State income tax apportionment is the method each state uses to figure out how much of a multistate company’s income it gets to tax. When your business earns money across state lines, no single state can tax the whole amount. Instead, every state where you have a taxable connection applies its own formula that measures how much of your business activity happens inside its borders, and taxes only that percentage of your total income. The formula varies from state to state, but the principle is uniform: a state’s share of your tax should track the share of business you actually do there.
The mechanics matter because two companies with identical total income can end up with very different state tax bills depending on where their customers, workers, and property sit, and depending on which states’ formulas apply to them.
When a State Can Tax You: Nexus
Apportionment only becomes relevant after a state establishes that it has the right to tax you at all. That right is called nexus. Historically, nexus required a physical footprint: an office, a warehouse, or employees in the state. That standard has expanded. A growing number of states now assert income tax nexus based purely on economic activity, using a “factor presence” test.
The Multistate Tax Commission’s model standard creates nexus in a state when a company exceeds any one of these thresholds during a tax year:
- $50,000 in property located in the state
- $50,000 in payroll paid in the state
- $500,000 in sales sourced to the state
- 25 percent of the company’s total property, payroll, or sales
Several states have adopted these thresholds or variations of them, and others set their own amounts.1Multistate Tax Commission. Factor Presence Nexus Standard for Business Activity Taxes A company with no physical presence in a state can owe corporate income tax there simply because its sales into the state are large enough. Remote sellers and digital businesses feel this most.
One federal limit still exists. Under Public Law 86-272, a state cannot impose a net income tax on a company whose only in-state activity is soliciting orders for tangible personal property, provided those orders are approved and filled from outside the state.2Office of the Law Revision Counsel. United States Code Title 15 Section 381 The protection is narrow. It covers tangible goods only, solicitation only, and nothing beyond that. Services, software licensing, and most digital activities fall outside the shield. More on how states are reading that in the internet era below.
The constitutional backdrop is the Supreme Court’s four-part test from Complete Auto Transit, Inc. v. Brady: a state tax on interstate commerce is valid only when it applies to an activity with substantial nexus to the state, is fairly apportioned, does not discriminate against interstate commerce, and is fairly related to services the state provides.3Justia Law. Complete Auto Transit Inc v Brady 430 US 274 (1977) Every apportionment formula has to answer to that “fairly apportioned” prong.
The Apportionment Formula
Once nexus is established, the state runs a formula to calculate the percentage of your income it can tax. The traditional formula, developed under the Uniform Division of Income for Tax Purposes Act (UDITPA) in the 1950s, weighted three factors equally: property, payroll, and sales. Each factor is a fraction comparing in-state activity to activity everywhere. Average the three fractions and you have the apportionment percentage.
That equal-weight approach is largely gone. About two-thirds of states with a corporate income tax now use a single sales factor formula, which ignores property and payroll and bases the entire apportionment percentage on where the company’s customers are located.4Multistate Tax Commission. Review of MTC Model Sales Receipts Sourcing and Special Industry Regulations A handful of remaining states use a weighted formula that emphasizes sales but still gives some weight to property and payroll. Only about 10 percent of taxing states still use the original equally weighted three-factor version.
The move to a single sales factor is deliberate economic policy. Counting property and payroll punishes companies for building facilities and hiring workers in a state, because those activities directly increase the apportionment percentage. Dropping the two factors removes that disincentive. The trade-off: states with large consumer markets capture more revenue from out-of-state sellers, while states that primarily host manufacturing or back-office operations lose tax base.
How the Three Factors Are Measured
Even if a state uses only the sales factor, the other two matter for nexus thresholds and for filing in states that still weight them.
Property
The property factor compares the value of real estate and tangible business property in the state to total property everywhere. Under UDITPA’s model rules, property is valued at original cost rather than depreciated book value. Rented property gets converted to an ownership-equivalent value by multiplying net annual rent by eight.5Multistate Tax Commission. Multistate Tax Compact The factor uses an average of beginning-of-year and end-of-year values.
Payroll
The payroll factor compares compensation paid to employees working in the state against total compensation paid everywhere. Compensation includes wages, salaries, commissions, and other remuneration. Pay generally follows the location where the employee performs most of their work. When an employee works in multiple states, the pay gets assigned to the state where the employee’s base of operations is located, or where the work is directed and controlled.
