination testing, higher costs, and far more administration. If you expect to hire soon, factor that in before you set up the plan.
Treating the choice as permanent
It is not. You can switch plans between years, and you can even hold both — though contributions across them share the same combined annual limit, so having both rarely helps. Many self-employed people start with a SEP-IRA for its simplicity and graduate to a Solo 401(k) once they understand the bigger deferral. Revisit the decision whenever your income or business structure changes.
A Simple Decision Framework
Strip away the detail and the choice usually comes down to a few questions:
- Are you a true solo operator (just you, maybe a spouse)? Both plans work; lean Solo 401(k) for the bigger contribution.
- Is your net income below roughly $200,000? The Solo 401(k) lets you save substantially more. This is the most common situation, and the Solo 401(k) wins.
- Do you want a Roth bucket or plan-loan access? Solo 401(k), without question.
- Do you do — or want to do — backdoor Roth IRA contributions? Solo 401(k), to keep the pro-rata rule from taxing your conversions.
- Did you forget to set up a plan and now it is past December 31? The SEP-IRA is your only retroactive option. Open one before your tax deadline and fund the prior year.
- Do you value absolutely zero paperwork over maximizing the deduction? SEP-IRA.
For the majority of one-person businesses, the Solo 401(k) is the stronger choice. The SEP-IRA earns its keep as the simple, flexible, last-minute fallback.
Keep Your Retirement Math Honest
Both of these plans live or die on one number: your net self-employment earnings. Get that wrong and every contribution calculation downstream is wrong too — which means either leaving deductions on the table or over-contributing into penalty territory.
That number comes straight from clean books. When your income and expenses are tracked accurately throughout the year, calculating your maximum contribution in December is a five-minute task instead of a guessing game. Beancount.io offers plain-text accounting that gives you complete transparency and control over your financial data — no black boxes, no vendor lock-in, and a clear running picture of the profit your retirement contribution depends on. Get started for free and see why developers and finance professionals are switching to plain-text accounting.