Form 1120 — U.S. Corporation Income Tax Return: Complete Practitioner Guide to C-Corporation Tax Planning, the 21% Flat Rate, and Avoiding the Accumulated Earnings Tax in 2026

The optimal salary for a C-corporation owner depends on the interplay between the corporate tax rate (21%), the individual income tax rate on the salary, and the payroll tax cost. Unlike S-corporations (where the IRS requires reasonable compensation to prevent under-reporting of employment taxes), C-corporations have a different optimization problem: the owner wants to pay enough salary to (1) avoid the accumulated earnings tax by reducing corporate taxable income, (2) maximize qualified plan contributions (which are based on W-2 compensation), and (3) potentially qualify for the QBI deduction if the owner has other pass-through income. However, paying too much salary increases payroll taxes (Social Security and Medicare) and moves income from the 21% corporate rate to the higher individual rate. The general framework is: pay enough salary to cover the owner’s personal living expenses and maximize retirement plan contributions, then retain the remaining earnings in the corporation for business needs. The retained earnings can be invested in business assets, used to fund expansion, or accumulated for a future QSBS-eligible exit. Practitioners should model the total tax cost at various salary levels to find the optimal point for each client.

The Corporate Alternative Minimum Tax (CAMT) was enacted by the Inflation Reduction Act of 2022 and imposes a 15% minimum tax on the “adjusted financial statement income” (AFSI) of corporations with average annual AFSI exceeding $1 billion. AFSI is based on the corporation’s financial statement income (book income) rather than taxable income, which means corporations that have significant book income but low taxable income (due to accelerated depreciation, NOL carryforwards, or other tax preferences) may be subject to the CAMT. The CAMT is calculated as 15% of AFSI minus the corporation’s regular tax liability. If the CAMT exceeds the regular tax, the corporation pays the CAMT. The CAMT does not apply to S-corporations, regulated investment companies (RICs), or real estate investment trusts (REITs). For most small and mid-size C-corporations with AFSI well below $1 billion, the CAMT is not relevant. However, practitioners advising large corporations or corporations that are subsidiaries of large corporate groups should analyze the CAMT exposure annually.

Converting a C-corporation to an S-corporation can be beneficial when the business owner wants to eliminate double taxation on future earnings, but the conversion comes with significant tax traps that practitioners must address. The most important trap is the built-in gains (BIG) tax under IRC §1374, which imposes a 21% corporate-level tax on gains from the disposition of assets that were held by the C-corporation at the time of the S-election, if those assets are sold within 5 years of the election. The BIG tax applies to the lesser of the recognized gain on the sale or the built-in gain at the time of the S-election. For a C-corporation with significant appreciated assets (real estate, equipment, goodwill), the BIG tax can be a substantial cost of conversion. The second major trap is the accumulated earnings and profits (E&P) from the C-corporation years. If the S-corporation makes distributions that exceed its accumulated adjustments account (AAA), those distributions are treated as dividends from the C-corporation E&P and taxed at the qualified dividend rate. Practitioners should model the BIG tax exposure, the E&P distribution risk, and the ongoing tax savings from S-corporation status before recommending a C-to-S conversion.