One of the most frequently tested areas on the REG section of the CPA exam is the distinction between S corporations and C corporations. The examiners love this topic because it requires candidates to understand two parallel but fundamentally different tax regimes. Getting these differences straight is not just important for the exam, it is essential knowledge for any practicing CPA who advises business clients on entity selection and tax planning.
This guide breaks down every major difference between S corps and C corps that you need to know, organized in a way that mirrors how the CPA exam tests these concepts. By the end, you will have a clear mental framework for tackling any S corp vs. C corp question that appears on your exam.
Understanding the C Corporation
A C corporation is the default corporate classification under the Internal Revenue Code. When a business incorporates at the state level, it is automatically treated as a C corporation for federal tax purposes unless it makes an affirmative election to be taxed as an S corporation. The defining characteristic of a C corporation is double taxation. The corporation itself pays income tax on its taxable income at the corporate rate, and then shareholders pay tax again when they receive dividends or sell their stock at a gain.
C corporations file Form 1120 and compute taxable income much like an individual, with some important differences. They can deduct the dividends received deduction (DRD), which is a concept unique to corporate taxation and frequently tested on the exam. The DRD percentages are 50 percent for ownership under 20 percent, 65 percent for ownership between 20 and 80 percent, and 100 percent for ownership of 80 percent or more within an affiliated group.
C corporations can also carry forward net operating losses, are subject to accumulated earnings tax if they retain earnings beyond reasonable business needs, and face the personal holding company tax if they meet certain passive income thresholds.
Understanding the S Corporation
An S corporation is a pass-through entity, meaning that income, deductions, gains, losses, and credits flow through to the shareholders' individual tax returns. The corporation itself generally does not pay federal income tax. Instead, shareholders report their distributive share of the corporation's income or loss on their personal returns, regardless of whether the corporation actually distributes any cash to them.
S corporations file Form 1120-S, which is an information return. Each shareholder receives a Schedule K-1 showing their share of income, deductions, and credits. This pass-through structure eliminates the double taxation problem that affects C corporations, making the S election attractive for many small and mid-sized businesses.
S Corporation Election Requirements
To qualify for S corporation status, the entity must meet all of the following requirements. The CPA exam tests these eligibility criteria frequently, so commit them to memory:
- Domestic corporation: The entity must be organized in the United States.
- Eligible shareholders only: Shareholders must be individuals, estates, certain trusts (grantor trusts, qualified subchapter S trusts, and electing small business trusts), and certain tax-exempt organizations. Partnerships, corporations, and nonresident aliens cannot be shareholders.
- One class of stock: The corporation can have only one class of stock, though differences in voting rights are permitted. Economic rights (distributions and liquidation proceeds) must be identical for all shares.
- Maximum 100 shareholders: The corporation cannot have more than 100 shareholders. Members of a family (up to six generations) are treated as a single shareholder for this purpose.
- Not an ineligible corporation: Certain entities such as financial institutions using the reserve method, insurance companies, and DISCs cannot elect S status.
The election is made on Form 2553 and must be filed by the 15th day of the third month of the tax year to be effective for that year. If filed after that date, the election takes effect for the following tax year. All shareholders must consent to the election.
Taxation Differences: The Core Distinction
The fundamental difference between S corps and C corps on the exam comes down to how income is taxed:
C Corporation Taxation
- The corporation pays tax at the flat 21 percent corporate rate on its taxable income.
- Dividends paid to shareholders are taxed again at the shareholder level, typically at qualified dividend rates (0, 15, or 20 percent depending on the shareholder's bracket).
- The corporation can use the dividends received deduction to reduce tax on dividends received from other corporations.
- Net operating losses can be carried forward indefinitely but are limited to 80 percent of taxable income in the carryforward year.
S Corporation Taxation
- Income and loss pass through to shareholders based on their stock ownership percentage on a per-share, per-day basis.
- Shareholders report income on their individual returns regardless of distributions.
- Losses are deductible only to the extent of the shareholder's basis in stock and debt.
- The S corporation itself does not pay federal income tax in most cases, though it may owe tax on built-in gains or excess passive investment income in certain situations.
