Solo 401(k): The Best Retirement Plan for Self-Employed Workers

A solo 401k is a retirement plan built for a self-employed person who has no employees other than a spouse. The IRS calls it a one-participant 401k, and you can read the official description on the IRS one-participant 401k page. The whole point of a solo 401k is that you wear two hats at once. You are the employee, and you are also the employer. That dual role lets you put money in from both sides, which is why a solo 401k beats almost every other self-employed retirement account on raw contribution room. A SEP IRA or a SIMPLE IRA only opens one of those two doors. The solo 401k opens both.

Here is the mechanics of how a solo 401k works in 2026. As the employee, you can defer up to 24,500 dollars of your earnings into the plan. As the employer, your business can kick in a profit-sharing contribution on top of that, up to 25 percent of your compensation. Add the two together and the combined cap under Section 415(c) is 72,000 dollars for the year. If you are age 50 or older, you get an extra 8,000 dollar catch-up, which pushes your personal ceiling to 80,000 dollars. Those figures come straight from the IRS 2026 contribution limit announcement, and they replace the older 23,500 and 70,000 numbers people still quote by mistake.

A worked example makes the solo 401k clearer. Say you run a single-member LLC and net 120,000 dollars after expenses. You first put in your full 24,500 dollar employee deferral. Then your business adds a profit-sharing contribution. For a sole proprietor the math runs off net earnings from self-employment, not gross revenue, so the employer piece lands near 22,300 dollars once the deduction for half your self-employment tax is folded in. That totals roughly 46,800 dollars socked away inside the solo 401k, and every dollar of a traditional contribution comes off your taxable income this year. At a 24 percent marginal rate, that single move trims your federal bill by more than 11,000 dollars.

The solo 401k also gives you a choice most self-employed accounts do not. You can route the employee deferral into a Roth bucket if your plan document allows it, meaning you pay tax now and pull the growth out tax-free in retirement. Many owners split the difference, putting the employee deferral in Roth and the employer profit-sharing in traditional pre-tax dollars. That mix gives you a pre-tax deduction today and a pool of tax-free money later, which is a flexibility a SEP IRA simply cannot match.

We see one mistake every single year with solo 401k plans. People assume the 25 percent employer contribution is 25 percent of their net profit. It is not. For an unincorporated owner the percentage is applied to a reduced base, and the IRS lays out the exact worksheet on its self-employed contribution calculation page. Skip that step and you over-contribute, which triggers a 10 percent excise tax on the excess. An S corporation owner has a cleaner path, because the 25 percent is figured on W-2 wages, but that only works if payroll is actually being run.

One more edge case worth flagging on a solo 401k. The plan loses its one-participant status the moment you hire a non-spouse employee who meets the eligibility rules, usually a worker who logs 1,000 hours in a year. At that point you are running a regular 401k with all the testing and disclosure that comes with it. If you expect to add staff, the way you set up the entity and payroll today shapes how painful that transition is later. Our entity formation and structuring team and our payroll compliance team handle exactly that handoff, so the plan you open now does not box you in. If you want us to model your solo 401k numbers against an S corporation election before year end, start at our new client inquiry page and we will run the comparison.

The short answer is up to 72,000 dollars in a solo 401k for 2026, or 80,000 dollars if you are 50 or older. That total is split between your employee deferral and your employer profit-sharing contribution, and understanding the split is where most solo 401k owners get tripped up. Treat them as two separate buckets that share one outer ceiling.

Start with the employee side. In 2026 you can defer 24,500 dollars of compensation into the solo 401k, a figure the IRS confirmed in its 401k limit announcement for 2026. This deferral is per person, not per plan, so if you also pay into a 401k at a separate W-2 job, the two deferrals share the same 24,500 dollar ceiling. The employer profit-sharing piece is separate. Your business can add up to 25 percent of compensation, and that employer money does not count against the 24,500 dollar deferral cap. It counts only against the combined 72,000 dollar Section 415(c) limit.

