Most CPAs spend the bulk of their careers in the pass-through world. S corporations, partnerships, sole proprietorships. The tax logic is familiar: income flows through to the owner, one level of tax, done. Then a C corp question surfaces, and the assumptions stop working.
The two entity types share a corporate structure but operate under fundamentally different tax rules. Here’s where the distinctions matter most.
One Level vs. Two
S corporations are pass-through entities. Income, deductions, and credits flow to shareholders via Schedule K-1 and are taxed at individual rates. The entity itself pays no federal income tax.
C corporations pay tax at the entity level: a flat 21% on taxable income under IRC Section 11(b). When the corporation distributes earnings as dividends, shareholders pay again at qualified dividend rates. That double taxation is the defining structural difference, and it drives every other comparison between the two (IRS Form 1120 instructions).
Compensation Runs in Opposite Directions
With S corps, practitioners typically minimize officer salaries to reduce payroll tax exposure. Distributions above reasonable compensation avoid FICA.
C corps flip that logic. Officer salaries are deductible at the entity level, directly reducing the 21% corporate tax. The incentive runs toward higher compensation, not lower. But the IRS scrutinizes C corp salaries that exceed what’s reasonable for the services performed, and excess compensation loses its deductibility. Getting the balance wrong creates exposure on both sides.
Losses Work Differently
S corp losses flow through to shareholders, subject to basis limitations, at-risk rules, passive activity rules, and the excess business loss limitation. Shareholders deduct losses on their individual returns if they have sufficient basis and meet the other hurdles.
C corp losses stay at the entity level. Net operating losses carry forward indefinitely but can only offset 80% of taxable income in any given year. Capital losses face a tighter restriction: they offset capital gains only, with a three-year carryback and five-year carryforward. There’s no mechanism for C corp losses to flow through to shareholders.
The QBI Deduction Doesn’t Apply
Pass-through owners can deduct up to 20% of qualified business income under Section 199A. That deduction effectively lowers the top rate on eligible pass-through income from 37% to 29.6%. C corporations don’t qualify. The flat 21% rate is the rate, with no QBI adjustment.
For practitioners who primarily work with pass-throughs, it’s easy to overvalue the C corp’s lower headline rate without factoring in the second layer of tax on distributions. The combined effective rate on distributed C corp earnings (21% at entity plus qualified dividend rates at shareholder) often exceeds the effective pass-through rate after QBI.
Items With No Pass-Through Equivalent
Several C corp provisions have no analog in the S corp world. The dividends received deduction (50%, 65%, or 100% depending on ownership percentage) reduces tax on dividends received from other domestic corporations. The accumulated earnings tax (20% on earnings retained beyond the reasonable needs of the business) penalizes corporations that hold profits to avoid shareholder-level tax. Personal holding company rules impose a 20% tax on undistributed personal holding company income when passive income dominates a closely held corporation’s earnings.
None of these exist in the pass-through context. CPAs encountering them for the first time in a C corp engagement are dealing with rules that have no familiar reference point.
When the Choice Matters
Entity selection isn’t permanent. S elections can be revoked. C corporations can elect S status (subject to eligibility rules). But conversion triggers its own tax consequences, and the decision rests on factors that shift over time: owner count, ownership structure, planned distributions, and whether the business intends to retain earnings or pay them out. Understanding both rulesets is the baseline for evaluating either one.