Fundamentals of the U.S. Federal Business Income Tax

In previous article, Fundamentals of the U.S. Federal Individual Income Tax, we introduced the fundamental concepts of the U.S. federal individual income tax system. We learned how individuals calculate taxable income by determining gross income, subtracting allowable deductions, applying the appropriate tax rates, and reducing the resulting tax with available credits. Although the Internal Revenue Code contains thousands of detailed rules, nearly every individual tax return follows this same logical framework.

Business taxation builds upon many of these same principles. Businesses also earn income, incur expenses, and calculate taxable income. However, one important question distinguishes business taxation from individual taxation:

Who actually pays the tax?

For some businesses, the business itself pays income tax. For others, the business merely calculates its income, while the owners report that income on their personal tax returns. Understanding this distinction is the key to understanding the U.S. business tax system.

In this part, we will introduce the basic business entities, explain how each is taxed, discuss how businesses calculate taxable income, and provide an overview of the major tax responsibilities faced by business owners.

Why Business Taxes Matter

Every business, regardless of its size, has tax responsibilities. Whether you operate a small online business from your home or manage a large manufacturing company with hundreds of employees, taxes become part of your daily operations.

Business taxes affect many important decisions, including:

• Choosing a business entity

• Hiring employees

• Purchasing equipment

• Paying business owners

• Expanding operations

• Raising capital

• Selling the business

Good tax planning does not mean avoiding taxes. Instead, it means understanding the tax consequences of business decisions so that owners can make informed choices while complying with the law.

The Four Common Business Entities

The federal tax system generally recognizes four common forms of business taxation.

Sole Proprietorship

A sole proprietorship is the simplest form of business ownership. The business and its owner are treated as the same taxpayer for federal income tax purposes. The business does not pay income tax separately. Instead, the owner reports all business income and expenses on Schedule C of Form 1040.

Partnership

A partnership generally exists when two or more people own a business together. Partnerships file an informational tax return (Form 1065), but they generally do not pay federal income tax. Instead, profits and losses pass through to the partners, who report their respective shares on their individual tax returns.

S Corporation

An S corporation is a corporation, or an eligible LLC, that elects pass-through taxation under Subchapter S of the Internal Revenue Code. Like a partnership, it generally does not pay federal income tax at the entity level. Instead, its income, deductions, credits, and other tax items pass through to its shareholders, who report them on their individual income tax returns.

S corporations must meet specific eligibility requirements established by the IRS. Despite these limitations, the S corporation has become a cornerstone of the U.S. small-business tax system because it combines pass-through taxation, limited liability protection, and potential payroll tax advantages. We will examine the S corporation in greater detail later in this part.

C Corporation

Unlike the other entities discussed above, a C corporation is a separate taxpayer. It files its own corporate income tax return and pays corporate income tax on its taxable income. If after-tax profits are later distributed to shareholders as dividends, the shareholders generally pay tax again on those dividends. This is commonly referred to as double taxation.

Limited Liability Companies (LLCs)

One of the most common misconceptions is that an LLC is a separate type of federal tax entity.

It is not!

An LLC is a legal entity created under state law. For federal income tax purposes, an LLC may be taxed as a sole proprietorship, partnership, S corporation, or C corporation, depending on the number of owners and elections made with the IRS.

For this reason, forming an LLC does not automatically reduce taxes. Instead, it provides legal flexibility while allowing the business to choose among several tax classifications.

Pass-Through Taxation vs. Corporate Taxation

Perhaps the most important concept in business taxation is understanding the difference between pass-through taxation and corporate taxation.

Under pass-through taxation, the business calculates its income but generally does not pay federal income tax. Instead, the income passes directly to the owners, who report it on their own tax returns.

Under corporate taxation, the corporation itself pays income tax. If the corporation later distributes profits as dividends, shareholders may also pay tax on those distributions. This distinction explains why two businesses earning the same profit may owe different amounts of tax depending on how they are organized.

The Basic Business Tax Formula

Although business tax returns may appear complicated, the basic calculation is remarkably straightforward.

Business Tax Formula

Gross Business Income

− Ordinary and Necessary Business Expenses

= Net Business Income

± Other Tax Adjustments

= Taxable Business Income

↓

Reported by the Business or the Owner

↓

Federal Income Tax

Like the individual income tax formula introduced in Part 1, this framework forms the foundation of virtually every federal business income tax return.

Ordinary and Necessary Business Expenses

Businesses are generally allowed to deduct expenses that are both ordinary and necessary in carrying on a trade or business.

