Corporate income tax is a federal tax on the profits of C-corporations, charged at a flat rate of 21 percent of taxable income. Because a corporation is treated as its own legal person under federal law, it files its own return and pays tax on what it earns, separate from the personal taxes owed by its owners or employees. The rate has been 21 percent since 2018, when the Tax Cuts and Jobs Act replaced the older graduated brackets that topped out at 35 percent, and unlike many individual provisions from that law, the corporate rate is permanent.1Office of the Law Revision Counsel. 26 U.S. Code 11 – Tax Imposed
Which Businesses Actually Owe It
C-corporations are the businesses that owe federal corporate income tax. A C-corporation holds its own assets, takes on its own debts, and files its own return, which produces what people call double taxation: the corporation pays tax on its profits, and shareholders then pay personal income tax on any dividends distributed from those already-taxed profits.
Most other business forms sidestep this second layer entirely. S-corporations, sole proprietorships, partnerships, and most limited liability companies are pass-through entities, meaning their profits land directly on the owners’ personal returns with no separate corporate-level tax. An LLC only owes corporate income tax if it specifically elects to be taxed as a C-corporation. So if you own an LLC or an S-corp and haven’t made that election, the 21 percent rate isn’t yours to worry about.
How the 21 Percent Rate Works
Every C-corporation pays the same 21 percent on its taxable income, whether the company earned ten thousand dollars or ten million. To find the federal bill, multiply taxable income by 0.21. A corporation with $500,000 in taxable income owes $105,000 before any credits are applied.
The key phrase is taxable income. That figure is not revenue, and it is usually much smaller than revenue.
How Taxable Income Is Calculated
A corporation starts with gross receipts — everything brought in from sales, services, and other sources — and subtracts allowable deductions. The usual ones include cost of goods sold, employee wages, rent, utilities, interest, advertising, depreciation, and most other taxes paid during the year. What’s left is taxable income.
Records matter here. The IRS can audit any return, and a corporation that overstates deductions faces penalties and interest. On the other side, a corporation that misses legitimate deductions ends up paying more than it owes.
Net Operating Losses
When deductions exceed revenue in a given year, the corporation has a net operating loss. That loss doesn’t disappear. It can be carried forward to offset taxable income in future years, and under current rules the carryforward never expires.2Office of the Law Revision Counsel. 26 U.S. Code 172 – Net Operating Loss Deduction There is a limit on how much can be used in any single year: the deduction is capped at 80 percent of that year’s taxable income, so a profitable year still generates some tax even when large losses are on the books.
Tax Credits
After the tax is calculated, credits reduce it dollar-for-dollar. That’s different from a deduction, which only reduces the income the tax is calculated on. Credits available to corporations include the research credit for qualified R&D spending, the Work Opportunity Tax Credit for hiring workers from certain targeted groups, clean vehicle credits, and the employer-provided childcare credit.3Internal Revenue Service. Credits and Deductions for Businesses Which credits a corporation qualifies for can change year to year, so it’s worth checking annually.
State Corporate Income Tax Sits on Top
The 21 percent federal rate is only part of the bill. Most states impose their own corporate income tax, with rates that run roughly from 2 percent to 11.5 percent, and many states use graduated brackets rather than a flat rate. A few states impose no corporate income tax at all, and a few others tax gross receipts instead of profits. A corporation’s total burden depends on where it operates and where it is incorporated.
Quarterly Payments and the Annual Return
Corporations don’t wait until filing day to hand over the whole year’s tax. If a corporation expects to owe $500 or more in federal tax for the year, it must make quarterly estimated payments.4Office of the Law Revision Counsel. 26 U.S. Code 6655 – Failure by Corporation to Pay Estimated Income Tax For a calendar-year corporation, the installments are due:
- April 15
- June 15
- September 15
- December 15
When a due date falls on a weekend or federal holiday, it shifts to the next business day.5Internal Revenue Service. Publication 509, Tax Calendars Underpaying a quarter triggers a penalty on the shortfall, though the penalty doesn’t apply if total annual tax comes in under $500.
The annual return is Form 1120. Every domestic C-corporation must file it, even in a year with no taxable income, unless a specific tax code provision exempts the corporation.6Internal Revenue Service. Instructions for Form 1120 The return is due by the 15th day of the fourth month after the tax year ends, which is April 15 for the calendar-year corporations that make up most filers.
A corporation that needs more time can file Form 7004 before the original deadline for an automatic six-month extension.7Internal Revenue Service. Instructions for Form 7004 The extension buys time to file, not time to pay. Any tax owed is still due on the original date, and interest and penalties start running from there on anything unpaid.
Penalties for Filing or Paying Late
Two separate penalties can hit at once. Filing late costs 5 percent of the unpaid tax for each month or partial month the return is overdue, capped at 25 percent.8Office of the Law Revision Counsel. 26 USC 6651 – Failure to File Tax Return or to Pay Tax Paying late — filing on time but not sending the full balance — costs an additional 0.5 percent of the unpaid tax for each month or partial month, also capped at 25 percent.9Internal Revenue Service. Failure to Pay Penalty A corporation that both files late and pays late owes both.
Interest also accrues on any unpaid balance from the original due date. The IRS can waive penalties if the corporation shows reasonable cause for the delay, but interest is not waivable.6Internal Revenue Service. Instructions for Form 1120
A Separate Rule for the Largest Corporations
One boundary worth naming: the Corporate Alternative Minimum Tax applies a 15 percent floor to the financial-statement income of very large corporations, and if that 15 percent figure exceeds what the corporation would owe under the regular 21 percent rules, the corporation pays the higher amount.10Office of the Law Revision Counsel. 26 U.S. Code 55 – Alternative Minimum Tax Imposed It only reaches “applicable corporations,” those with average annual adjusted financial statement income above $1 billion over the prior three years.11Internal Revenue Service. New Simplified Method for Determining Status for Corporate Alternative Minimum Tax For the overwhelming majority of C-corporations, the CAMT never comes into play, and the 21 percent rate is the whole federal story.