S-Corp vs. Sole Proprietor for Locum Physicians: The Tax and Retirement Trade-offs

If you're a locum physician, 1099 moonlighter, or independent contractor picking up shifts between W-2 jobs, at some point a friend, a CPA, or a podcast has almost certainly delivered the same recommendation: become an S-corp. Sometimes that's right. Sometimes it's expensive advice. And sometimes it's a wash, a lot of extra paperwork for a tax savings that quietly gets eaten by payroll fees, state franchise taxes, and a smaller retirement contribution than expected.

This article walks through how the S-corp versus sole proprietor decision actually works for locum and 1099 physicians, where the tax savings come from, where they quietly disappear, and how each structure interacts with the retirement-plan options physicians care most about. It's educational only. The right answer for any individual physician depends on income, state, existing W-2 job, spouse's situation, and long-term plans, so this is a decision to work through with your CPA and your financial planner together. If you'd like our help coordinating that conversation alongside the rest of your plan, our tax strategies for doctors overview is a good place to start, and you can also see how this fits into broader wealth management for doctors.

Why This Question Comes Up for Locum Physicians

Most physicians start their careers as W-2 employees. Your hospital or group withholds federal tax, FICA, and state tax, drops money into a 401(k) or 403(b), and the entity question never comes up. Then a locum assignment, a telemedicine side gig, a medical directorship, or an expert-witness engagement lands in your lap, and suddenly you're receiving 1099 income with nothing withheld. That's self-employment income, and how you structure the business that receives it has real tax and retirement consequences.

Two structures dominate the conversation for locum and 1099 physicians: operating as a sole proprietor (or a single-member LLC, which is taxed as a sole proprietor by default) and operating as an S-corporation (or an LLC that has elected S-corp tax treatment). The IRS S-corporation page lays out the federal rules for S-corp elections, and the IRS Publication 334 (Tax Guide for Small Business) covers how sole proprietor income flows through Schedule C.

The trade-offs aren't abstract. They show up in your self-employment tax bill, the size of your solo 401(k) contribution, your eligibility for the Section 199A Qualified Business Income deduction, and how much time and money you spend every year on compliance. Physicians with a full-time W-2 job plus light 1099 income often land in a very different place than physicians whose entire income is locum work.

How the Sole Proprietor Structure Works

As a sole proprietor, you and the business are the same legal entity for tax purposes. Your 1099 income lands on Schedule C of your personal return. After business expenses (malpractice tail, CME, travel, licensing, home office if applicable, solo 401(k) contributions through a separate line, and so on), the net profit flows to Schedule 1 and gets added to the rest of your taxable income.

That net profit is also subject to self-employment tax, essentially the employer and employee halves of Social Security and Medicare rolled into one. The IRS self-employment tax overview (Topic 554) explains the mechanics: 12.4% Social Security tax up to the annual wage base, and 2.9% Medicare tax on all net earnings, plus the 0.9% Additional Medicare Tax for high earners. A sole proprietor pays the full amount on Schedule SE.

Here's the nuance many locum physicians miss: if you also have a W-2 job paying you above the Social Security wage base, you've already maxed out the 12.4% Social Security portion through your W-2 withholding. Your 1099 income in that same year isn't hit again with Social Security tax, only the 2.9% Medicare plus the 0.9% Additional Medicare Tax. For a high-earning W-2 physician with moderate side 1099 income, that dramatically shrinks the "SE tax savings" an S-corp is often sold on, because there's less SE tax to save in the first place.

How the S-Corporation Structure Works

An S-corporation is a pass-through entity for federal tax purposes, but it's a separate legal entity that you own as a shareholder and work for as an employee. When your practice is an S-corp, the money coming in gets split into two buckets: a reasonable W-2 salary you pay yourself as the physician-employee, and the remaining profit, which passes through as a K-1 distribution.

The W-2 salary portion is subject to payroll taxes (Social Security, Medicare, and any applicable state and federal unemployment taxes), withheld and remitted through a payroll service. The K-1 distribution portion is subject to federal and state income tax but is not subject to self-employment or payroll tax. That spread is where the "S-corp tax savings" story comes from.

The catch is the word "reasonable." The IRS requires S-corp owner-employees to pay themselves a reasonable compensation for the services actually performed before taking distributions. A physician practicing medicine through an S-corp can't simply declare a $40,000 salary and take the rest as a K-1; that's a textbook IRS audit flag, and adjustments can include back payroll taxes, penalties, and interest. Reasonable compensation for a physician generally reflects what a comparable physician would be paid for the same work in the same market, informed by MGMA or similar compensation surveys your CPA can reference.

