SEP IRA vs. Solo 401(k)

If you’re self-employed, retirement planning can feel like building the plane while you’re flying it. You are earning the income, paying the taxes, covering health insurance, and somehow you’re also supposed to become your own “employer benefits department.”

The good news: you have access to two of the most powerful retirement accounts out there, the SEP IRA and the Solo 401(k). Both can let you put away far more than a typical IRA, and both can lower your tax bill when used correctly.

This guide will walk you through the differences in plain English so you can pick the account that fits how you actually work and earn.

A self-employed freelancer working at a kitchen table with a laptop, a notebook, and a cup of coffee.

The quick takeaway

If you want the shortest, most practical summary, here it is.

Choose a SEP IRA if you want simplicity, minimal paperwork, and you like the idea of deciding your contribution after you know your profit. Roth SEP contributions are allowed under current law, but availability is still limited at many providers.
Choose a Solo 401(k) if you want the highest possible contribution at lower income levels, want the option of Roth contributions, or want features like employee deferrals (and potentially loans, depending on your plan document and provider).

Who these plans are for

SEP IRA

A SEP IRA is designed for small businesses, including one-person businesses. Contributions are made by the “employer,” which is you.

A SEP IRA can also work if you have employees, as long as you’re comfortable with the rule that employees who qualify generally must receive the same contribution percentage you give yourself.

Solo 401(k)

A Solo 401(k) is a 401(k) for a business with no employees other than the owner and, if applicable, the owner’s spouse.

If you hire eligible employees (not counting your spouse), a Solo 401(k) usually stops being a “solo” plan and you may need to convert to a traditional 401(k) setup with more testing and administration.

Contribution limits

This is usually the deciding factor. The confusing part is that the contribution formulas are different.

Two guardrails to keep in mind as you read:

There is an overall annual cap on total additions to a 401(k) style plan (employee plus employer), and it changes by year.
Your business type matters. Sole proprietors and partners calculate “compensation” differently than an S corp owner paying W-2 wages.

SEP IRA contributions

With a SEP IRA, contributions are employer contributions only. The amount is based on a percentage of compensation.

General rule of thumb: up to 25% of compensation. For many sole proprietors and partners, the effective maximum often works out closer to about 20% of net self-employment earnings (after the self-employment tax adjustment).
Practical feel: SEP contributions scale with profit. If you have a modest-profit year, the maximum you can contribute may feel smaller than a Solo 401(k).

Solo 401(k) contributions

A Solo 401(k) lets you contribute in two roles:

Employee deferral: you, as the employee, can contribute up to the annual employee limit. If you are 50 or older, you can typically add catch-up contributions (subject to the IRS rules for the year).
Employer contribution: you, as the employer, can also contribute up to 25% of compensation (similar concept to a SEP). For many sole proprietors and partners, the effective maximum often lands closer to about 20% of net self-employment earnings due to how the formula works.

Why this matters: the employee deferral is a big advantage at lower income levels. Even if your business profit is not huge, you may still be able to defer a meaningful amount as the “employee,” then add an employer contribution on top, as long as you stay under the annual total cap.

A simplified example (not tax advice): If your net self-employment income is $60,000, a SEP IRA maximum contribution might feel like “around 20% of that number.” With a Solo 401(k), you could potentially make an employee deferral first, then add an employer contribution, which often produces a higher total at that income level. The exact maximum depends on your entity type and your final numbers.

A small business owner sitting at a desk reviewing tax documents with a calculator and pen.

Roth option

This is one of the clearest differences, with one important update under current law:

SEP IRA: historically, SEP contributions were pre-tax only. Under the SECURE 2.0 Act, Roth SEP contributions are now allowed, but many major brokerages and plan providers have been slow to offer this in practice. So the option may exist on paper, but not at the place you want to open the account.
Solo 401(k): many providers allow Roth employee deferrals. That means you pay taxes now, but qualified withdrawals in retirement can be tax-free.

If you like the idea of “tax-free income later,” a Solo 401(k) is often the more flexible tool today simply because Roth options are more widely available.

