Can Locum Physicians Use a Solo 401(k) to Save More for Retirement?

You took the locum tenens route for autonomy. You wanted to escape hospital politics, dictate your own schedule, and maximize your hourly rate. But there's a catch nobody warns you about in medical school. You traded the W-2 safety net for a 1099 tax bomb.

Understanding the specific financial hurdles of this path is essential, and resources like the guide on common locum tax mistakes physicians make in their first year can help you avoid costly errors.

No employer 401(k) match. No automatic payroll deductions. Just you, a massive tax bill, and a lingering feeling that your retirement savings are dangerously behind. I see this constantly. High-earning locum physicians pulling in $300,000+ a year but saving like they're still PGY-2s.

The fix isn't picking up more weekend shifts. It's restructuring your tax-advantaged accounts. While a SEP IRA is the default "easy" button for independent contractors, it's essentially training wheels. The Solo 401(k) is the heavy artillery you need to actually fix your retirement trajectory.

The following information is for educational purposes only. It is not financial advice, not legal advice, and not tax advice. Because individual circumstances and tax laws change, figures vary, and you should always consult a qualified professional before making financial decisions.

For physicians weighing their options, the debate between locum vs employed a practical framework for your first job choice offers a structured way to compare career paths.

What Is a Solo 401(k) and Do You Qualify?

A Solo 401(k) is a retirement plan designed specifically for self-employed individuals. The magic of this account is that it allows you to play both sides of the table. You contribute as the employee, and you contribute as the employer.

Do you qualify? If you are a locum physician operating as a sole proprietor, a single-member LLC, or a PLLC, and you have zero full-time W-2 employees, you are eligible. Your spouse doesn't count against you, provided they are the only other person working in the business. If you have a W-2 admin or a part-time nurse practitioner on your payroll, you are disqualified.

Most locums default to a SEP IRA because it requires zero paperwork to set up. That's a mistake. A SEP IRA only allows employer profit-sharing contributions. A Solo 401(k) allows both employee elective deferrals and employer profit-sharing in one account. This structural difference is what allows you to max out your contributions at a much lower income threshold than a SEP IRA.

Maximizing Contributions: The Numbers That Matter

Let's look at the hard numbers for 2024. This is where the Solo 401(k) crushes the competition.

As the "employee," you can defer up to $23,000 of your compensation ($30,500 if you are age 50 or older). As the "employer," you can make a profit-sharing contribution of up to 25% of your net self-employment income.

The total combined contribution cap is $69,000 ($76,500 with the catch-up contribution).

Look at the chart above. You might notice the SEP IRA technically shares the same $69,000 maximum limit. Here is the catch: because a SEP IRA lacks the $23,000 employee deferral bucket, you have to earn significantly more net income to actually reach that $69,000 cap. With a Solo 401(k), the $23,000 employee deferral gets you a third of the way there immediately, regardless of your profit-sharing math.

The deadline to fund your contributions is your tax filing deadline, including extensions (typically October 15 of the following year). But there is a hard, unforgiving deadline for setting up the plan itself. The plan document must be adopted by December 31 of the tax year you want to contribute for. Miss that date, and you are locked out for the year.

Roth vs. Traditional: A Tax Strategy for Locums

Locum income is notoriously variable. You might work 40 hours a week in Q1 and take the entire summer off. This volatility is actually a massive strategic advantage if you use Roth and Traditional buckets correctly.

Traditional contributions are made with pre-tax dollars. You get the deduction today, and pay taxes when you withdraw in retirement. Roth contributions are made with after-tax dollars. You get no deduction today, but the money grows tax-free and is withdrawn tax-free.

Here is the protocol:

• High-income years: Max out your Traditional Solo 401(k) deferrals. Shave $23,000 off your taxable income when you are sitting in the highest marginal tax brackets.
• Low-income years: Switch your deferrals to Roth. If you took three months off to travel and your income dropped, lock in those lower tax rates by paying the tax now and letting the money grow tax-free forever.

There is another critical reason to use a Solo 401(k). If you are a high-earning locum, you are likely using the Backdoor Roth IRA strategy. If you have pre-tax money sitting in a Traditional IRA or Rollover IRA, the IRS pro-rata rule will trigger a massive tax bill when you execute your backdoor conversion.

The fix? Roll your pre-tax IRA money into your Solo 401(k). The pro-rata rule ignores money held in employer-sponsored plans like a 401(k). The Solo 401(k) acts as a tax-shielded vault, keeping your Backdoor Roth IRA strategy clean and penalty-free.

How to Set Up Your Solo 401(k) in 5 Steps

Stop overthinking this. It is a straightforward administrative process. Follow this exact sequence.