An ordinary expense is one that is common and accepted within a particular industry.

A necessary expense is one that is helpful and appropriate for operating the business.

Common deductible business expenses include:

• Employee wages

• Rent

• Utilities

• Office supplies

• Advertising and marketing

• Insurance

• Professional fees

• Business travel

• Vehicle expenses

• Interest on business loans

• Depreciation of business property

  
  

Not every business expenditure is immediately deductible. Certain long-term assets, such as buildings, machinery, and equipment, must generally be recovered over time through depreciation. We will examine depreciation in greater detail in a later part of this series.

Payroll Taxes

Many new business owners assume that business taxes consist only of income tax.

In reality, employers often have significant payroll tax responsibilities.

Businesses with employees are generally responsible for:

• Withholding federal income tax from employee wages

• Withholding Social Security and Medicare taxes

• Paying the employer's share of Social Security and Medicare taxes

• Paying federal unemployment tax (FUTA)

• Paying applicable state unemployment taxes

  
  

These taxes are separate from the business's income tax and are subject to their own filing requirements and payment deadlines.

Choosing the Right Business Entity

Choosing a business entity is one of the first decisions an entrepreneur makes, and it can affect taxes, liability protection, administrative responsibilities, and future growth.

The following table summarizes the four common business entities.

Business Entity	Federal Income Tax	Liability Protection	Typical Use
Sole Proprietorship	Owner pays tax	Generally No	Individual owners
Partnership	Partners pay tax	Varies	Multiple owners
S Corporation	Shareholders pay tax	Yes	Small and medium businesses
C Corporation	Corporation pays tax	Yes	Larger businesses and companies seeking outside investors

There is no single business entity that is best for every situation. The appropriate choice depends on many factors, including the nature of the business, the number of owners, financing needs, liability concerns, and long-term objectives.

S Corporations: The Most Popular Choice for Growing Small Businesses

Among all business entities in the United States, the S corporation has become one of the most popular choices for small and medium-sized businesses. Although sole proprietorships remain the simplest form of business organization, many businesses transition to S corporation taxation as they grow because it offers a combination of liability protection, pass-through taxation, and potential payroll tax advantages.

It is important to understand that an S corporation is not a different type of corporation under state law. Instead, it is a corporation—or, in many cases, an LLC—that has elected to be taxed under Subchapter S of the Internal Revenue Code. This election is made by filing IRS Form 2553 and meeting specific eligibility requirements established by the Internal Revenue Service.

Like partnerships and sole proprietorships, S corporations are generally pass-through entities. The corporation calculates its taxable income and files an informational tax return on Form 1120-S, but it generally does not pay federal income tax at the corporate level. Instead, the corporation issues Schedule K-1 to each shareholder, reporting that shareholder's share of the corporation's income, deductions, and credits. The shareholders then report these amounts on their individual income tax returns.

One of the primary reasons business owners elect S corporation status is the treatment of owner compensation.

Unlike a sole proprietor, who generally pays self-employment tax on all net business income (15.3% on your net business earnings. This is split into 12.4% for Social Security and 2.9% for Medicare), a shareholder who actively works for an S corporation is generally treated as an employee. The corporation must pay the shareholder a reasonable salary for services performed. That salary is subject to payroll taxes, including Social Security and Medicare taxes, just like the wages of any other employee.

After paying reasonable compensation, however, additional business profits may generally be distributed as shareholder distributions rather than wages. These distributions are generally not subject to self-employment tax or FICA taxes, although they remain subject to federal and, where applicable, state income taxes.

For many profitable small businesses, this distinction can produce meaningful payroll tax savings. Consider the following simplified example.

Example

Suppose ABC Consulting, Inc., an S corporation, earns $250,000 before paying its owner. The owner actively manages the business and pays herself a reasonable salary of $120,000. The remaining $130,000 is distributed as shareholder distributions.

The salary is subject to payroll taxes and is reported on Form W-2.

The shareholder distribution generally is not subject to Social Security and Medicare payroll taxes, although it is still included in taxable income through Schedule K-1.

This difference explains why many successful service businesses—including accounting firms, consulting firms, engineering firms, medical practices, law firms, and marketing agencies—often elect S corporation taxation once they become consistently profitable.

However, the payroll tax benefit is not unlimited. Some business owners mistakenly believe they can pay themselves an artificially low salary while receiving the remainder of the profits as distributions. The IRS actively scrutinizes this practice.

