Retirement accounts
SEP IRA vs Solo 401(k): Where the Self-Employed Ceiling Actually Sits
A SEP IRA allows 20% of net self-employment earnings. A solo 401(k) adds a flat deferral layer on top, so it permits far more until income runs high.
A SEP IRA and a solo 401(k) hold the same investments and receive the same tax treatment. The difference that matters is the formula that decides how much can go in, and at most income levels the solo 401(k) formula permits far more.
The reason is structural. A SEP IRA is funded by employer contributions only, capped at 25% of compensation. A solo 401(k) allows that same employer contribution and stacks an employee elective deferral of up to $24,500 for 2026 on top of it.
That second layer is flat. It does not scale with profit. So the lower the self-employment income, the wider the gap between the two ceilings.
The contribution formula is the entire difference
Start with what each vehicle is.
A SEP, short for simplified employee pension, is an IRA that an employer funds. The employer can be you, funding your own account, and a business of any size including a one-person business can establish one. There are no salary deferrals in a SEP and no catch-up contributions. The IRS states it plainly: elective salary deferrals and catch-up contributions are not permitted in SEP plans.
A solo 401(k), which the IRS calls a one-participant 401(k), is an ordinary 401(k) that happens to cover one person. It is available to a business owner with no employees, or that owner and a spouse. Because it is a 401(k), the owner wears two hats. As the employee, you make an elective deferral. As the employer, you make a profit-sharing contribution of up to 25% of compensation.
Both layers land in the same account and both count against the same outer limit.
Why a 25% employer rate is really 20% for an unincorporated owner
This is where the arithmetic surprises people the first time they run it themselves.
Plan documents state the employer contribution rate as a percentage of compensation. For a sole proprietor or a partner, compensation means net earnings from self-employment after deducting both one half of self-employment tax and the plan contribution itself.
The contribution sits inside the base it is measured against. That is circular, and the IRS resolves the circle by reducing the rate. Writing C for the contribution and E for net earnings before it, the plan says C = 0.25 × (E − C), which solves to C = 0.20 × E.
The IRS publishes that conversion as a rate table for the self-employed. A 25% plan contribution rate becomes a self-employed rate of 0.200000. The headline "25%" and the working figure "20% of net earnings" describe the same dollars.
Everything below measures net earnings after the deduction for half of self-employment tax, which is step 3 of the IRS deduction worksheet. That keeps self-employment tax out of the comparison, where it would only add noise, since it applies identically to both vehicles.
What the stacking is worth in dollars
Two formulas, both for 2026:
- SEP IRA: 20% of net earnings, capped at $72,000.
- Solo 401(k): $24,500 plus 20% of net earnings, capped at $72,000.
The $72,000 is the section 415(c) limit on annual additions, which binds any defined contribution arrangement and is the same ceiling that sets how much room a mega backdoor Roth has inside a 401(k). Both vehicles run into it. They run into it at very different income levels.
At $50,000 of net earnings the SEP permits $10,000 and the solo 401(k) permits $34,500. That is not a marginal difference. It is 3.45 times as much, and the entire excess is the flat deferral layer.
The gap holds at exactly $24,500 until the solo 401(k) reaches the cap at $237,500 of net earnings. From there the solo ceiling flattens while the SEP keeps climbing. The two meet at $360,000, which is where 20% of net earnings first equals $72,000.
The advantage decays as income rises
Dividing both ceilings by net earnings makes the decay legible in a way the dollar chart cannot.
At $50,000 the solo 401(k) permits 69% of net earnings into the plan. At $100,000 that falls to 44.5%. At $200,000 it is 32.3%. The SEP holds flat at 20% the whole way.
Which reframes the comparison. The solo 401(k) is not generically better by some fixed amount. It is dramatically better for a side business, a consultant in a thin year, or a practice whose profit swings, and it is functionally identical to a SEP for someone clearing $360,000.
One boundary condition sits at the other end of the range. The deferral layer cannot exceed earned income. Below roughly $31,000 of net earnings the solo formula would call for more than 100% of compensation, and section 415(c) tests annual additions against compensation as well as against the dollar cap. Both charts start at $50,000 to stay clear of that zone.
