The federal corporate tax rate for C-Corporations is a flat 21% as established by the Tax Cuts and Jobs Act of 2017. This rate applies to all taxable income regardless of amount. Pass-through entities (S-Corps, LLCs, Partnerships) do not pay corporate tax; instead, income passes through to owners who pay at individual rates.

For C-Corporations with a calendar year-end, Form 1120 is due April 15th. S-Corporations and Partnerships must file by March 15th. Extensions are available: 6 months for C-Corps (October 15th) and 6 months for S-Corps/Partnerships (September 15th). An extension to file is not an extension to pay.

C-Corporation: Pays corporate tax at 21% on profits. When dividends are distributed, shareholders pay tax again (double taxation).
S-Corporation: No corporate-level tax. Profits and losses pass through to shareholders who report them on personal returns. S-Corps must meet specific requirements including having no more than 100 shareholders who are U.S. citizens/residents.

Common deductions include: employee salaries and benefits, rent and utilities, business insurance, depreciation of assets (Section 179, Bonus Depreciation), interest on business loans, advertising and marketing costs, professional fees (legal, accounting), travel and vehicle expenses, research and development costs, and charitable contributions (up to 25% of taxable income).

Corporations expecting to owe $500 or more in taxes must make quarterly estimated tax payments. Use Form 1120-W to calculate amounts. Payments are due April 15, June 15, September 15, and December 15 for calendar-year corporations. Pay via EFTPS (Electronic Federal Tax Payment System).

Key business tax credits include: Research & Development Credit (R&D Credit), Work Opportunity Tax Credit (WOTC), Disabled Access Credit, Small Employer Health Insurance Credit, Energy Investment Tax Credit, Employee Retention Credit (COVID-related, now expired for most), and General Business Credit (Form 3800).

Most states impose corporate income tax in addition to federal tax. Rates range from 2.5% (North Carolina) to 11.5% (New Jersey). Some states have no corporate income tax: Nevada, Ohio, South Dakota, Texas, Washington, and Wyoming. State taxes are typically deductible for federal purposes after the TCJA $10,000 SALT cap for pass-throughs.

The Section 199A QBI deduction allows eligible pass-through entities (S-Corps, LLCs, Partnerships, Sole Proprietorships) to deduct up to 20% of qualified business income. Phase-outs apply for specified service trades or businesses (SSTB) and are based on taxable income thresholds. This deduction does not apply to C-Corporations.

Maintain records for at least 7 years including: income statements, expense receipts, bank statements, payroll records, asset purchase documentation, depreciation schedules, prior tax returns, contracts, loan documents, corporate minutes, and stock transfer records. Digital copies are acceptable if properly maintained.

Late filing penalty: 5% of unpaid taxes per month (max 25%). Late payment penalty: 0.5% per month (max 25%). Interest accrues on unpaid taxes at the federal short-term rate plus 3%. S-Corps and Partnerships face a $220 per shareholder/partner per month penalty for late filing. File for an extension before the deadline to avoid failure-to-file penalties (you must still pay estimated tax due).