Step 1: Choose a provider. Standard brokerages like Vanguard, Fidelity, and Charles Schwab offer free, off-the-shelf Solo 401(k) plans. They are great for basic Traditional and Roth deferrals. If you want advanced features like the Mega Backdoor Roth (more on this below) or checkbook control for real estate investing, you need a custom plan provider like My Solo 401k or Ubiquity.

Step 2: Adopt the plan document. This is just digital paperwork. You are formally establishing the plan. This must be completed by December 31. Do it in November to avoid the holiday rush and potential processing delays.

Step 3: Open the account and link your bank. Once the plan is adopted, open the actual brokerage account. Link your business checking account. Do not link your personal checking account; keep your business and personal finances strictly separated.

Step 4: Calculate and fund the account. Run the math for your employee deferral and employer profit-sharing. Transfer the funds. You have until your tax filing deadline (including extensions) to actually move the cash.

Step 5: File Form 5500-EZ annually. Once your Solo 401(k) assets cross the $250,000 threshold, you must file Form 5500-EZ with the IRS every year by July 31. It's a simple one-page form, but ignoring it is a disaster.

Advanced Strategy: The Mega Backdoor Roth for Locums

If you are maxing out your $23,000 employee deferral and your 25% employer profit-sharing, but you still have cash left over and want to shelter more from taxes, the Mega Backdoor Roth is your next move.

Standard Solo 401(k) plans only have two buckets: Pre-Tax (Traditional) and Roth. But the IRS actually allows a third bucket: After-Tax contributions. This is entirely different from Roth.

Here is the play:

1. Calculate the gap. The total 2024 limit is $69,000. If your employee deferral is $23,000 and your employer profit-sharing is $15,000, you have $31,000 of unused space left.
2. Make an After-Tax contribution. You contribute that remaining $31,000 into the After-Tax bucket.
3. Execute an in-plan Roth rollover. You immediately convert that After-Tax money into the Roth bucket.

Because you convert it almost instantly, there are virtually no taxable earnings to worry about. You have just successfully shoved an extra $31,000 into a tax-free Roth environment. Over a 20-year career, the tax-free compounding on those extra contributions will be staggering.

Warning: Standard brokerages like Fidelity or Schwab usually do not support the in-plan Roth rollover feature for their off-the-shelf Solo 401(k)s. You will need to pay a third-party administrator for a custom plan document that explicitly includes this feature. It costs a few hundred dollars a year, but the tax savings pay for it immediately.

Common Pitfalls and How to Avoid Them

I have watched brilliant physicians lose tens of thousands of dollars to entirely preventable administrative errors. Avoid these traps.

• Missing the December 31 adoption deadline. You cannot retroactively create a Solo 401(k) for a prior year. If it is January 5th and you want to lower your tax bill for the previous year, a Solo 401(k) is off the table. You are stuck with a SEP IRA. Set a calendar reminder for November 1st to adopt the plan.
• Botching the net earnings math. Your 25% employer profit-sharing is based on net self-employment income. You must subtract half of your self-employment tax and your employee deferral contributions before calculating the 25%. Use a dedicated Solo 401(k) calculator or hire a CPA. Guessing will trigger an IRS audit.
• Ignoring Form 5500-EZ. The IRS penalty for failing to file this form when your assets exceed $250,000 is $250 per day, up to $150,000 per missed return. I am not exaggerating. If you cross the $250k mark, put the July 31st deadline in your calendar with three separate alerts.
• Accidentally hiring a W-2 employee. If your locum business grows and you hire a W-2 medical assistant or biller, your Solo 401(k) is instantly invalid. You must amend it to a standard 401(k) and offer it to them, or terminate the plan. Stick to 1099 subcontractors if you need help.

Your Action Plan: Start Saving More Today

You have the blueprint. Now execute it. Here is your immediate checklist to secure your financial future.

1. Verify your income structure. Confirm your locum income is strictly 1099 self-employment income. If you are receiving a W-2 from a staffing agency, you do not qualify for a Solo 401(k) for that specific income.
2. Adopt the plan immediately. Do not wait until December 30th. Log into your chosen brokerage or third-party administrator today and sign the plan adoption agreement.
3. Run the contribution math. Calculate your exact employee deferral and employer profit-sharing limits based on your projected net income for the year.
4. Automate the funding. Set up automatic transfers from your business checking to your Solo 401(k) to ensure you hit your targets before the tax filing deadline.
5. Evaluate the Mega Backdoor. If you are maxing out the standard limits and want more tax-free growth, upgrade to a custom plan document that allows after-tax contributions and in-plan Roth rollovers.
6. Hire a professional. Tax codes are dense. Pay a CPA who specializes in physician finances to review your Solo 401(k) calculations and optimize your Roth vs. Traditional split.

You became a locum to take control of your career. Take control of your retirement with the exact same aggression. Set up the Solo 401(k), max it out, and get back to work.