Now the catch-up. If you turn 50 or older during 2026, you get an extra 8,000 dollars of deferral room on top, which is why your personal ceiling rises to 80,000 dollars. There is a newer wrinkle from the SECURE 2.0 law. Owners who reach age 60, 61, 62, or 63 in the year get a larger catch-up of 11,250 dollars instead of 8,000. So a 61-year-old consultant with enough income can target 83,250 dollars in a single year inside a solo 401k. That is a meaningful jump for someone in the final stretch before retirement who wants to pack away as much as the rules allow.

One number caps the whole thing from above. Compensation that counts toward the 25 percent employer contribution is itself limited to 360,000 dollars for 2026. Even a high earner pulling 500,000 dollars cannot use the slice above 360,000 dollars to compute the profit-sharing piece. In practice that ceiling, combined with the 72,000 dollar Section 415(c) limit, is what stops a very high income from translating into an unlimited contribution. Knowing the comp cap matters because it tells you the income level at which extra earnings stop adding to your plan.

Here is a worked example for a solo 401k contribution. You operate as an S corporation and pay yourself 90,000 dollars in W-2 wages. Your employee deferral is the full 24,500 dollars. Your employer profit-sharing contribution is 25 percent of the 90,000 dollar wage, which is 22,500 dollars. Add them and you have placed 47,000 dollars into the solo 401k, well under the 72,000 dollar cap, with room to raise your salary next year if you want to push closer to the ceiling. The wage you choose drives the whole calculation, which is why salary setting matters so much for an S corporation owner.

The mistake we correct every year is double counting compensation across plans, or running the 25 percent off the wrong base. For a sole proprietor the employer percentage is not 25 percent of Schedule C profit. It is roughly 20 percent of net earnings after the self-employment tax adjustment, and the IRS publishes the exact contribution and deduction worksheet so you can verify it line by line. Get the base wrong and you either leave money on the table or over-fund and owe a penalty. One edge case on timing closes this out. Your employee deferral generally has to be elected by December 31, but the employer profit-sharing contribution can be funded as late as your business tax filing deadline, including extensions. That gives you a planning window after the year closes to true up the solo 401k once your final numbers are known. If you want help setting an S corporation salary that maximizes your solo 401k without overpaying payroll tax, our tax strategy consulting team runs that calculation, and our payroll compliance group makes sure the wages are documented. Reach us through the new client inquiry page.

You have to file Form 5500-EZ for your solo 401k once the plan holds 250,000 dollars or more in total assets at the end of the plan year. Below that line you file nothing for an ongoing plan, which is one of the quiet advantages of a solo 401k over a more complex employer plan. The official rule lives on the IRS About Form 5500-EZ page. For a brand-new plan that starts small, this means years can pass before you ever touch a federal filing.

The threshold is a combined number, and that catches people. If you maintain more than one one-participant plan, you add up the assets across all of them. So a solo 401k worth 180,000 dollars plus a separate defined benefit plan worth 100,000 dollars puts you at 280,000 dollars combined, over the line, and you file a 5500-EZ for each plan even though neither one alone tops 250,000 dollars. The IRS spells this aggregation rule out on its one-participant 401k page, and it is the single most missed detail on solo 401k filings.

A worked example helps. You opened a solo 401k three years ago. End of year one it held 90,000 dollars, year two 170,000 dollars, year three 265,000 dollars. You owed no Form 5500-EZ for years one and two, because each year-end balance sat under 250,000 dollars. For year three you cross the threshold, so a 5500-EZ is due by the last day of the seventh month after your plan year ends. For a calendar-year plan that is July 31. Once you start filing, you generally keep filing every year going forward, even if the balance later dips back below the line.

The return itself is short. Form 5500-EZ asks for basic plan identification, the number of participants, and the plan assets at the beginning and end of the year. There is no income statement and no audit. Most solo 401k owners can complete it in under an hour, and it is filed electronically through the EFAST2 system or on paper with the IRS. The contrast with a full 5500 for a staffed plan, which can require an independent audit once participant counts climb, is part of why the solo 401k stays so cheap to run.

There is also a final-year rule that applies regardless of size. The year you terminate the solo 401k and distribute all the assets, you must file a final Form 5500-EZ even if the balance is only 40,000 dollars. So a plan that never crossed 250,000 dollars during its life still owes one return at the very end. People who roll a small solo 401k into an IRA and forget this filing get a surprise notice later.

