Taxes must be applied among the states under a set of constitutional rules that split the taxing power between Washington and the states and then fence in both. Federal indirect taxes like excise taxes have to be charged at the same rate in every state. Federal direct taxes have to be divided among the states in proportion to population. States can tax activity within their borders, but they cannot discriminate against businesses from other states, reach taxpayers with no real connection to the state, or single out imports and exports. Those rules shape almost every tax bill you see, from the federal gas tax to state sales and income taxes.
Uniformity for Federal Indirect Taxes
Article I, Section 8 lets Congress impose taxes, duties, and excises, but requires that all such indirect taxes be “uniform throughout the United States.”1Legal Information Institute. U.S. Constitution Annotated Article I, Section 8, Clause 1 An indirect tax is one built into the price of goods or services rather than billed directly to a person. The federal excise tax on gasoline, currently 18.4 cents per gallon, is a familiar example, along with federal taxes on tobacco and alcohol.2U.S. Energy Information Administration. How Much Tax Do We Pay on a Gallon of Gasoline and on a Gallon of Diesel Fuel?
Geographic uniformity means the rate and the collection rules must be identical wherever the taxed activity occurs. A federal excise on a particular chemical hits the Ohio manufacturer at the same rate as the Georgia one. Congress cannot use excise taxes to hand one region a competitive edge. One state may generate more revenue than another simply because more of the taxed activity happens there, and that disparity does not break uniformity. What matters is that the text and application of the law are the same everywhere.
Apportionment for Federal Direct Taxes
Direct taxes face a much stiffer rule. Article I, Section 9 provides that “no Capitation, or other direct, Tax shall be laid, unless in Proportion to the Census.”3Legal Information Institute. U.S. Constitution Annotated Article I, Section 9, Clause 4 – Prohibition on Direct Taxation Overview A direct tax is one imposed directly on a person or on property, such as a head tax or a tax on land ownership. The total collected from each state has to match that state’s share of the national population under the most recent census.
That math creates trouble. If Congress wanted to raise $1 billion through a direct property tax, a state with 10 percent of the population would owe exactly $100 million, no matter how much property sat inside it. A state with expensive real estate and few residents could see very high per-person rates. A crowded state with cheaper land would pay far less per person. Congress rarely imposes direct taxes for exactly this reason. Two neighbors across a state line could face wildly different bills on identical property.
The 16th Amendment Carve-Out for Income Taxes
Apportionment nearly killed the federal income tax. In 1895 the Supreme Court held in Pollock v. Farmers’ Loan & Trust Co. that a tax on income from property was a direct tax that had to be divided by population, which made a broad income tax essentially unworkable. The 16th Amendment, ratified in 1913, fixed the problem by authorizing Congress to “lay and collect taxes on incomes, from whatever source derived, without apportionment among the several States, and without regard to any census or enumeration.”4Legal Information Institute. Amendment XVI – Income Tax Deductions and Exemptions Your federal income tax rate depends on what you earn, not the state you live in. Other kinds of direct taxes still have to follow the original apportionment math.
The Federal Ban on Taxing Exports
Article I, Section 9, Clause 5 flatly prohibits Congress from imposing any tax or duty on goods exported from any state.5Legal Information Institute. Prohibition on Taxes on Exports This is one of the Constitution’s few absolute bans. Southern states, dependent on exporting agricultural products, feared that a Northern-dominated Congress would tax their exports into the ground, and the clause was their protection. It kicks in once goods enter the stream of exportation to a foreign country. A general property tax that happens to cover goods sitting in a warehouse before export does not violate the clause; a tax specifically targeting exports does. User fees for services like customs inspections can still apply because they compensate for a specific service rather than functioning as a tax on the goods.
Commerce Clause Limits on State Taxes
The Constitution gives Congress the power to regulate interstate commerce, and courts have long read that grant as an implied limit on state taxing power even when Congress has not acted. This is the dormant Commerce Clause. It stops states from using tax codes to discriminate against out-of-state businesses or to burden the flow of goods and services across state lines.6Legal Information Institute. State Taxation and the Dormant Commerce Clause A state cannot impose a higher tax on goods shipped in from another state than on identical goods produced locally.
The Supreme Court’s 1977 decision in Complete Auto Transit, Inc. v. Brady set a four-part test that any state tax touching interstate commerce must pass:7Justia U.S. Supreme Court. Complete Auto Transit, Inc. v. Brady, 430 U.S. 274 (1977)
- A substantial nexus between the taxed activity and the state.
- Fair apportionment, so the state taxes only the share of business activity actually occurring in the state and multiple states do not tax the same dollar of income.
- No discrimination against interstate commerce or out-of-state competitors.
