SEP IRA vs. Solo 401(k) for Side Hustlers

If you have a side business or freelance income, both accounts let you shelter far more than an IRA alone. They differ in how much, how much administration, and what else they interfere with. The right answer changes as the business grows, and switching later is possible but tedious, so it is worth getting right.

This assumes you have no employees other than a spouse. Hiring anyone changes the analysis completely — a solo 401(k) stops being available and a SEP starts requiring proportional contributions for staff.

How they differ

The contribution gap, and why it exists

This is the reason most people choose the solo 401(k), and the mechanism is worth understanding rather than memorising.

A SEP contribution is capped at a percentage of your compensation. For an unincorporated business that works out to roughly 20% of net self-employment income after the deduction for half your self-employment tax. Earn a modest side income and 20% of it is a modest number.

A solo 401(k) adds a second channel. You are both the employer and the employee, so you can make an employee deferral — a flat dollar amount, not a percentage — and then add the same employer profit-share a SEP would have allowed. The flat deferral is what changes the arithmetic at lower incomes.

Concretely: on a side income of $30,000, a SEP allows roughly $6,000. A solo 401(k) allows that same employer contribution plus a deferral that can take the total to a large multiple of it, subject to the annual limits and to not exceeding your earned income. The gap narrows as income rises and both approach the same overall cap, which is why the solo 401(k) advantage is largest for exactly the people this article is written for.

The dollar limits are indexed annually and the self-employed calculation is circular — the contribution affects the income the contribution is based on. Use the IRS worksheet linked below or your accountant’s software rather than a rule of thumb. Our contribution guide covers how this stacks with a workplace plan.

The pro-rata trap

Here is the interaction almost nobody is warned about at the point of opening a SEP.

The backdoor Roth IRA only works cleanly if you hold no pre-tax money in any traditional, SEP or SIMPLE IRA at year end. A SEP IRA is an IRA. Fund one for a few years and you have built a pre-tax balance that makes every future backdoor Roth conversion mostly taxable.

A solo 401(k) is not an IRA and sits entirely outside that calculation. It can even absorb an existing pre-tax IRA balance by rollover, which is the standard way of clearing the pot.

So if your income is high enough that you are using a backdoor Roth — which is plausible for anyone with a salary plus a side business — the solo 401(k) is not merely the bigger account. It is the one that does not break something else you are doing.

Where the SEP genuinely wins

The deadline. A solo 401(k) generally has to exist by 31 December of the year you want to contribute for. A SEP can be opened and funded up to your filing deadline including extensions — so in April, looking at an unexpectedly large tax bill for last year, the SEP is often the only option still open. This single fact accounts for a lot of SEPs.

No filing, ever. A solo 401(k) requires Form 5500-EZ once plan assets pass $250,000. It is a short form, but it is an annual obligation with a penalty for missing it. A SEP has none at any size.

Genuinely less to understand. No plan document, no deferral election, no distinction between two contribution types. For a small and irregular side income where the contribution would be modest either way, simplicity has real value.

If you might hire someone. A SEP continues to work with employees, at the cost of contributing for them. A solo 401(k) has to be terminated or converted.

Deciding

Choose the solo 401(k) if your side income is modest but you want to shelter most of it, you are over 50 and want catch-up contributions, you want Roth treatment on the deferral, or you use a backdoor Roth. That covers most people reading this.

Choose the SEP if it is already April and you need a deduction for last year, you expect to hire employees, or the contribution will be small enough that the administrative difference outweighs the contribution difference.

You can have both, though the overall annual limit applies across plans for the same business, so it rarely helps. What does work is a solo 401(k) alongside a workplace 401(k) at a day job — but the employee deferral limit is per person, not per plan, so only the employer side has room. Get that wrong and you have an excess contribution to unwind.

Opening one

Both are available at the large brokerages with no account fee. A solo 401(k) requires adopting a plan document, which the provider supplies, and getting an EIN if you do not have one — the same EIN process as for a business bank account, and free.

Check two things before choosing a provider: whether the solo 401(k) permits Roth deferrals, and whether it accepts incoming rollovers. Several large providers’ free plans historically did neither, and both matter — the first for tax diversification, the second for clearing a pre-tax IRA balance out of the pro-rata pot.

Frequently asked questions

Can I contribute more to a solo 401(k) than a SEP IRA?

At the same self-employment income, almost always yes. A solo 401(k) has two contribution channels — a flat-dollar employee deferral plus an employer profit-share — while a SEP has only the employer percentage. The advantage is largest at lower incomes and narrows as both approach the overall annual cap.

Does a SEP IRA affect the backdoor Roth?

Yes, and this catches people out. SEP balances count in the pro-rata calculation that governs Roth conversions, so a pre-tax SEP balance makes most of any conversion taxable. Solo 401(k) balances are excluded from that calculation.

What is the deadline to open each one?

A SEP can be opened and funded up to your tax filing deadline including extensions. A solo 401(k) generally has to be established by 31 December of the contribution year, with limited relief for sole proprietors under SECURE 2.0.

Do I need an EIN?

For a solo 401(k), yes. For a SEP as a sole proprietor you can often use your Social Security number, though an EIN is free and takes about ten minutes.

What happens if I hire an employee?

A solo 401(k) stops qualifying and has to be converted to a conventional plan or terminated. A SEP keeps working, but you must make proportional contributions for every eligible employee.

Can I have a solo 401(k) and a workplace 401(k)?

Yes, but the employee deferral limit applies per person across all plans, not per plan. If you are already deferring the maximum at a day job, only the employer profit-share side of your solo 401(k) has room.