The Internal Revenue Code requires shareholder-employees to receive reasonable compensation based on the services they provide to the business. Factors considered in determining reasonable compensation include:

• The owner's education and experience

• Duties and responsibilities

• Time devoted to the business

• Comparable salaries for similar positions

• The business's size and profitability

• Compensation paid to non-owner employees

  
  

If the IRS determines that compensation is unreasonably low, it may reclassify distributions as wages, assess additional payroll taxes, and impose interest and penalties.

For this reason, S corporation status should be viewed as a legitimate business structure—not simply as a tax-saving strategy.

Despite these requirements, S corporations remain one of the most valuable tools available to growing small businesses. They combine pass-through taxation, limited liability, and potential payroll tax efficiencies while avoiding the double taxation generally associated with traditional C corporations.

Side-by-Side Comparison

Assumptions

• Business Earns $250,000 Before Paying the Owner

• Single owner

• Active in the business

• No other income

• No state income tax

• Ignore QBI deduction and certain tax credits to keep the comparison simple

• For the S corporation, the owner receives a reasonable salary of $120,000

• Remaining profit is distributed to the shareholder

Item	Sole Proprietorship	S Corporation	C Corporation
Business profit before owner pay	$250,000	$250,000	$250,000
Owner salary	N/A	($120,000)	($120,000)
Remaining business profit	N/A	$130,000	$130,000
Business pays corporate income tax	No	No	Yes
Income passes to owner	$250,000	$130,000 (via K-1) + $120,000 W-2	Salary only (dividends taxed separately if distributed)
Payroll/Social Security & Medicare taxes	Generally apply to substantially all business earnings through self-employment tax	Apply to the $120,000 salary only	Apply to the $120,000 salary only
Potential double taxation	No	No	Yes, if profits are distributed as dividends

In this simplified example:

• Sole Proprietorship: Approximately $250,000 of business earnings is generally subject to self-employment tax.

Step	Calculation	Amount
Net business income		$250,000.00
Net earnings subject to SE tax (92.35%)	$250,000 × 92.35%	$230,875.00
Social Security portion (12.4%)	First $184,500 × 12.4%	$22,878.00
Medicare portion (2.9%)	$230,875 × 2.9%	$6,695.38
Additional Medicare Tax (0.9%)*	$230,875 − $200,000 = $30,875 × 0.9%	$277.88
Total SE-related taxes		≈ 29,851.26

• S Corporation: Only the $120,000 reasonable salary is generally subject to Social Security and Medicare payroll taxes. The remaining $130,000 shareholder distribution is generally not subject to those payroll taxes (though it remains subject to income tax).

Tax	Employee	Employer	Total
Social Security (6.2%)	$7,440	$7,440	$14,880
Medicare (1.45%)	$1,740	$1,740	$3,480
Total FICA Payroll Tax	$9,180	$9,180	$18,360

• C Corporation: The $120,000 salary is subject to payroll taxes. If the remaining after-tax corporate earnings are later distributed as dividends, they may be taxed again at the shareholder level.

Based on the calculations above, it is clear that an S corporation offers significant tax advantages over a sole proprietorship, partnership, or C corporation in many situations. However, an S corporation is not a tax shelter. It does not eliminate income tax; rather, it provides a more tax-efficient structure by reducing certain employment taxes while still requiring shareholders to pay income tax on their share of the corporation's taxable income.

Instead, its principal advantage is that it may reduce employment taxes by distinguishing between:

• Compensation for services (salary, which is generally subject to payroll taxes), and

• Return on ownership (shareholder distributions, which generally are not subject to payroll taxes).

  
  

The IRS requires shareholder-employees to receive reasonable compensation before taking distributions. Paying an unreasonably low salary in an attempt to avoid payroll taxes can lead to additional tax assessments, interest, and penalties.

Conclusion

Although the U.S. business tax system appears more complex than the individual income tax system, it is built upon many of the same principles. Businesses calculate income, subtract allowable expenses, determine taxable income, and report that income to the Internal Revenue Service. The principal difference is determining who ultimately pays the tax – the business itself, the owners, or, in some cases, both.

Understanding this framework provides the foundation for every area of business taxation. Whether discussing payroll taxes, depreciation, business credits, or owner compensation, each topic begins with knowing how the business is organized and how it is taxed.

With this foundation in place, we are now ready to explore the other major taxes that affect individuals and businesses in future articles.

By: Vien Vo, CPA

July 21, 2026