The Tax Comparison: Where the Savings Actually Come From

The core tax question is how much of your net 1099 earnings escape the 15.3% (or, if your Social Security wage base is already met through a W-2 job, roughly 2.9% Medicare plus the 0.9% Additional Medicare Tax) self-employment layer. As a sole proprietor, the entire net profit is exposed. As an S-corp, only the W-2 salary portion is exposed; the remaining K-1 distribution escapes that layer. The bigger the spread between a defensible reasonable salary and total net business income, the bigger the potential savings. For a locum physician doing modest part-time 1099 work on top of a full-time W-2 job, the spread is often small and the administrative cost is often large. For a full-time locum physician with no W-2 job, the math frequently tips the other way.

Side-by-Side Comparison

Here's how the two structures typically stack up across the factors physicians care about most. These are generalizations; your CPA will model your specific numbers, and state rules (California's 1.5% S-corp franchise tax, for example) can change the picture materially.

The QBI Deduction: The Curveball for High-Earning Physicians

The Section 199A Qualified Business Income deduction, created by the 2017 Tax Cuts and Jobs Act, allows many pass-through business owners to deduct up to 20% of qualified business income. It's the single biggest reason many CPAs recommend S-corp elections for small-business owners, but medicine is a special case. The IRS classifies health services, including physicians, as a "Specified Service Trade or Business" (SSTB) under Section 199A, as detailed in the IRS Instructions for Form 8995-A. That means the QBI deduction phases out above certain taxable income thresholds and disappears entirely above the upper threshold.

For most attending-level locum physicians, the QBI deduction is fully phased out regardless of whether income flows through a sole proprietorship or an S-corp. The TCJA's individual provisions have also been the subject of ongoing legislative changes, so the QBI landscape is genuinely in flux. Choosing an S-corp structure specifically to chase QBI is rarely the right reason for a physician well above the SSTB phase-out; the conversation becomes more nuanced for physicians in residency, fellowship, or early-career years where taxable income may temporarily sit below the threshold, or for dual-earner households with specific income-bunching considerations. This is another layer where a CPA review of the current-year numbers matters.

Where Retirement Plan Math Gets Interesting

The retirement-plan interaction is the piece physicians tend to underestimate. The solo 401(k) is the workhorse retirement vehicle for 1099 physicians, and the IRS rules for contribution calculation differ depending on entity structure. The IRS one-participant 401(k) plan page walks through the contribution rules, and the formulas are detailed further in IRS Publication 560.

As a sole proprietor, you can contribute the full employee elective deferral (up to the annual 402(g) limit) plus an employer profit-sharing contribution up to roughly 20% of net self-employment earnings, all the way up to the overall 415(c) limit, plus a catch-up if you're 50+. Net SE earnings means net Schedule C income minus half of SE tax minus the retirement contribution itself, a slightly circular formula that your CPA or payroll software handles automatically.

As an S-corp, your solo 401(k) contributions are calculated off your W-2 salary. The employer contribution can go up to 25% of W-2 wages, a higher percentage than the sole prop formula, but off a smaller base, because only your reasonable salary counts, not the K-1 distribution. If your reasonable salary is set low to maximize SE-tax savings, you're also capping your retirement contribution. Many physicians are surprised to discover that electing S-corp status reduced their maximum solo 401(k) contribution compared to filing as a sole proprietor on the same gross revenue.

This is a place where tax-savings enthusiasm and retirement-savings goals can quietly collide. Physicians who treat their 1099 income primarily as a retirement-savings engine, maxing the solo 401(k), layering on a spouse's contribution if they're legitimately on payroll, and using after-tax mega-backdoor-style contributions where the plan document allows, often find the sole proprietor route produces a larger tax-advantaged contribution than an S-corp with a modest reasonable salary. Our retirement planning for doctors framework walks through how this typically gets modeled alongside W-2 retirement accounts.

The Hidden Administrative Cost

On paper, S-corp SE-tax savings can look attractive. In practice, physicians often underestimate the administrative friction that comes with maintaining the structure. A running S-corp typically involves a payroll provider, a separate Form 1120-S tax return (often meaningfully higher in CPA fees compared to a personal return alone), quarterly payroll tax filings, annual W-2 and W-3 issuance, corporate formalities, and in many states a franchise tax or minimum annual entity fee. The Small Business Administration's guide to choosing a business structure summarizes the entity-choice landscape.

California is the classic example of administrative cost eating into S-corp tax benefits: the state imposes a 1.5% S-corporation franchise tax on net income with an $800 annual minimum, which means the federal SE-tax savings have to overcome a state tax that sole proprietors simply don't pay. Other states vary; some are friendly to pass-through entities, and some are not. A structure decision that pencils out in Texas might not pencil out in California, New York, or Massachusetts without adjustment.