One kitchen-table way to think about it: Traditional is “I want a tax break today.” Roth is “I want fewer tax surprises later.” Many people end up using a mix over time.

Backdoor Roth and the pro-rata rule

If you are a higher earner who uses, or might use, the backdoor Roth IRA strategy, pay close attention.

Money in a SEP IRA counts as pre-tax IRA money for the pro-rata rule. In plain English, having a SEP IRA balance can make backdoor Roth conversions partly taxable.

A Solo 401(k) generally does not count in that same IRA pro-rata calculation, which can make it easier to keep the backdoor Roth clean.

This is not a reason everyone should avoid SEP IRAs. It is just a common “surprise” I’ve seen trip people up after they’ve already built a large SEP balance.

Deadlines

Deadlines matter because self-employed income is often lumpy. You might not know what you can afford to contribute until you’ve closed the books.

SEP IRA deadlines

SEP IRAs are famous for flexibility. In most cases, you can open and fund a SEP IRA up to your tax filing deadline, including extensions. That makes the SEP a favorite for people who want to decide contributions after the year ends.

Solo 401(k) deadlines

Solo 401(k)s used to come with a big “do it by year-end” warning for employee deferrals. Under SECURE 2.0, there is now a helpful exception that is especially relevant to many sole proprietors and single-member LLCs: you may be able to start a new Solo 401(k) and make first-year employee deferrals up to your tax filing deadline (without extensions).

Two real-world cautions:

Provider policies vary. Some custodians still have internal cutoffs or processing timelines that effectively move your deadline earlier.
Setup takes time. Even when the law gives you more runway, paperwork and onboarding can be the bottleneck.

Employer contributions may have more flexibility, often tracking the business tax filing deadline. Because rules and operational steps vary by provider, the safest move is to start the Solo 401(k) setup earlier in the year you want it to count.

Paperwork

I’m a planner, and even I like simple when simple is available.

SEP IRA administration

Typically easy to open
Minimal ongoing paperwork
No annual filing like Form 5500 for most SEP setups

Solo 401(k) administration

More setup decisions (traditional vs Roth, incoming rollovers, whether the plan document allows loans, and more)
More recordkeeping
If your plan assets exceed certain IRS thresholds, you may need to file an annual Form 5500-EZ

This is not meant to scare you away from a Solo 401(k). Plenty of self-employed folks run them smoothly. It’s just important to be honest about your tolerance for extra steps.

If you have employees

This is where many business owners accidentally choose the wrong tool for the business they want to build.

SEP IRA with employees

With a SEP IRA, if employees meet eligibility rules, you generally must contribute for them if you contribute for yourself, and at the same percentage of pay. That can get expensive fast if you have a team.

Solo 401(k) with employees

A Solo 401(k) is meant for a business with no employees other than you and potentially your spouse. If you bring on eligible employees, you may need to shift into a different plan type.

Friendly neighbor advice: If hiring is even a “maybe” in the next year or two, pause and think. The best retirement plan is the one you can actually keep using as your business grows.

How taxes work

Both SEP IRAs and traditional Solo 401(k) contributions can reduce your taxable income, but they do it through different mechanics depending on your business type (sole proprietor, LLC, S corp, etc.).

SEP IRA: employer contribution is generally deductible as a business expense (details depend on entity type). If Roth SEP contributions are available through your provider, those Roth amounts are generally after-tax.
Solo 401(k): employee deferrals reduce taxable income if traditional, and employer contributions are generally deductible by the business. Roth employee deferrals are after-tax.

In retirement, distributions from pre-tax accounts are generally taxable as ordinary income. Roth Solo 401(k) distributions can be tax-free if qualified.

RMDs

Most people do not pick a plan based on required minimum distributions, but it is worth knowing what you are signing up for.

SEP IRA and traditional Solo 401(k): generally subject to required minimum distributions (RMDs) under current rules.
Roth Solo 401(k): RMD rules have changed in recent years. Depending on the current rules for the year you retire, you may be able to avoid lifetime RMDs on Roth 401(k) money, which is one reason the Roth option gets so much attention.