Sales
The sales factor compares revenue sourced to the state against total revenue everywhere. Because the sales factor now carries all or most of the weight in the majority of states, sourcing rules for sales have become the single most consequential piece of the entire calculation.
Sourcing the Sales Factor
Sourcing rules assign a particular sale to a particular state. For tangible goods the rule is easy: the sale is sourced to where the product is delivered. Services and intangible property are the hard part, and two competing approaches divide the states.
Cost-of-Performance
The older method sources a service sale to the state where the company incurs the costs of delivering the service. A consulting firm doing most of the work at its headquarters in State A sources that revenue to State A, even if the client is in State B. Where costs are split, the entire sale typically goes to whichever state has the largest share. That all-or-nothing structure created planning opportunities: concentrate service employees in a low-tax state, source most revenue there.
Market-Based Sourcing
Most taxing states have replaced cost-of-performance with market-based sourcing, which assigns service revenue to the state where the customer receives the benefit.4Multistate Tax Commission. Review of MTC Model Sales Receipts Sourcing and Special Industry Regulations The logic is that income comes from the market, not the back office. A New York law firm advising a Texas client sources that revenue to Texas.
Market-based sourcing works cleanly for services with identifiable customers and gets messy with digital products. Where is the benefit received when a company licenses software accessible nationwide, or sells advertising viewed across dozens of states? Software revenue often gets sourced to where the user accesses it, which can require tracking login locations. Digital advertising tends to follow the audience. The specific methodology for measuring “benefit received” still varies from state to state.
Certain industries operate under their own sourcing regimes. Financial institutions, airlines, railroads, and trucking companies typically apportion using industry-specific factors like deposits, revenue miles, or originated loans rather than the general sales factor.
Business Income vs. Non-Business Income
Not all corporate income runs through the apportionment formula. In states following UDITPA, only business income is apportioned. Non-business income is allocated to a single state.
Business income is revenue arising from the company’s regular operations or from property integral to those operations. If a manufacturer sells a warehouse it has used for years, the gain is business income because the property was part of the ongoing enterprise. Non-business income is everything else, typically passive investment returns disconnected from the core operations, such as interest on a short-term bank deposit or rent from investment-only property.
Under UDITPA’s model rules, allocation of non-business income follows the type of income:
- Real property rents, royalties, and gains: allocated to the state where the property is located
- Tangible personal property rents and gains: allocated to the state where the property is used, or to the company’s commercial domicile if the company isn’t taxable in the state of use
- Interest and dividends: allocated to the state of the company’s commercial domicile
- Patent and copyright royalties: allocated to the state where the payer uses the patent or copyright
The commercial domicile is generally the state where the company’s principal place of business is located.6Multistate Tax Commission. UDITPA Issues to Consider for Revision Whether income is business or non-business can shift where it gets taxed dramatically. States tend to read business income broadly, and many apply both a “transactional test” (was the income earned in the regular course of business?) and a “functional test” (was the underlying property integral to business operations?) to pull as much income as possible into the apportionment formula.
Combined Reporting vs. Separate Filing
Large corporations often operate through networks of subsidiaries, and a state’s treatment of related entities can change the apportionment result substantially.
Under separate entity reporting, each subsidiary files independently, and only subsidiaries that individually have nexus with the state need to file there. A parent can shift income to a subsidiary incorporated in a low-tax or no-tax state to reduce the overall bill.
Under combined reporting, all members of a “unitary” group pool their income and apportion it together. A unitary group exists when the subsidiaries are integrated enough that the operations of one contribute to the profitability of the others. Roughly half the states with a corporate income tax now require combined reporting, largely to shut down the income-shifting that separate filing allows.7Institute on Taxation and Economic Policy. Combined Reporting of State Corporate Income Taxes A Primer
Multinationals face a further question: which entities get pulled into the combined group? Under a “water’s-edge” election, foreign subsidiaries and certain U.S. entities that do most of their business overseas are excluded. Under worldwide combined reporting, every affiliate everywhere is included. Most states that require combined reporting default to water’s-edge or allow the company to elect it.