Shareholder Basis Rules
Basis calculation is one of the most heavily tested areas for both entity types, but the rules differ significantly:
For S corporation shareholders, basis starts with the initial investment and is adjusted annually. Basis increases for the shareholder's pro rata share of income and additional capital contributions. Basis decreases for distributions, the shareholder's pro rata share of losses, and nondeductible expenses. Importantly, S corporation shareholders get basis for direct loans they make to the corporation but do not get basis for corporate-level debt, even if they personally guarantee it.
For C corporation shareholders, basis is generally the amount paid for the stock (or the adjusted basis of property contributed under Section 351). Shareholder basis in a C corporation does not fluctuate with corporate income or loss since the corporation is a separate taxpaying entity.
Distributions
The treatment of distributions is another area where the exam draws sharp distinctions:
C corporation distributions are treated as dividends to the extent of the corporation's earnings and profits (E&P). The ordering is: first from current E&P, then from accumulated E&P. Distributions in excess of E&P are treated as a return of capital reducing the shareholder's stock basis, and any excess beyond basis is capital gain.
S corporation distributions follow a different ordering. For an S corporation that has never been a C corporation (and thus has no accumulated E&P), distributions are tax-free to the extent of the shareholder's stock basis and capital gain thereafter. If the S corporation has accumulated E&P from prior C corporation years, the distribution ordering uses the Accumulated Adjustments Account (AAA) first, then E&P, then Other Adjustments Account (OAA), and finally return of basis and capital gain.
Built-In Gains Tax
When a C corporation converts to S corporation status, it may be subject to the built-in gains (BIG) tax. This tax applies if the S corporation sells an asset within five years of the conversion and the asset had a built-in gain at the time of the S election (meaning the fair market value exceeded the adjusted basis at the conversion date). The BIG tax is imposed at the highest corporate rate (21 percent) on the recognized built-in gain. This prevents corporations from converting to S status simply to avoid corporate-level tax on appreciated assets.
The exam may test your ability to calculate the built-in gains tax or to identify when it applies. Remember: the BIG tax only applies to assets held at the time of conversion, only for gains that existed at the conversion date, and only during the five-year recognition period.
Quick Comparison Chart
Here is a side-by-side summary of the major differences:
- Tax level: C corp pays entity-level tax; S corp passes through to shareholders.
- Tax rate: C corp flat 21 percent; S corp income taxed at individual rates.
- Number of shareholders: C corp unlimited; S corp maximum 100.
- Shareholder types: C corp any entity or individual; S corp limited to eligible individuals, estates, and certain trusts.
- Stock classes: C corp multiple classes allowed; S corp only one class (voting differences okay).
- Loss deductions: C corp losses stay at entity level; S corp losses pass through to shareholders (limited by basis).
- Distributions: C corp dividends from E&P taxed as ordinary or qualified dividends; S corp distributions generally tax-free to extent of basis.
- Fringe benefits: C corp can deduct fringe benefits for shareholder-employees; S corp cannot deduct certain fringe benefits for shareholders owning more than 2 percent.
Common Exam Traps
Watch out for these frequently tested tricky areas:
- S corp shareholder basis from debt: Unlike partnerships, S corp shareholders do NOT get basis from entity-level debt unless they personally lend money to the corporation.
- Termination of S election: The S election terminates if any eligibility requirement is violated, if passive investment income exceeds 25 percent of gross receipts for three consecutive years while the corp has accumulated E&P, or if shareholders holding more than 50 percent of shares revoke the election.
- Reasonable compensation: S corp shareholder-employees must receive reasonable compensation before taking distributions to avoid self-employment tax issues.
- AAA ordering: Know the distribution ordering rules when an S corporation has accumulated E&P from prior C corporation years.
Study Strategy for S Corp vs. C Corp Questions
When you see a question comparing these two entity types, first identify which entity you are dealing with. Then apply the correct set of rules. Many wrong answers on the exam result from candidates accidentally applying S corporation rules to a C corporation question or vice versa. Create flashcards that clearly separate the two regimes, and practice calculation problems for both.
Think CPA's REG course walks through dozens of practice scenarios comparing S corps and C corps, with detailed explanations for each answer choice. If you want to feel truly confident on these questions, structured practice with immediate feedback is the fastest path to mastery.
By locking down the differences outlined in this guide and practicing calculation-heavy questions, you will be well prepared for whatever the CPA exam throws at you in this area.