Real-world scenarios

Scenario 1: Set it and forget it

If you want maximum simplicity and you like the idea of making one contribution after you see your final profit, a SEP IRA is often a clean fit.

Scenario 2: Backdoor Roth user

If you’re using backdoor Roth contributions and want to avoid the SEP IRA pro-rata trap, a Solo 401(k) often plays nicer.

Scenario 3: Catch-up mode

If you are behind and want to contribute as much as possible, especially at moderate income levels, the Solo 401(k) often allows higher contributions because of the employee deferral bucket.

Scenario 4: Hiring soon

Neither a SEP IRA nor a Solo 401(k) may be the long-term final answer if you’re building a team. This is where it can help to talk through SIMPLE IRAs, safe harbor 401(k)s, or other designs. But if you are truly solo today, you can still start with the right short-term plan and upgrade later.

A small business owner meeting with two employees at a conference table, discussing documents.

At a glance

Best for simplicity: SEP IRA
Best for maximizing contributions at lower profit levels: Solo 401(k)
Roth option: Solo 401(k) (often available); Roth SEP may be possible under SECURE 2.0 but depends on provider
Backdoor Roth friendliness: Solo 401(k) (often better)
Easy late-year funding: SEP IRA (often more flexible); Solo 401(k) first-year deferrals may be possible by the filing deadline without extensions for many self-employed filers, but provider policies vary
Works if you have employees: SEP IRA can, but contributions for employees can be costly; Solo 401(k) is generally not for employee-based businesses
Paperwork: SEP IRA is usually lighter; Solo 401(k) is more involved

How to choose

Confirm your employee situation. If you have eligible employees (other than a spouse), a Solo 401(k) is likely not an option.
Estimate your net profit for the year. You don’t need perfection, just a ballpark.
Decide if Roth is important to you. If yes, lean toward Solo 401(k), or confirm whether your SEP provider actually supports Roth SEP contributions.
Think about backdoor Roth IRA contributions. If you do them now or might later, be cautious about building a large SEP IRA balance.
Be honest about admin tolerance. If paperwork makes you shut down, SEP IRA is often the better “do something and keep doing it” solution.
Pick a provider and open the plan early. Especially for Solo 401(k), earlier is smoother even when the law gives you more time.

Common questions

Can I have a SEP IRA and a Solo 401(k) at the same time?

Sometimes, but it can get complicated fast because plan limits can interact depending on how the business is structured and whether the plans cover the same income. If you’re considering both, it’s worth getting personalized guidance so you don’t accidentally over-contribute.

Can I contribute if my income is irregular?

Yes. Both plans can work with irregular income. A SEP IRA is often easier if you want to decide your contribution after you know your final profit. A Solo 401(k) can still work well, but you’ll want to stay organized about timing, especially for employee deferrals.

What if I have a side gig and a W-2 job?

You may be able to contribute to a Solo 401(k) for your self-employment income while also contributing to a workplace 401(k). The key catch is that your employee deferral limit is shared across plans, while employer contributions are tied to each employer. This is a very common spot for mistakes, so track contributions carefully.

Do I need an LLC to open either plan?

Not necessarily. Many sole proprietors open a SEP IRA or Solo 401(k) using their Social Security number or an EIN. Provider requirements vary, but you do not need a fancy business structure to start saving.

I already have a SEP IRA. Can I move it into a Solo 401(k)?

Sometimes. Many Solo 401(k) plans can accept rollovers from SEP IRAs, but not all providers allow incoming rollovers, and the rollover has to be handled correctly. If your goal is to clear out pre-tax IRA balances to make backdoor Roth IRA conversions cleaner, this is a conversation worth having before you move money, because timing and plan rules matter.

The bottom line

If you want the easiest path to higher retirement contributions, a SEP IRA is hard to beat for simplicity.

If you want more flexibility, especially Roth contributions and potentially higher savings at moderate income levels, a Solo 401(k) is often the better long-term engine.

Whichever you choose, the biggest win is getting a plan in place and funding it consistently. Retirement confidence is built one boring, repeatable contribution at a time. And that is good news, because boring is something we can all do.