Recordkeeping is the unglamorous part that prevents the late-filing nightmare. Keep a simple log of your solo 401k year-end balance for every year the plan exists, because the day you cross 250,000 dollars sets the first filing year, and that fact is easy to lose track of a decade out. Brokerage statements usually show the December 31 value, so save the year-end statement each year in one folder. When the plan eventually terminates, that same record tells you the final balance you report on the closing return. Five minutes of filing once a year saves you from reconstructing a decade of balances under penalty pressure.

The mistake we see every year is a late or missed 5500-EZ, and the penalty is brutal. The IRS can assess 250 dollars per day up to 150,000 dollars per plan year for a return that is not filed. The saving grace is a dedicated relief program for one-participant plans that lets you file delinquent returns for a flat 500 dollars per year, capped at 1,500 dollars, but only if you come forward before the IRS contacts you first. Once a notice arrives, that cheap fix is gone. One edge case on valuation closes this out. The 250,000 dollar test uses fair market value at year-end, including any hard-to-value assets like real estate held inside the solo 401k. If your plan owns property, get a defensible valuation before you decide you are under the line. Our tax strategy consulting team monitors that threshold for solo 401k clients each year. If you think you may already be past it, start at our new client inquiry page and we will check.

For most self-employed people earning under about 350,000 dollars, a solo 401k beats a SEP IRA, and the reason is the employee deferral. A SEP IRA only allows the employer profit-sharing contribution, capped at 25 percent of compensation. A solo 401k gives you that same 25 percent employer piece plus a flat 24,500 dollar employee deferral on top. At moderate income levels that extra deferral is the difference between a decent contribution and a large one.

Run the numbers and the gap is obvious. Take an independent contractor netting 100,000 dollars. Inside a SEP IRA the contribution is roughly 18,600 dollars, because the 25 percent is applied to the reduced self-employment base described on the IRS self-employed retirement contribution page. Inside a solo 401k that same person adds the 24,500 dollar employee deferral first, then the employer profit-sharing piece on top, reaching roughly 43,100 dollars. Same income, more than double the tax-deferred savings, simply because the solo 401k has two contribution channels and the SEP IRA has one.

The solo 401k carries other features a SEP IRA lacks. You can take a participant loan from a solo 401k of up to 50,000 dollars or half the balance, whichever is less. A SEP IRA flatly prohibits loans. Most solo 401k plans also accept Roth employee deferrals, so you can split contributions between pre-tax and after-tax buckets, while a SEP IRA is traditional pre-tax only. And the higher catch-up contributions for the 50-plus and 60-to-63 age bands flow through the solo 401k, not the SEP. For an owner who values access to their money or wants Roth treatment, those features alone settle the question.

Where does the SEP IRA win. Simplicity. A SEP IRA has no Form 5500-EZ filing at any asset level, while a solo 401k picks up that annual return once it crosses 250,000 dollars, a rule you can confirm on the About Form 5500-EZ page. A SEP IRA can also be opened and funded all the way up to your extended filing deadline with almost no paperwork, which makes it a useful catch-up vehicle if you missed setting up a solo 401k before December 31. For a high earner who only wants the employer percentage anyway, the simpler SEP IRA can be the practical pick.

The income level matters too. Once your earnings climb high enough that the employer 25 percent alone fills the 72,000 dollar Section 415(c) limit, the employee deferral advantage of the solo 401k disappears, because you have already hit the combined cap from the employer side. That crossover sits around 290,000 to 360,000 dollars of compensation depending on your entity type. Below it, the solo 401k wins clearly. Above it, the two plans land in roughly the same place and the SEP IRA wins on paperwork. Knowing where you fall on that curve is the heart of the decision.

Administration cost is the last factor on the scale. A solo 401k and a SEP IRA both cost nothing to open at most major brokerages, so the ongoing expense difference is small until the solo 401k crosses 250,000 dollars and picks up the annual Form 5500-EZ. Even then the return is short and the filing is free to submit. For a self-employed person who wants the largest possible deduction at a moderate income, the modest extra paperwork of the solo 401k is a price worth paying. For someone who values zero filings above all and earns enough that the employer side alone fills the cap, the SEP IRA stays attractive.

