For a fractional CFO or CMO billing several steady retainers, the S corp question usually comes down to one trade: less self-employment tax in exchange for more administration. Whether that trade wins depends almost entirely on how much profit your practice clears.
What an S corp changes for a multi-retainer practice
As a sole proprietor or single-member LLC, every dollar of net profit owes 15.3% self-employment tax up to the Social Security wage base, with Medicare continuing beyond it. With an S corp election, you become an employee of your own company. You pay yourself a reasonable salary, and that salary owes normal payroll taxes. Profit above the salary comes out as shareholder distributions, and distributions do not owe self-employment or payroll tax at all.
Say your practice nets a healthy six figures across your retainers. If a defensible market salary for the work you actually do is meaningfully lower than your total profit, the payroll-tax savings on the gap can be substantial, year after year. The IRS requires the salary to be reasonable for your role and hours, and fractional executives have an unusually easy time documenting that, because your own client contracts show what companies pay for a portion of an executive's time.
Filing Form 2553 and running real payroll
The election itself is Form 2553. An existing LLC can elect S corp treatment for a tax year generally by filing within the first two months and fifteen days of that year, and late-election relief exists if you have a reasonable cause. But the form is the easy part. Once elected, you must run actual payroll for yourself: quarterly payroll filings, W-2 issuance in January, state unemployment registration, and a separate S corp tax return, Form 1120-S, with a Schedule K-1 flowing to your personal return. That means payroll software or a bookkeeper, and a real filing calendar.
There are also side effects worth weighing. Retirement contributions through a solo 401k are calculated off your W-2 salary rather than total profit, so an aggressively low salary can shrink how much you can put away. And some states levy their own taxes or fees on S corps that eat into the savings.
When fractional income is too small or too lumpy for the switch
If you are in your first year, still ramping to two or three retainers, or your income swings hard between engagements, the fixed costs of payroll and an extra return can outweigh the savings. The election makes the most sense when profit is high, recurring, and predictable, exactly the pattern of a mature fractional practice with annual or evergreen retainers. Many fractional executives start as an LLC taxed as a sole proprietorship, watch profit for a year, then file Form 2553 once the math clearly clears the administrative cost. Run the numbers on your actual profit before you commit, not on a generic rule of thumb.