Joyce vs. Finnigan
When a combined group files in a state, one more question arises: what about sales made into that state by a group member that doesn’t individually have nexus there? Two competing rules govern.
Under Joyce, only sales by group members that individually have nexus count in the state’s sales factor numerator. If Subsidiary A has nexus in Minnesota but Subsidiary B does not, Minnesota ignores Subsidiary B’s sales into the state. Under Finnigan, the combined group is treated as a single taxpayer, and if any member has nexus, all members’ sales into the state count.8Multistate Tax Commission. Finnigan Briefing Book Finnigan generally produces a higher apportionment percentage and a larger bill, which is why revenue-hungry states tend to prefer it.
Throwback and Throwout Rules
Apportionment can leave gaps where some income isn’t taxed by any state. A company ships goods from State A to customers in State B, but has no nexus in State B. State B can’t tax the income. State A might not include those sales in its own formula because the goods went elsewhere. The result is “nowhere income.” Throwback and throwout rules close that gap.
About twenty-two states and the District of Columbia use a throwback rule. When a company ships tangible goods from a state into a destination state where it isn’t taxable, the sale gets thrown back and included in the originating state’s sales factor numerator.9Multistate Tax Commission. Notes on Throwback Rule Ship from an Illinois warehouse to a customer in a state where you have no nexus, and Illinois counts that sale as an Illinois sale. Throwback applies only to tangible personal property.
A smaller number of states use a throwout rule instead. Rather than adding “nowhere” sales to the numerator, throwout removes them from both the numerator and the denominator. The apportionment percentage still rises because the denominator shrinks. States with throwout rules tend to apply them more broadly, sometimes covering intangibles and services in addition to tangible goods.
Whether a sale gets thrown back or thrown out depends on whether the company is “taxable” in the destination state, and that determination often hinges on P.L. 86-272. If your only activity in the destination state is soliciting orders for tangible goods, P.L. 86-272 shields you from income tax there, making you not taxable and triggering the throwback in the origin state. Cross the line beyond pure solicitation and the destination state can tax you, and the throwback rule no longer applies.
Public Law 86-272 in the Digital Economy
P.L. 86-272 was written in 1959, when interstate commerce meant shipping physical goods. Its protection reaches only companies whose sole in-state activity is soliciting orders for tangible personal property. Services, digital products, licensing, and any transaction involving intangible property have never been covered.2Office of the Law Revision Counsel. United States Code Title 15 Section 381
The Multistate Tax Commission has issued revised guidance identifying common internet activities that strip away P.L. 86-272 protection even for companies that sell tangible goods. Activities beyond protected solicitation include:
- Providing post-sale customer support via live chat or email initiated through the company’s website
- Soliciting and processing branded credit card applications online, which generates interest and fee income separate from product sales
- Allowing in-state job applicants to submit applications and resumes through the website
- Placing tracking cookies on in-state customers’ devices to collect browsing data used for product development, inventory planning, or targeted marketing
- Transmitting remote software updates or upgrades to products already purchased by in-state customers
- Offering and selling extended warranty plans through the website
Any of these can create nexus in every state where in-state customers interact with the company’s website.10Multistate Tax Commission. Statement of Information Concerning Practices of Multistate Tax Commission and Supporting States Under Public Law 86-272 For e-commerce companies that once counted on P.L. 86-272 to limit their state exposure, this is a substantial expansion of taxing authority. A company that only sells physical products but uses its website to collect customer data or provide support may now have income tax obligations in states it previously ignored.
Who Actually Deals With Apportionment
Apportionment primarily affects C-corporations operating in more than one state. S-corporations, partnerships, and LLCs structured as pass-throughs generally don’t pay corporate income tax at the entity level; income flows through to the owners, who report it on their personal state returns. A growing number of states have enacted pass-through entity taxes that allow the entity itself to pay state income tax on behalf of its owners, and apportionment can apply to those elections as well.
Six states impose no corporate income tax at all. Nevada, Ohio, Texas, and Washington levy gross receipts taxes instead, while South Dakota and Wyoming impose neither a corporate income tax nor a gross receipts tax. Companies operating exclusively in those states don’t face apportionment for corporate income tax purposes, though gross receipts taxes have their own sourcing rules to work through.