- A fair relation to services the state provides the taxpayer, such as roads, police, and courts.8Legal Information Institute. The Benefit Prong of the Complete Auto Test for Taxes on Interstate Commerce
Fail any one prong and a court can strike the tax down. Companies operating across state lines rely on this test when a state tries to claim a larger share of their income than their in-state activity supports.
State Taxes on Foreign Trade
Article I, Section 10 adds a separate restriction on the states: no state can impose duties on imports or exports without Congressional consent, except for charges strictly necessary to run its inspection programs.9Legal Information Institute. Import-Export Clause Any revenue a state does collect from permitted inspection charges has to go to the U.S. Treasury, and Congress can override these laws at any time.
The clause is narrower than it looks. It covers goods moving to or from foreign countries and does not reach goods shipped between states. It also does not block every tax that happens to touch imported goods. A general sales tax applied equally to domestic and imported products is fine. What the clause prohibits is a tax aimed specifically at imports or exports because of their cross-border character.
When a State Can Reach You: Nexus
Even a tax that clears the Commerce Clause has to satisfy the Due Process Clause of the 14th Amendment. Due process requires a minimum connection between the state and the person or business it wants to tax. You must have purposefully taken advantage of the state’s economy or legal protections in a way that makes the tax obligation foreseeable and fair. The due process standard and the Commerce Clause’s substantial nexus prong overlap without being identical. Due process asks whether it is basically fair to pull this taxpayer into the state’s system. The Commerce Clause asks whether the state is overreaching in a way that hurts interstate trade. In most cases, a tax that satisfies one satisfies the other.
Economic Nexus After South Dakota v. Wayfair
For decades the Supreme Court’s 1992 decision in Quill Corp. v. North Dakota held that a state could not require a business to collect sales tax unless the business had a physical presence in the state, such as an office, warehouse, or employees. The internet dismantled that framework. Online retailers were shipping billions of dollars of goods into states where they had no physical footprint, and states were losing significant tax revenue.
In 2018 the Supreme Court overruled Quill in South Dakota v. Wayfair, Inc., holding that physical presence is not required for sales tax nexus.10Supreme Court of the United States. South Dakota v. Wayfair, Inc., 585 U.S. ___ (2018) A substantial economic connection to the state is enough. The South Dakota law upheld in the case applied only to sellers delivering more than $100,000 in goods or services into the state, or completing 200 or more separate transactions there, in a year. The Court treated those thresholds as evidence the law targeted sellers with a real presence in the state’s economy rather than small or occasional vendors.
Nearly every state with a sales tax has since adopted economic nexus rules. The most common threshold is $100,000 in annual sales revenue, though California and New York use $500,000. Many states also have a separate 200-transaction trigger, though some have dropped it. If you sell into multiple states, you have to track each state’s threshold separately. Crossing the line in a state obligates you to register, collect sales tax from customers there, and remit it, even if you have never set foot in the state.
The Federal Shield for Interstate Sellers: Public Law 86-272
Federal law adds a separate protection for certain businesses selling physical products across state lines. Under 15 U.S.C. ยง 381, known as Public Law 86-272, a state cannot impose a net income tax on an out-of-state business if the company’s only in-state activity is soliciting orders for tangible goods and those orders are approved and shipped from outside the state.11Office of the Law Revision Counsel. 15 USC 381 – Imposition of Net Income Tax A sales representative can visit customers, hand out samples, and take orders without triggering the state’s income tax, as long as the orders are sent out of state for approval and filled from somewhere else.
The protection has real boundaries. It covers only sales of tangible personal property, so companies selling services, digital products, software licenses, or leasing arrangements get no shelter. It does not apply to businesses incorporated in the taxing state or to individuals who live there. It only blocks net income taxes, not sales taxes, gross receipts taxes, or franchise taxes measured by something other than net income. If employees do anything beyond solicitation in the state, such as repairing products, collecting payments, or conducting training unrelated to sales, the protection disappears. Many states have also taken the position that internet-based activities like cookies, app-based interactions, and remote employee access to in-state customers push a company past mere solicitation, so the practical scope of this protection is narrowing.
How States Handle Double Taxation of Income
Live in one state and earn income in another, and both states have a legitimate claim to tax that income. The home state can tax you as a resident. The work state can tax you because the income was earned there. Without relief you would pay in full to both. Most states with an income tax solve this by giving residents a credit for taxes paid to another state. If you live in State A and pay income tax to State B on wages earned there, State A reduces your bill by what you already paid to State B, up to what State A would have charged on that same income.
The credit is typically nonrefundable, so it can zero out your home-state tax on the out-of-state income but will not produce a refund. Some states have reciprocal agreements that simplify things further: your employer withholds tax only for your home state, and you skip filing in the work state. If you work remotely or split time across states, the rules get more complicated. Some states tax based on where you physically perform the work, others on where your employer is located. Tracking your work days by state becomes the basis for what you owe each one.