One person, two contribution slots
The solo 401(k), which the IRS calls a one-participant 401(k), runs on a simple trick: when you are self-employed, you are both the employee and the employer, and the tax code lets you contribute in both roles.
As the employee, you can defer up to $24,500 of compensation for 2026, the same limit any 401(k) participant gets. As the employer, your business can add up to 25% of compensation on top. Combined, the 2026 cap is $72,000 under IRS limits, and the $8,000 catch-up for people 50 and older stacks above that, taking the total to $80,000.
Those are enormous numbers for an account a freelancer can open at a mainstream broker for free. The catch is in the name: one participant. The plan is for businesses with no employees, though a spouse earning income from the business can join, which doubles a household’s room.
Why it beats the SEP for most freelancers
The SEP IRA is the solo 401(k)‘s main rival, and at the top end they tie: both cap at $72,000 for 2026. The difference lives at ordinary income levels, and it is not subtle.
A SEP is percentage-only. Net $50,000 from self-employment and the SEP’s effective roughly-20% math lets you shelter around $10,000. The solo 401(k) starts with the flat $24,500 employee deferral, available from the first dollars of compensation, then adds the employer percentage on top. Same freelancer, same income, more than double the sheltered amount.
The solo 401(k) also offers things the SEP structurally cannot: the age-50 catch-up, because catch-ups attach to employee deferrals, and at most providers a Roth option for the deferral portion, letting you buy tax-free retirement income in your low-bracket years.
So the honest scorecard: solo 401(k) wins on contribution room at low and middle incomes, catch-ups, and Roth access. SEP wins on setup simplicity, zero ongoing filings, and the ability to open it as late as your tax filing deadline. High steady earners who will hit $72,000 either way can take the SEP’s simplicity. Most other self-employed savers are leaving room on the table if they do.
The paperwork, honestly stated
A solo 401(k) is a real 401(k) plan, and it carries real plan obligations, scaled down.
You adopt a plan document through your provider, generally before year-end for the year you want deferrals, so this is not a decision to leave for April. You track employee versus employer contributions separately, because different limits and deadlines apply to each. And once plan assets pass the IRS threshold, currently $250,000, you file a short Form 5500-EZ each year. The form takes minutes. Forgetting it is expensive, with penalties that accrue per day, so set the reminder the day you open the account.
None of this requires an administrator or a lawyer for a one-person plan. It requires a calendar and ten minutes of attention a year. Price that against the extra contribution room and it is usually a bargain.
Making it run
Open the plan at a major broker, adopt the document, and invest like every sensible retirement account invests: broad, cheap index funds or a target-date fund. The wrapper is exotic. The contents should be boring.
Compare providers on features, not just brand. Roth deferral options, loan provisions, and rollover acceptance all vary by plan document, and switching providers later means plan paperwork. Ten minutes of comparison up front saves an afternoon of forms in year three.
Then handle the self-employment problem the plan does not solve: income arrives in lumps, and retirement contributions should never be money you might need back. Early withdrawals from a 401(k) generally cost income tax plus a 10% additional tax, which makes the plan a terrible emergency fund.
So build the real one first. Keep your tax reserve plus three to six months of expenses in a high-yield savings account, where it earns interest and stays reachable on a day’s notice. Let income land there, let the buffer fill, and contribute to the solo 401(k) from the surplus, with the employee deferral funded through the year and the employer piece trued up when you know the year’s numbers. Buffer, then deferral, then employer top-up. That sequence captures the account’s huge ceiling without ever betting the rent on it.