The mistake we see every year is a self-employed client running a SEP IRA out of habit and leaving 20,000 dollars of deferral room on the floor. They set up the SEP because it was easy, never revisited it, and missed years of larger solo 401k contributions. The fix is simple but the lost deferrals from prior years cannot be recovered. One edge case worth knowing closes this out. You generally cannot run a SEP IRA and a solo 401k for the same business in the same year without coordinating the combined 415(c) limit, so do not assume you can stack both to double up. If you want us to compare a solo 401k against a SEP IRA using your actual numbers and entity type, our tax strategy consulting team builds that side-by-side, and our entity formation and structuring team makes sure the business is set up to support it. Start at the new client inquiry page.

To set up a solo 401k you need self-employment income, an EIN for the business, a written plan document, and the plan established by your tax filing deadline. The setup is more involved than a SEP IRA but far lighter than a full employer 401k, and once it is running a solo 401k takes very little annual maintenance until you cross the 250,000 dollar filing threshold.

Walk through the steps. First, confirm you qualify. You need earned income from self-employment and no full-time non-spouse employees, the one-participant test the IRS describes on its one-participant 401k page. Second, get an EIN if your business does not already have one, since the solo 401k is sponsored by the business, not by you personally. Third, adopt a written plan document, either a prototype from a brokerage or a custom document if you want features like Roth deferrals or participant loans. Fourth, open the funding accounts and make your elections before the relevant deadlines.

Deadlines are where the solo 401k rules changed in your favor. Under the old law the plan had to exist by December 31 to count for that year. The SECURE Act loosened this. A solo 401k is now treated as established for a tax year if you adopt it by your business tax return deadline, including extensions, for that year. So a sole proprietor can open a solo 401k as late as the following October and still make employer profit-sharing contributions for the prior year. The one catch is the employee deferral. To make an employee deferral for a given year you generally must have elected it by December 31 of that year, which is the part the new flexibility does not fully cover.

A worked example ties it together. You had a strong 2026 as a freelance designer and want to shelter income, but you never opened a plan. In early 2027, before your extended filing deadline, you adopt a solo 401k retroactive to 2026. You can fund the employer profit-sharing contribution for 2026, perhaps 18,000 dollars on your income. What you cannot do is add the 24,500 dollar employee deferral for 2026, because you did not elect it by the end of 2026. The lesson is to open the solo 401k early so both channels stay available.

Spousal participation is a step many owners miss. If your spouse earns income from the same business, the spouse can join the same solo 401k and make a full employee deferral plus receive an employer contribution, effectively doubling the household contribution room. A married couple running one business can move well past 90,000 dollars into the plan in a single year when both participate. The spouse needs legitimate compensation from the business for this to hold, so the wages or self-employment income have to be real and documented, not a paper allocation made after the fact.

Choosing a provider matters more than people expect. A brokerage prototype plan is free and easy but often blocks the features that make a solo 401k powerful, namely Roth deferrals, participant loans, and the ability to roll in outside accounts. A custom plan document costs a few hundred dollars up front but unlocks all of it. If you ever expect to borrow from the plan or hold alternative assets, pay for the document that allows it from day one rather than amending later. We help solo 401k clients pick the document tier that fits how they actually plan to use the account.

The mistake we see every year is owners confusing the employer deadline with the employee deadline and assuming the whole solo 401k can be funded after year-end. It cannot. Only the employer side enjoys the extended window. The other frequent error is forgetting that an S corporation must run actual payroll before any solo 401k contribution is valid, because the deferral comes out of W-2 wages that have to be processed during the year. One edge case on entity choice closes this out. If you are deciding between a sole proprietorship, an LLC, and an S corporation, the structure changes how your solo 401k contribution is calculated and how much payroll tax you pay along the way. Our entity formation and structuring team sets up the business correctly, our corporate returns team handles the S corporation filing, and we coordinate the plan around both. To get your solo 401k opened before the next deadline, start at our new client inquiry page.