Everything the formula does not cover
The contribution math settles most of the comparison. The rest of it lives in features that have nothing to do with the ceiling.
| SEP IRA | Solo 401(k) | |
|---|---|---|
| Who funds it | Employer only | Employee deferral plus employer profit sharing |
| Catch-up at age 50+ | Not permitted | $8,000 for 2026 |
| Participant loans | Not permitted, and assets cannot be pledged as collateral | Permitted if the plan document allows, up to the lesser of 50% of the vested balance or $50,000 |
| Roth treatment | Permitted since 2023 by election, but the contribution is includible in gross income | Designated Roth account, the standard 401(k) mechanism |
| Annual filing | Generally none | Form 5500-EZ once plan assets reach $250,000 |
| Plan paperwork | IRS Form 5305-SEP is enough for many arrangements | A written plan document, adopted from a provider |
| Employees | Every eligible employee must be covered on the same terms | Owner and spouse only |
| Vesting of employer money | Always 100%, immediately | Set by the plan document |
The loan difference is structural, not a technicality
IRAs and IRA-based plans cannot offer participant loans. The IRS treats a loan from an IRA as a prohibited transaction, and pledging part of an IRA as collateral makes the pledged portion a distribution.
A 401(k) can offer loans where the plan document provides for them, capped at the lesser of 50% of the vested balance or $50,000. That same 50% rule is why an unvested employer match quietly shrinks a plan loan ceiling in an ordinary workplace plan.
Whether that access has any value depends on how likely the business is to need working capital, and borrowing against a retirement balance carries repayment terms of its own. The narrow point is that one vehicle has the option and the other cannot acquire it.
Roth exists in both now, on unequal terms
Before 2023, a SEP could not be a Roth account at all. Section 601 of the SECURE 2.0 Act removed that bar. A Roth IRA can now receive SEP contributions, provided the participant affirmatively elects it before the contribution is made. An employer cannot make the election on the participant's behalf.
The tax treatment is where the two diverge. A SEP contribution made to a Roth IRA is not excludable from gross income, and it is reported on Form 1099-R as though it had gone to a traditional IRA and been converted immediately. A solo 401(k) reaches Roth through a designated Roth account, the same route a workplace 401(k) uses, and custodians have supported it for years. The practical gap is coverage rather than statute. The same counting logic applies here as elsewhere in the Roth world, where holding several Roth IRAs does not raise the limit.
The employee question decides it for some businesses
This is the part that overrides the contribution math for anyone who is not strictly a one-person shop.
A SEP is an employer plan, and an employer that contributes for itself must contribute for its eligible employees on the same terms. The eligibility test is age 21, work for the business in at least 3 of the last 5 years, and at least $800 of compensation from the business in 2026. Same terms means the same plan contribution rate, so a 20% contribution for the owner corresponds to a 25% contribution for every eligible employee's compensation.
SEP contributions are also always 100% vested. An employee who leaves the following month keeps every dollar. There is no vesting schedule available to slow that down, which is the opposite of how a workplace 401(k) match is usually structured.
A solo 401(k) carries none of that exposure, for the plain reason that it cannot carry employees. The IRS defines the plan as covering a business owner with no employees, or that owner and a spouse. Hire one eligible common-law employee and it stops being a one-participant plan. It becomes a regular 401(k), with nondiscrimination testing and a full Form 5500.
So the vehicles answer different questions. One is built to distribute employer money across a workforce and is being borrowed by solo operators. The other is built for exactly one household and breaks if the business grows.
Where the ceiling stops being the point
Both vehicles are containers. Neither one earns anything.
Two consequences follow, and they cut against treating the limit as the deciding factor.
The ceiling only matters to the extent it binds. Someone contributing $12,000 a year lands in the same place under either formula, because $12,000 sits below both ceilings at any plausible income. The formula difference is worth precisely nothing until the intended contribution is large enough to collide with the SEP's 20%.
The cost of the container is a separate question from its size. Solo 401(k) providers vary on setup fees, annual administration, and the investment menu, while a SEP IRA is an ordinary IRA at whatever custodian holds it. Fund-level costs are the part that compounds against the balance every year, and fund fees are routinely larger than the account-level fees people scrutinize.
The mechanism in one line
A SEP IRA is a single multiplicative layer. A solo 401(k) is that same layer with a fixed additive layer stacked beneath it, and additive terms dominate multiplicative ones whenever the base is small.
That is the whole comparison. At $50,000 of net earnings the additive layer is worth 49 percentage points of income. At $360,000 it is worth nothing, because the section 415(c) cap has already taken over from both formulas. Where a business sits on that curve, and whether it has employees, settles the question well before any secondary feature does.