Why Career Stage and W-2 Status Matter So Much

The typical S-corp savings pitch assumes 100% of your professional income flows through the entity. Most physicians don't look like that. The most common physician-1099 patterns:

• Full-time W-2 plus light 1099 moonlighting. Your W-2 paycheck usually already exceeds the Social Security wage base, so SE tax on your 1099 income is effectively just Medicare (2.9%) plus the 0.9% Additional Medicare Tax for high earners. The federal SE-tax spread between sole prop and S-corp shrinks in many cases, and administrative costs can consume most of it.
• Full-time locum (no W-2 job). All Social Security, Medicare, and federal income tax runs through your 1099 income. This is where S-corp math most often becomes genuinely compelling, and also where reasonable-compensation documentation matters most, because there's no W-2 job benchmarking your pay.
• Transitional year (residency-to-attending, or between jobs). Partial-year 1099 income often doesn't justify the administrative setup of an S-corp, even if full-year projections would. Sole proprietor with a solo 401(k) can often do the heavy lifting through the transition.
• Dual-physician households. If both spouses have 1099 income, structure decisions can differ between the two and need to coordinate with a shared retirement strategy.

A Quick Note on Liability

Some physicians consider an LLC or S-corp specifically for liability protection. For medicine, malpractice liability attaches to the physician personally regardless of entity structure; an LLC or S-corp doesn't shield you from malpractice claims for your own clinical work. Entity structures can offer some separation from non-clinical business liabilities (contract disputes, vendor claims, premises issues), but the primary physician-protection tool is adequate malpractice and umbrella coverage. Entity selection should be driven primarily by tax and retirement planning, not by misunderstood liability claims.

How Physicians Typically Work Through This Decision

In planning conversations with locum and 1099 physicians, the discussion usually moves through a few layers in order:

• What is the realistic annual 1099 net income, and is it stable year over year?
• Is there a concurrent W-2 job that's already exhausting the Social Security wage base?
• What state does the physician live and practice in, and what are the state-level entity costs?
• What's the primary goal for this income: current-year tax reduction, retirement-account maximization, or current cash flow?
• Is the physician prepared for the ongoing administrative lift of payroll, separate tax returns, and reasonable-compensation documentation?
• Are there spouse-employment, family-employment, or cash-balance-plan opportunities that change the retirement-plan math?

Only after those layers are clear does the S-corp-versus-sole-proprietor question have a real answer. The answer also evolves. A physician often starts as a sole proprietor while 1099 income is modest or unstable, and later elects S-corp treatment once the income base is large and stable enough to absorb the administrative cost and still leave meaningful savings. The reverse also happens; some physicians unwind an S-corp when their practice pattern changes and the structure no longer earns its keep.

Coordinating With Your CPA and Your Planner

Entity decisions sit squarely at the intersection of tax preparation and financial planning, which is why they tend to fall through the cracks. Your CPA is focused on this year's tax return. Your financial planner is focused on long-term retirement, cash flow, and family goals. The best decisions get made when both seats at the table are looking at the same projection, modeling the net after-tax, after-retirement-contribution outcome of each structure over several years, not just one tax season.

At Physician Family, we don't run a formal entity-structure or reasonable-compensation service, but we do keep an eye on these pieces inside the broader plan. If something looks off with how an entity, owner comp, or retirement plan design fits the rest of the picture, we'll raise it with you and your CPA so it can be addressed. We don't prepare tax returns; we work alongside the CPA who does. If you're navigating a new locum arrangement, a transition out of a W-2 job, or an existing S-corp that you're not sure is earning its keep, our financial planning for new physicians framework and broader planning engagements both address this kind of structural question.

The Bottom Line

There is no universal "physicians should be S-corps" answer, and there is no universal "it's never worth it" answer either. The right structure depends on how much 1099 income flows through, whether a W-2 job is already soaking up Social Security tax, what state you practice in, how much you want to contribute to retirement plans, and how much administrative complexity you're willing to carry. For some locum physicians the math is genuinely compelling; for others, it's a wash or worse once fees and lost retirement capacity are counted.

The most useful thing a physician can do before making, keeping, or unwinding an S-corp election is run the numbers both ways against specific income, state, and retirement-contribution goals, with a CPA and a planner in the same conversation.

If you'd like help thinking this through as part of a broader plan, not as a one-off tax tactic, you can get started with Physician Family and we'll walk through your locum income, retirement goals, and entity structure together with your CPA.